1.
Rolf Huisgen
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Rolf Huisgen is a German chemist. He was born in Gerolstein in the Rhineland-Palatinate and studied in Munich under the supervision of Heinrich Otto Wieland, after completing his Ph. D. in 1943 and his habilitation in 1947, he was named professor at the University of Tübingen in 1949. He returned to the University of Munich in 1952 where he remained dedicated to long after attaining emeritus status in 1988. One of his achievements was the development of the 1, 3-Dipolar cycloaddition reaction. The Huisgen reaction is of paramount importance to the synthesis of heterocyclic compounds, ivar Karl Ugi, Johann Mulzer, Bernd Giese, Johann Gasteiger, Herbert Mayr, Hans-Ulrich Reissig, Jürgen Sauer and Reinhard Brückner are but a few of them. Rüchardt, Christoph, Sauer, Jürgen, Sustmann, Reiner, Rolf Huisgen, Some Highlights of His Contributions to Organic Chemistry. Houk, Kendall N. Rolf Huisgens Profound Adventures in Chemistry
2.
Royal Society of Chemistry
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The Royal Society of Chemistry is a learned society in the United Kingdom with the goal of advancing the chemical sciences. At its inception, the Society had a membership of 34,000 in the UK. The headquarters of the Society are at Burlington House, Piccadilly and it also has offices in Thomas Graham House in Cambridge where RSC Publishing is based. The Society has offices in the United States at the University City Science Center, Philadelphia, the organisation carries out research, publishes journals, books and databases, as well as hosting conferences, seminars and workshops. The designation FRSC is given to a group of elected Fellows of the society who have made contributions to chemistry. The names of Fellows are published each year in The Times, Honorary Fellowship of the Society is awarded for distinguished service in the field of chemistry. The rim of the wheel is gold, and the spokes are of non-tarnishable metals. The current president is Sir John Holman, AMRSC, Associate Member, Royal Society of Chemistry The entry level for RSC membership, AMRSC is awarded to graduates in the chemical sciences. HonFRSC, Honorary Fellow of the Society Honorary Fellowship is awarded for distinguished service in the field of chemistry, CChem, Chartered Chemist The award of CChem is considered separately from admission to a category of RSC membership. Candidates need to be MRSC or FRSC and demonstrate development of specific attributes and be in a job which requires their chemical knowledge. CSci, Chartered Scientist The RSC is a licensed by the Science Council for the registration of Chartered Scientists, eurChem, European Chemist The RSC is a member of the European Communities Chemistry Council, and can award this designation to Chartered Chemists. MChemA, Mastership in Chemical Analysis The RSC awards this postgraduate qualification which is the UK statutory qualification for practice as a Public Analyst and it requires candidates to submit a portfolio of suitable experience and to take theory papers and a one-day laboratory practical examination. The qualification GRSC was awarded from 1981 to 1995 for completion of college courses equivalent to a chemistry degree. It replaced the GRIC offered by the Royal Institute of Chemistry, the society is organised around 9 divisions, based on subject areas, and local sections, both in the United Kingdom and overseas. Divisions cover broad areas of chemistry but also many special interest groups for more specific areas. Analytical Division for analytical chemistry and promoting the aims of the Society for Analytical Chemistry. Dalton Division, named after John Dalton, for inorganic chemistry, Faraday Division, named after Michael Faraday, for physical chemistry and promoting the original aims of the Faraday Society. There are 12 subjects groups not attached to a division, there are 35 local sections covering the United Kingdom and Ireland
3.
Chemical reaction
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A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Nuclear chemistry is a sub-discipline of chemistry that involves the reactions of unstable. The substance initially involved in a reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, and they yield one or more products. Reactions often consist of a sequence of individual sub-steps, the elementary reactions. Chemical reactions are described with chemical equations, which present the starting materials, end products. Chemical reactions happen at a characteristic reaction rate at a given temperature, typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium, Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of energy to go forward. Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product, in biochemistry, a consecutive series of chemical reactions form metabolic pathways. These reactions are catalyzed by protein enzymes. Chemical reactions such as combustion in fire, fermentation and the reduction of ores to metals were known since antiquity, in the Middle Ages, chemical transformations were studied by Alchemists. They attempted, in particular, to lead into gold, for which purpose they used reactions of lead. The process involved heating of sulfate and nitrate minerals such as sulfate, alum. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid, further optimization of sulfuric acid technology resulted in the contact process in the 1880s, and the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of an element called phlogiston, which was contained within combustible bodies. This proved to be false in 1785 by Antoine Lavoisier who found the explanation of the combustion as reaction with oxygen from the air
4.
Reaction mechanism
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In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism is a conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases and it also describes each reactive intermediate, activated complex, and transition state, and which bonds are broken, and which bonds are formed. A complete mechanism must also explain the reason for the reactants and catalyst used, the stereochemistry observed in reactants and products, all products formed and the amount of each. The electron or arrow pushing method is used in illustrating a reaction mechanism, for example. A reaction mechanism must also account for the order in which molecules react, often what appears to be a single-step conversion is in fact a multistep reaction. Reaction intermediates are often free radicals or ions, the kinetics are explained in terms of the energy needed for the conversion of the reactants to the proposed transition states. Information about the mechanism of a reaction is provided by the use of chemical kinetics to determine the rate equation. Consider the following reaction for example, CO + NO2 → CO2 + NO In this case and this form suggests that the rate-determining step is a reaction between two molecules of NO2. The elementary steps should add up to the original reaction, when determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step is the slowest step, it is the rate-determining step, because it involves the collision of two NO2 molecules, it is a bimolecular reaction with a rate law of r = k 2. Other reactions may have mechanisms of several consecutive steps, in organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth. There are also more complex such as chain reactions, in which the propagation steps of the chain form a closed cycle. A correct reaction mechanism is an important part of accurate predictive modeling, for many combustion and plasma systems, detailed mechanisms are not available or require development. Rate constants or thermochemical data are not available in the literature. Computational chemistry methods can also be used to calculate potential energy surfaces for reactions, molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step. A reaction step involving one molecular entity is called unimolecular, a reaction step involving two molecular entities is called bimolecular. A reaction step involving three molecular entities is called termolecular
5.
Azide
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Azide is the anion with the formula N−3. It is the base of hydrazoic acid. N−3 is an anion that is isoelectronic with CO2, NCO−, N2O, NO+2. Per valence bond theory, azide can be described by resonance structures. Azide is also a group in organic chemistry, RN3. The dominant application of azides is as a propellant in air bags, for example, lead azide, used in detonators, may be prepared from the metathesis reaction between lead nitrate and sodium azide. An alternative route is direct reaction of the metal with silver azide dissolved in liquid ammonia, some azides are produced by treating the carbonate salts with hydrazoic acid. The principal source of the moiety is sodium azide. As a pseudohalogen compound, sodium azide generally displaces a leaving group to give the azido compound. Aryl azides may be prepared by displacement of the diazonium salt with sodium azide, or trimethylsilyl azide, nucleophilic aromatic substitution is also possible. Anilines and aromatic hydrazines undergo diazotization, as do alkyl amines and hydrazines, appropriately functionalized aliphatic compounds undergo nucleophilic substitution with sodium azide. Aliphatic alcohols give azides via a variant of the Mitsunobu reaction, the azo-transfer compounds, trifluoromethanesulfonyl azide and imidazole-1-sulfonyl azide, are prepared from sodium azide as well. Azide salts can decompose with release of gas as discussed under Applications. The decomposition temperatures of the alkali metal azides are, NaN3, KN3, RbN3 and this method is used to produce ultrapure alkali metals. They decompose with sodium nitrite when acidified and this is a method of destroying residual azides, prior to disposal. 2 NaN3 +2 HNO2 →3 N2 +2 NO +2 NaOH Many inorganic covalent azides have been described, the azide anion behaves as a nucleophile, it undergoes nucleophilic substitution for both aliphatic and aromatic systems. It reacts with epoxides, causing a ring-opening, it undergoes Michael-like conjugate addition to 1, Azides can be used as precursors of the metal nitrido complexes. Azide complexes thus is induced to release N2, generating a complex in unusual oxidation states
6.
Alkyne
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In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name also refers specifically to C2H2. Like other hydrocarbons, alkynes are generally hydrophobic but tend to be more reactive, alkynes are characteristically more unsaturated than alkenes. Thus they add two equivalents of bromine whereas an alkene adds only one equivalent in the reaction, in some reactions, alkynes are less reactive than alkenes. For example, in a molecule with an -ene and an -yne group, possible explanations involve the two π-bonds in the alkyne delocalising, which would reduce the energy of the π-system or the stability of the intermediates during the reaction. They show greater tendency to polymerize or oligomerize than alkenes do, the resulting polymers, called polyacetylenes are conjugated and can exhibit semiconducting properties. In acetylene, the H–C≡C bond angles are 180°, by virtue of this bond angle, alkynes are rod-like. The C≡C bond distance of 121 picometers is much shorter than the C=C distance in alkenes or the C–C bond in alkanes, the triple bond is very strong with a bond strength of 839 kJ/mol. The sigma bond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol, bonding usually discussed in the context of molecular orbital theory, which recognizes the triple bond as arising from overlap of s and p orbitals. In the language of valence bond theory, the atoms in an alkyne bond are sp hybridized. Overlap of an sp orbital from each atom forms one sp–sp sigma bond, each p orbital on one atom overlaps one on the other atom, forming two pi bonds, giving a total of three bonds. The remaining sp orbital on each atom can form a bond to another atom. The two sp orbitals project on opposite sides of the carbon atom, internal alkynes feature carbon substituents on each acetylenic carbon. Symmetrical examples include diphenylacetylene and 3-hexyne, terminal alkynes have the formula RC2H. Terminal alkynes, like itself, are mildly acidic, with pKa values of around 25. They are far more acidic than alkenes and alkanes, which have pKa values of around 40 and 50, the acidic hydrogen on terminal alkynes can be replaced by a variety of groups resulting in halo-, silyl-, and alkoxoalkynes. The carbanions generated by deprotonation of terminal alkynes are called acetylides, in systematic chemical nomenclature, alkynes are named with the Greek prefix system without any additional letters
7.
1,2,3-triazole
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1,2, 3-Triazole is one of a pair of isomeric chemical compounds with molecular formula C2H3N3, called triazoles, which have a five-membered ring of two carbon atoms and three nitrogen atoms. 1,2, 3-Triazole is an aromatic heterocycle. Substituted 1,2, 3-triazoles can be produced using the azide alkyne Huisgen cycloaddition in which an azide and it is a surprisingly stable structure compared to other organic compounds with three adjacent nitrogen atoms. However, flash vacuum pyrolysis at 500 °C leads to loss of nitrogen to produce aziridine. Certain triazoles are relatively easy to cleave due to so-called ring-chain tautomerism, one manifestation is found in the Dimroth rearrangement. 1,2, 3-Triazole finds use in research as a block for more complex chemical compounds
8.
Regioselectivity
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In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. A specific example is a halohydrin formation reaction with 2-propenylbenzene, Because of the preference for the formation of one product over another and this reaction is regioselective because it selectively generates one constitutional isomer rather than the other. Certain examples of regioselectivity have been formulated as rules for classes of compounds under certain conditions. Regioselectivity in ring-closure reactions is subject to Baldwins rules, if there are two or more orientations that can be generated during a reaction, one of them is dominant Regioselectivity can also be applied to specific reactions such as addition to pi ligands. Selectivity also occurs in carbene insertions, for example in the Baeyer-Villiger reaction, in this reaction an oxygen is regioselectively inserted near an adjacent carbonyl group. In ketones this insertion is directed toward the carbon which is more highly substituted, for example, in a study involving acetophenones this oxygen was preferentially inserted in-between the carbonyl and the aromatic ring to give acetyl aromatic esters, not methyl benzoates. Zaitsevs rule Cryptoregiochemistry Chemoselectivity Stereoselectivity Enantioselectivity Keto–enol tautomerism
9.
Stereoselectivity
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The selectivity arises from differences in steric effects and electronic effects in the mechanistic pathways leading to the different products. Stereoselectivity can vary in degree but it can never be total since the energy difference between the two pathways is finite. However, in cases, the minor stereoisomer may not be detectable by the analytic methods used. The degree of selectivity is measured by the enantiomeric excess, a diastereoselective reaction is one in which one diastereomer is formed in preference to another, establishing a preferred relative stereochemistry. The degree of selectivity is measured by the diastereomeric excess. Stereoconvergence can be considered an opposite of stereoselectivity, when the reaction of two different stereoisomers yield a single product stereoisomer, the quality of stereoselectivity is concerned solely with the products, and their stereochemistry. Of a number of possible products, the reaction selects one or two to be formed. An example of modest stereoselectivity is the dehydrohalogenation of 2-iodo-butane which yields 60% trans-2-butene, since alkene geometric isomers are also classified as diastereomers, this reaction would also be called diastereoselective. The chiral center need not be pure, as the relative stereochemistry will be the same for both enantiomers. In the case of chiral alcohols, kinetic resolution results. Another example is Sharpless asymmetric dihydroxylation, in the example below the achiral alkene yields only one of possible 4 stereoisomers. With a stereogenic center next to the carbocation the substitution can be stereoselective in inter-, the first dirigent protein was discovered in Forsythia intermedia. This protein has found to direct the stereoselective biosynthesis of -pinoresinol from coniferyl alcohol monomers. Recently, a second, enantiocomplementary dirigent protein was identified in Arabidopsis thaliana, stereospecific Dynamic stereochemistry Torquoselectivity Regioselectivity Chemoselectivity
10.
Heterocyclic compound
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A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring. Heterocyclic chemistry is the branch of chemistry dealing with the synthesis, properties. Examples of heterocyclic compounds include all of the acids, the majority of drugs, most biomass. Although heterocyclic compounds may be inorganic, most contain at least one carbon, while atoms that are neither carbon nor hydrogen are normally referred to in organic chemistry as heteroatoms, this is usually in comparison to the all-carbon backbone. But this does not prevent a compound such as borazine from being labelled heterocyclic, IUPAC recommends the Hantzsch-Widman nomenclature for naming heterocyclic compounds. Heterocyclic compounds can be classified based on their electronic structure. The saturated heterocycles behave like the acyclic derivatives, thus, piperidine and tetrahydrofuran are conventional amines and ethers, with modified steric profiles. Therefore, the study of heterocyclic chemistry focuses especially on unsaturated derivatives, included are pyridine, thiophene, pyrrole, and furan. Another large class of heterocycles are fused to rings, which for pyridine, thiophene, pyrrole, and furan are quinoline, benzothiophene, indole. Fusion of two benzene rings gives rise to a large family of compounds, respectively the acridine, dibenzothiophene, carbazole. The unsaturated rings can be classified according to the participation of the heteroatom in the pi system, heterocycles with three atoms in the ring are more reactive because of ring strain. Those containing one heteroatom are, in general, stable and those with two heteroatoms are more likely to occur as reactive intermediates. Five-membered rings with one heteroatom, The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are called the azoles. Thiazoles and isothiazoles contain a sulfur and an atom in the ring. A large group of 5-membered ring compounds with three heteroatoms also exists, one example is dithiazoles that contain two sulfur and a nitrogen atom. Five-member ring compounds with four heteroatoms, With 5-heteroatoms, the compound may be considered rather than heterocyclic. With 7-membered rings, the heteroatom must be able to provide an empty pi orbital for normal aromatic stabilization to be available, otherwise, for example, with the benzo-fused unsaturated nitrogen heterocycles, pyrrole provides indole or isoindole depending on the orientation. The pyridine analog is quinoline or isoquinoline, for azepine, benzazepine is the preferred name
11.
Concerted reaction
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A concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup of charge in the transition state, the reaction is said to progress through a concerted mechanism as all bonds are formed and broken in concert. Pericyclic reactions, the SN2 reaction, and some rearrangements - such as the Claisen rearrangement - are concerted reactions, the rate of the SN2 reaction is second order overall due to the reaction being bimolecular. The reaction does not have any intermediate steps, only a transition state and this means that all the bond making and bond breaking takes place in a single step. In order for the reaction to occur both molecules must be situated correctly
12.
Pericyclic reaction
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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions, pericyclic reactions often have related stepwise radical processes associated with them. Some pericyclic reactions, such as the cycloaddition, are controversial because their mechanism is not definitively known to be concerted, a large photoinduced hydrogen sigmatropic shift was utilized in a corrin synthesis performed by Albert Eschenmoser containing a 16π system. Due to the principle of microscopic reversibility, there is a set of retro pericyclic reactions. Thus substituents in the diene can significantly affect the rate of the not only by their electronic character. Thus for example cis I -substituted butadiene I is less reactive than its trans isomer II since a bulky R disfavors the cisoid conformation, bulky 2-substituents in the diene favor the cisoid conformation more than the transoid and thus the diene in this case is more reactive
13.
Cycloaddition
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A cycloaddition is a pericyclic chemical reaction, in which two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity. The resulting reaction is a cyclization reaction, many but not all cycloadditions are concerted. As a class of reaction, cycloadditions permit carbon–carbon bond formation without the use of a nucleophile or electrophile. Cycloadditions can be described using two systems of notation, an older, but still common, notation is based on the size of linear arrangements of atoms in the reactants. It uses parentheses, where the variables are the numbers of atoms in each reactant. The product is a cycle of size, in this system, the standard Diels-Alder reaction a cycloaddition, the 1, 3-dipolar cycloaddition is a cycloaddition and cyclopropanation of a carbene with an alkene a cycloaddition. A more recent, IUPAC-preferred notation uses square brackets to indicate the number of electrons, rather than carbon atoms, in the notation, the standard Diels-Alder reaction is a cycloaddition, the 1, 3-dipolar cycloaddition is. Thermal cycloadditions are those cycloadditions where the reactants are in the electronic state. They usually have π electrons participating in the material, for some integer n. These reactions occur, for reasons of symmetry, in a suprafacial-suprafacial or antarafacial-antarafacial manner. Cycloadditions in which 4n π electrons participate can also occur via photochemical activation, here, one component has an electron promoted from the HOMO to the LUMO. Orbital symmetry is then such that the reaction can proceed in a suprafacial-suprafacial manner, an example is the DeMayo reaction. Another example is shown below, the dimerization of cinnamic acid. The two trans alkenes react head-to-tail, and the isomers are called truxillic acids. Supramolecular effects can influence these cycloadditions, the cycloaddition of trans-1, 2-bisethene is directed by resorcinol in the solid-state in 100% yield. Some cycloadditions instead of π bonds operate through strained cyclopropane rings, for example, an analog for the Diels-Alder reaction is the quadricyclane-DMAD reaction, In the cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation a Diels-Alder reaction is a cycloaddition and a 1, the IUPAC preferred notation however, with takes electrons into account and not atoms. In this notation the DA reaction and the dipolar reaction both become a cycloaddition, the reaction between norbornadiene and an activated alkyne is a cycloaddition
14.
Radical (chemistry)
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In chemistry, a radical is an atom, molecule, or ion that has an unpaired valence electron. Most radicals are reasonably stable only at low concentrations in inert media or in a vacuum. A notable example of a radical is the hydroxyl radical. Two other examples are triplet oxygen and triplet carbene which have two unpaired electrons, free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The latter can be affected by any process that puts energy into the parent molecule, such as ionizing radiation, heat, electrical discharges, electrolysis. Radicals are intermediate stages in many chemical reactions, free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes. In living organisms, the free radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and they also play a key role in the intermediary metabolism of various biological compounds. Such radicals can even be messengers in a process dubbed redox signaling, a radical may be trapped within a solvent cage or be otherwise bound. The qualifier free was then needed to specify the unbound case, following recent nomenclature revisions, a part of a larger molecule is now called a functional group or substituent, and radical now implies free. However, the old nomenclature may still appear in some books, the term radical was already in use when the now obsolete radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase radical in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie, a radical was then identified as the root base of certain acids. Historically, the radical in radical theory was also used for bound parts of the molecule. These are now called functional groups, for example, methyl alcohol was described as consisting of a methyl radical and a hydroxyl radical. In a modern context the first organic free radical identified was triphenylmethyl radical and this species was discovered by Moses Gomberg in 1900 at the University of Michigan USA. In 1933 Morris Kharash and Frank Mayo proposed that free radicals were responsible for anti-Markovnikov addition of hydrogen bromide to allyl bromide. It should be noted that the electron of the breaking bond also moves to pair up with the attacking radical electron. Free radicals also take part in addition and radical substitution as reactive intermediates. Chain reactions involving free radicals can usually be divided into three distinct processes and these are initiation, propagation, and termination
15.
Molecular symmetry
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Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a concept in chemistry, as it can predict or explain many of a molecules chemical properties, such as its dipole moment. Many university level textbooks on chemistry, quantum chemistry. While various frameworks for the study of symmetry exist, group theory is the predominant one. This framework is useful in studying the symmetry of molecular orbitals, with applications such as the Hückel method, ligand field theory. Another framework on a scale is the use of crystal systems to describe crystallographic symmetry in bulk materials. Many techniques for the assessment of molecular symmetry exist, including X-ray crystallography and various forms of spectroscopy. Spectroscopic notation is based on symmetry considerations, the study of symmetry in molecules is an adaptation of mathematical group theory. The symmetry of a molecule can be described by 5 types of symmetry elements, symmetry axis, an axis around which a rotation by 360 ∘ n results in a molecule indistinguishable from the original. This is also called a rotational axis and abbreviated Cn. Examples are the C2 axis in water and the C3 axis in ammonia, a molecule can have more than one symmetry axis, the one with the highest n is called the principal axis, and by convention is aligned with the z-axis in a Cartesian coordinate system. Plane of symmetry, a plane of reflection through which a copy of the original molecule is generated. This is also called a plane and abbreviated σ. Water has two of them, one in the plane of the molecule itself and one perpendicular to it, a symmetry plane parallel with the principal axis is dubbed vertical and one perpendicular to it horizontal. A third type of symmetry plane exists, If a vertical symmetry plane additionally bisects the angle between two 2-fold rotation axes perpendicular to the axis, the plane is dubbed dihedral. A symmetry plane can also be identified by its Cartesian orientation, center of symmetry or inversion center, abbreviated i. A molecule has a center of symmetry when, for any atom in the molecule, in other words, a molecule has a center of symmetry when the points and correspond to identical objects. For example, if there is an atom in some point
16.
Transition state theory
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Transition state theory explains the reaction rates of elementary chemical reactions. The theory assumes a special type of equilibrium between reactants and activated transition state complexes. TST is used primarily to understand qualitatively how chemical reactions take place and this theory was developed simultaneously in 1935 by Henry Eyring, then at Princeton University, and by Meredith Gwynne Evans and Michael Polanyi of the University of Manchester. TST is also referred to as theory, absolute-rate theory. Before the development of TST, the Arrhenius rate law was used to determine energies for the reaction barrier. Therefore, further development was necessary to understand the two associated with this law, the pre-exponential factor and the activation energy. During that period, many scientists and researchers contributed significantly to the development of the theory, the basic ideas behind transition state theory are as follows, Rates of reaction can be studied by examining activated complexes near the saddle point of a potential energy surface. The details of how these complexes are formed are not important, the saddle point itself is called the transition state. The activated complexes are in an equilibrium with the reactant molecules. The activated complexes can convert into products, and kinetic theory can be used to calculate the rate of this conversion, by the early 20th century many had accepted the Arrhenius equation, but the physical interpretation of A and Ea remained vague. In 1910, French chemist René Marcelin introduced the concept of standard Gibbs energy of activation and they proposed the following rate constant equation k ∝ exp exp However, the nature of the constant was still unclear. In early 1900, Max Trautz and William Lewis studied the rate of the reaction using collision theory, Lewis applied his treatment to the following reaction and obtained good agreement with experimental result. 2HI → H2 + I2 However, later when the treatment was applied to other reactions. Statistical mechanics played a significant role in the development of TST and it was not until 1912 when the French chemist A. Berthoud used the Maxwell–Boltzmann distribution law to obtain an expression for the rate constant. D ln k d T = a − b T R T2 where a and b are constants related to energy terms. Two years later, René Marcelin made a contribution by treating the progress of a chemical reaction as a motion of a point in phase space. He then applied Gibbs statistical-mechanical procedures and obtained a similar to the one he had obtained earlier from thermodynamic consideration. In 1915, another important contribution came from British physicist James Rice, based on his statistical analysis, he concluded that the rate constant is proportional to the critical increment
17.
Diazo
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Diazo refers to a type of organic compound called diazo compound that has two linked nitrogen atoms as a terminal functional group. The simplest example of a compound is diazomethane. The electronic structure of diazo compounds involves a positive charge on the central nitrogen, some of the most stable diazo compounds are α-diazo-ß-diketones and α-diazo-ß-diesters since the negative charge is delocalized into the carbonyls. In contrast, most alkyldiazo compounds are explosive, a commercially relevant diazo compound is ethyl diazoacetate. A group of compounds with only few similar properties are the diazirines. Four resonance structures can be drawn, Diazo compounds should not be confused with azo compounds of the type R-N=N-R or with diazonium compounds of the type R-N2+. Several laboratory methods exist for the preparation of compounds, Alpha-acceptor-substituted primary aliphatic amines R-CH2-NH2 react with nitrous acid to generate the diazo compound. An example of an electrophilic substitution using a compound is that of a reaction between an acyl halide and diazomethane, for example the first step in the Arndt-Eistert synthesis. In diazo transfer certain carbon acids can be reacted with tosyl azide, examples are the synthesis of tert-butyl diazoaceate and di-tert-butyl diazomalonate. Other oxidizing reagents are lead tetraacetate, manganese dioxide and the Swern reagent, tosylhydrazones RRC=N-NHTs are reacted with base for example triethylamine in the synthesis of crotyl diazoacetate and in the synthesis of phenyldiazomethane from PhCHNHTs and sodium methoxide. These triazenes result from coupling of aromatic diazonium salts with primary amines, one method is described for the synthesis of diazo compounds from azides using phosphines, Diazo compounds react as 1, 3-dipoles in diazoalkane 1, 3-dipolar cycloadditions. Diazo compounds are used as precursors to carbenes, which are generated by thermolysis or photolysis, as such they are used in cyclopropanation for example in the reaction of ethyl diazoacetate with styrene. Certain diazo compounds can couple to form alkenes in a formal carbene dimerization reaction, intramolecular reactions of diazocarbonyl compounds provide access to cyclopropanes. In the Buchner ring expansion diazo compounds react with aromatic rings with ring-expansion, the Buchner-Curtius-Schlotterbeck Reaction yields ketones from aldehydes and aliphatic diazo compounds, The reaction type is nucleophilic addition. Two families of naturally occurring products feature the group, kinamycin and lomaiviticin. These molecules are DNA-intercalators, with diazo functionality as their warheads, loss of N2, induced reductively, generates a DNA-cleaving fluorenyl radical. Azo compound Diazoalkane 1, 3-dipolar cycloaddition Diazonium compound Reprography Whiteprint
18.
Alkene
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In organic chemistry, an alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. The words alkene, olefin, and olefine are used often interchangeably, acyclic alkenes, with only one double bond and no other functional groups, known as mono-enes, form a homologous series of hydrocarbons with the general formula CnH2n. Alkenes have two hydrogen atoms less than the corresponding alkane, the simplest alkene, ethylene, with the International Union of Pure and Applied Chemistry name ethene, is the organic compound produced on the largest scale industrially. Aromatic compounds are drawn as cyclic alkenes, but their structure and properties are different. This double bond is stronger than a single covalent bond and also shorter, each carbon of the double bond uses its three sp2 hybrid orbitals to form sigma bonds to three atoms. The unhybridized 2p atomic orbitals, which lie perpendicular to the created by the axes of the three sp² hybrid orbitals, combine to form the pi bond. This bond lies outside the main C–C axis, with half of the bond on one side of the molecule, rotation about the carbon–carbon double bond is restricted because it incurs an energetic cost to break the alignment of the p orbitals on the two carbon atoms. As a consequence, substituted alkenes may exist as one of two isomers, called cis or trans isomers, more complex alkenes may be named with the E–Z notation for molecules with three or four different substituents. For example, of the isomers of butene, the two groups of -but-2-ene appear on the same side of the double bond, and in -but-2-ene the methyl groups appear on opposite sides. These two isomers of butene are slightly different in their chemical and physical properties, a 90° twist of the C=C bond requires less energy than the strength of a pi bond, and the bond still holds. This contradicts a common assertion that the p orbitals would be unable sustain such a bond. In truth, the misalignment of the p orbitals is less than expected because pyramidalization takes place, trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a dihedral angle of 137° and a degree of pyramidalization of 18°. The trans isomer of cycloheptene is stable only at low temperatures, as predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes includes bond angles about each carbon in a double bond of about 120°. The angle may vary because of steric strain introduced by nonbonded interactions between groups attached to the carbons of the double bond. For example, the C–C–C bond angle in propylene is 123. 9°, for bridged alkenes, Bredts rule states that a double bond cannot occur at the bridgehead of a bridged ring system unless the rings are large enough. The physical properties of alkenes and alkanes are similar and they are colourless, nonpolar, combustable, and almost odorless. The physical state depends on mass, like the corresponding saturated hydrocarbons, the simplest alkenes, ethene, propene. Linear alkenes of approximately five to sixteen carbons are liquids, Alkenes are relatively stable compounds, but are more reactive than alkanes, either because of the reactivity of the carbon–carbon pi-bond or the presence of allylic CH centers
19.
Solvent effects
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In chemistry, solvent effects refers to the influence of a solvent on chemical reactivity or molecular associations. Solvents can have an effect on solubility, stability and reaction rates, a solute dissolves in a solvent when it forms favorable interactions with the solvent. This dissolving process all depends upon the energy change of both solute and solvent. The free energy of solvation is a combination of several factors, first, a cavity must be created in the solvent. The creation of the cavity will be entropically and enthalpically unfavorable as the structure of the solvent decreases. Second, the solute must separate out from the bulk solute and this is enthalpically unfavorable as solute-solute interactions are breaking but is entropically favorable. Third, the solute must occupy the cavity created in the solvent and this results in favorable solute-solvent interactions and is also entropically favorable as the mixture is more disordered than when the solute and solvent are not mixed. Dissolution often occurs when the solute-solvent interactions are similar to the solvent-solvent interactions, hence, polar solutes dissolve in polar solvents, whereas nonpolar solutes dissolve in nonpolar solvents. There is no one measure of solvent polarity and so classification of solvents based on polarity can be carried out using different scales, different solvents can affect the equilibrium constant of a reaction by differential stabilization of the reactant or product. The equilibrium is shifted in the direction of the substance that is preferentially stabilized, the ionization equilibrium of an acid or a base is affected by a solvent change. A change in the ability or dielectric constant can thus influence the acidity or basicity. In the table above, it can be seen that water is the most polar-solvent, followed by DMSO, various 1, 3-dicarbonyl compounds can exist in the following tautomeric forms as shown. 1, 3-dicarbonyl compounds most often undergo tautomerization between the enol form and the diketo form. The intramolecular H bond formed in the cis form is more pronounced when there is no competition for intermolecular H bonding with the solvent. As a result, solvents of low polarity that do not readily form H bonds allow cis enolic stabilization by intramolecular H bonding, often, reactivity and reaction mechanisms are pictured as the behavior of isolated molecules in which the solvent is treated as a passive support. However, solvents can actually influence reaction rates and order of a chemical reaction, solvents can affect rates through equilibrium-solvent effects that can be explained on the basis of the transition state theory. In essence, the rates are influenced by differential solvation of the starting material. When the reactant molecules proceed to the state, the solvent molecules orient themselves to stabilize the transition state
20.
Stereochemistry
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Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. An important branch of stereochemistry is the study of chiral molecules, stereochemistry is also known as 3D chemistry because the prefix stereo- means three-dimensionality. The study of stereochemistry focuses on stereoisomers and spans the spectrum of organic, inorganic, biological, physical. This property, the physical property in which the two types of tartrate salts differed, is due to optical isomerism. In 1874, Jacobus Henricus van t Hoff and Joseph Le Bel explained optical activity in terms of the arrangement of the atoms bound to carbon. Cahn–Ingold–Prelog priority rules are part of a system for describing a molecules stereochemistry and they rank the atoms around a stereocenter in a standard way, allowing the relative position of these atoms in the molecule to be described unambiguously. A Fischer projection is a way to depict the stereochemistry around a stereocenter. An often cited example of the importance of stereochemistry relates to the thalidomide disaster, thalidomide is a pharmaceutical drug, first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug was discovered to be teratogenic, causing genetic damage to early embryonic growth and development. Some of the proposed mechanisms of teratogenecity involve a different biological function for the -. In the human body however, thalidomide undergoes racemization, even if one of the two enantiomers is administered as a drug, the other enantiomer is produced as a result of metabolism. Accordingly, it is incorrect to state one of the stereoisomer is safe while the other is teratogenic. Thalidomide is currently used for the treatment of diseases, notably cancer. Strict regulations and controls have been enabled to avoid its use by pregnant women and this disaster was a driving force behind requiring strict testing of drugs before making them available to the public. Torsional strain results from resistance to twisting about a bond
21.
Stereospecificity
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A stereoselective process will normally give multiple products even if only one mechanism is operating on an isomerically pure starting material. In the latter sense, the term stereospecific reaction is commonly misused to mean highly stereoselective reaction, chiral synthesis is built on a combination of stereospecific transformations and stereoselective ones, where also the optical activity of a chemical compound is preserved. Of stereoisomeric reactants, each behaves in its own specific way, the choice of mechanism adopted by a particular reactant combination depends on other factors. For example, tertiary centres react almost exclusively by the SN1 mechanism whereas primary centres react almost exclusively by the SN2 mechanism. When a nucleophilic substitution results in inversion, it is because of a competition between the two mechanisms, as often occurs at secondary centres, or because of double inversion. The addition of singlet carbenes to alkenes is stereospecific in that the geometry of the alkene is preserved in the product, for example, dibromocarbene and cis-2-butene yield cis-2, 3-dimethyl-1, 1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane. This addition remains stereospecific even if the alkene is not isomerically pure. The disrotatory ring closing reaction of conjugated trienes is stereospecific in that isomeric reactants will give isomeric products
22.
Sigma bond
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In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals, sigma bonding is most simply defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis, by this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2. Quantum theory also indicates that molecular orbitals of identical symmetry actually mix or hybridize, as a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this depends on the relative energies of the MOs of like symmetry. For homodiatomics, bonding σ orbitals have no nodal planes at which the wavefunction is zero, the corresponding antibonding, or σ* orbital, is defined by the presence of one nodal plane between the two bonded atoms. Sigma bonds are the strongest type of covalent bonds due to the overlap of orbitals. The symbol σ is the Greek letter sigma, when viewed down the bond axis, a σ MO has a circular symmetry, hence resembling a similarly sounding s atomic orbital. Typically, a bond is a sigma bond while a multiple bond is composed of one sigma bond together with pi or other bonds. A double bond has one sigma plus one pi bond, sigma bonds are obtained by head-on overlapping of atomic orbitals. The concept of bonding is extended to describe bonding interactions involving overlap of a single lobe of one orbital with a single lobe of another. For example, propane is described as consisting of ten sigma bonds, transition metal complexes that feature multiple bonds, such as the dihydrogen complex, have sigma bonds between the multiple bonded atoms. These sigma bonds can be supplemented with other bonding interactions, such as donation, as in the case of W32. Organic molecules are often cyclic compounds containing one or more rings, such as benzene, according to the sigma bond rule, the number of sigma bonds in a molecule is equivalent to the number of atoms plus the number of rings minus one. Nσ = Natoms + Nrings −1 A molecule with no rings can be represented as a tree with a number of equal to the number of atoms minus one. There is no more than 1 sigma bond between any two atoms, molecules with rings have additional sigma bonds, such as benzene rings, which have 6 C−C sigma bonds within the ring for 6 carbon atoms. The anthracene molecule, C14H10, has three rings so that the rule gives the number of bonds as 24 +3 −1 =26. In this case there are 16 C−C sigma bonds and 10 C−H bonds
23.
Transition state
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The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the corresponding to the highest potential energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules always go on to form products and it is often marked with the double dagger ‡ symbol. The concept of a state has been important in many theories of the rates at which chemical reactions occur. This started with the state theory, which was first developed around 1935 by Eyring, Evans and Polanyi. A collision between reactant molecules may or may not result in a successful reaction, the outcome depends on factors such as the relative kinetic energy, relative orientation and internal energy of the molecules. Even if the partners form an activated complex they are not bound to go on and form products. Because of the rules of quantum mechanics, the state cannot be captured or directly observed. This is sometimes expressed by stating that the state has a fleeting existence. However, cleverly manipulated spectroscopic techniques can get us as close as the timescale of the technique allows, femtochemical IR spectroscopy was developed for precisely that reason, and it is possible to probe molecular structure extremely close to the transition point. Often along the reaction coordinate reactive intermediates are present not much lower in energy from a transition state making it difficult to distinguish between the two, Transition state structures can be determined by searching for first-order saddle points on the potential energy surface of the chemical species of interest. A first-order saddle point is a point of index one, that is. This is further described in the geometry optimization. The Hammond–Leffler Postulate states that the structure of the state more closely resembles either the products or the starting material. A transition state resembles the reactants more than the products is said to be early. Thus, the Hammond–Leffler Postulate predicts a transition state for an endothermic reaction. A dimensionless reaction coordinate that quantifies the lateness of a state can be used to test the validity of the Hammond–Leffler Postulate for a particular reaction. One demonstration of principle is found in the two bicyclic compounds depicted below
24.
Allyl group
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An allyl group is a substituent with the structural formula H2C=CH−CH2R, where R is the rest of the molecule. It consists of a methylene bridge attached to a vinyl group, the name is derived from the Latin word for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil, the term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride. A site on the carbon atom is called the allylic position or allylic site. A group attached at this site is described as allylic. Thus, CH2=CHCH2OH has a hydroxyl group. Allylic C−H bonds are about 15% weaker than the C−H bonds in ordinary sp3 carbon centers and are more reactive. This heightened reactivity has many practical consequences, benzylic and allylic are related in terms of structure, bond strength, and reactivity. Other reactions that tend to occur with compounds are allylic oxidations, ene reactions. Benzylic groups are related to groups, both show enhanced reactivity. A CH2 group connected to two groups is said to be doubly allylic. The bond dissociation energy of C−H bonds on a doubly allylic centre is about 10% less than the bond energy of a C−H bond that is allylic. The weakened C−H bonds reflect the high stability of the resulting pentadienyl radicals, compounds containing the C=C−CH2−C=C linkages, e. g. linoleic acid derivatives, are prone to autoxidation, which can lead to polymerization or form semisolids. This reactivity pattern is fundamental to the behavior of the drying oils. The term homoallylic refers to the position on a skeleton next to an allylic position. In but-3-enyl chloride CH2=CHCH2CH2Cl, the chloride occupies a homoallylic position, the allyl group is widely encountered in organic chemistry. Allylic radicals, anions, and cations are often discussed as intermediates in reactions, all feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance. Each species can be presented by two structures with the charge or unpaired electron distributed at both 1,3 positions
25.
Propargyl
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In organic chemistry, propargyl is an alkyl functional group of 2-propynyl with the structure HC≡C−CH2−, derived from the alkyne propyne. The term propargylic refers to a position on a molecular framework next to an alkynyl group. The name comes from mix of propene and argentum, which refers to the reaction of the terminal alkynes with silver salts. The term homopropargylic designates in the manner a saturated position on a molecular framework next to a propargylic group. A 3-butynyl fragment, HC ≡ C-CH2CH2-, or substituted homologue, alkenyl groups Allyl Vinyl Ethynyl Propargyl chloride Propargyl alcohol Propargyl bromide Propiolic acid
26.
Allene
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An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes, the parent compound of allene is propadiene. Compounds with a structure but with more than three carbon atoms are called cumulenes. Allenes are much more reactive than most other alkenes, for example, their reactivity with gaseous chlorine is more like the reactivity of alkynes than that of alkenes. The central carbon atom of allene forms two sigma bonds and two pi bonds, the central carbon is sp-hybridized, and the two terminal carbon atoms are sp2-hybridized. The bond angle formed by the three carbon atoms is 180°, indicating linear geometry for the atoms of allene. It can also be viewed as a tetrahedral with a similar shape to methane. The symmetry and isomerism of allenes has long fascinated organic chemists, for allenes with four identical substituents, there exist two twofold axes of rotation through the center carbon, inclined at 45° to the CH2 planes at either end of the molecule. The molecule can thus be thought of as a two-bladed propeller, a third twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH2 planes. Thus this class of molecules belong to the D2d point group, because of the symmetry, an unsubstituted allene has no net dipole moment. An allene with two different substituents on each of the two carbon atoms will be chiral because there no longer be any mirror planes. For the bottom, only the group of higher priority need be considered, chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties. Laboratory methods for the formation of allenes include, from geminal dihalocyclopropanes, from reaction of certain terminal alkynes with formaldehyde, copper bromide and added base from dehydrohalogenation of certain dihalides. From reaction of an ester with an acid halide, a Wittig reaction accompanied by dehydrohalogenation Allenes function as ligands. Ni reagents catalyze the cyclooligomerization of allene, using a suitable catalyst, it is possible to reduce just one of the double bonds of an allene. Many rings or ring systems are known by names that assume a maximum number of noncumulative bonds. This may be combined with the λ-convention for specifying nonstandard valency states, e. g. 2λ4δ2, compounds with three or more adjacent carbon–carbon double bonds are called cumulenes. The allene motif is frequently encountered in carbomers, IUPAC, Compendium of Chemical Terminology, 2nd ed. Online corrected version, allenes
27.
Zwitterion
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In chemistry, a zwitterion, formerly called a dipolar ion, is a neutral molecule with both positive and negative electrical charges. Zwitterions are distinct from molecules that have dipoles at different locations within the molecule, zwitterions are sometimes called inner salts. Unlike simple amphoteric compounds that may form either a cationic or anionic species. Amino acids are the examples of zwitterions. These compounds contain an ammonium and a group, and can be viewed as arising via a kind of intramolecular acid–base reaction. NH 2RCHCO 2H ⇌ NH+ 3RCHCO−2 The zwitterionic structure of glycine in the state has been confirmed by neutron diffraction measurements. At least in cases, the zwitterionic form of amino acids also persists in the gas phase. In addition to the acids, many other compounds that contain both acidic and basic centres tautomerize to the zwitterionic form. Examples, such as bicine and tricine, contain a secondary or tertiary amine fragment together with a carboxylic acid fragment. Neutron diffraction measurements show that solid sulfamic acid exists as a zwitterion, many alkaloids, such as LSD and psilocybin, exist as zwitterions because they contain carboxylates and ammonium centres. Many zwitterions contain quaternary ammonium cations, since it lacks N–H bonds, the ammonium center cannot participate in tautomerization. Zwitterions containing quaternary-ammonium centers are common in biology, an example are the betaines. The membrane-forming phospholipids are also commonly zwitterions, the polar head groups in these compounds are zwitterions, resulting from the presence of the anionic phosphate and cationic quaternary ammonium centres. Dipolar compounds and amine oxides are not classified as zwitterions, zwitterionic compounds are neutral compounds having unit electrical charges on atoms
28.
Molecular geometry
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Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism and biological activity. The angles between bonds that an atom forms depend only weakly on the rest of molecule, i. e. they can be understood as approximately local, the molecular geometry can be determined by various spectroscopic methods and diffraction methods. IR, microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques. X-ray crystallography, neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density, gas electron diffraction can be used for small molecules in the gas phase. NMR and FRET methods can be used to determine complementary information including relative distances, dihedral angles, angles, molecular geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries. Larger molecules often exist in multiple stable geometries that are close in energy on the energy surface. Geometries can also be computed by ab initio quantum chemistry methods to high accuracy, the molecular geometry can be different as a solid, in solution, and as a gas. The position of each atom is determined by the nature of the bonds by which it is connected to its neighboring atoms. Since the motions of the atoms in a molecule are determined by quantum mechanics, the overall quantum mechanical motions translation and rotation hardly change the geometry of the molecule. In addition to translation and rotation, a type of motion is molecular vibration. The molecular vibrations are harmonic, and the atoms oscillate about their equilibrium positions, at higher temperatures the vibrational modes may be thermally excited, but they oscillate still around the recognizable geometry of the molecule. At 298 K, typical values for the Boltzmann factor β are, β =0.089 for ΔE =500 cm−1, β =0.008 for ΔE =1000 cm−1, β = 7×10−4 for ΔE =1500 cm−1. When an excitation energy is 500 cm−1, then about 8.9 percent of the molecules are excited at room temperature. To put this in perspective, the lowest excitation vibrational energy in water is the bending mode, thus, at room temperature less than 0.07 percent of all the molecules of a given amount of water will vibrate faster than at absolute zero. As stated above, rotation hardly influences the molecular geometry, but, as a quantum mechanical motion, it is thermally excited at relatively low temperatures. From a classical point of view it can be stated that at temperatures more molecules will rotate faster. In quantum mechanical language, more eigenstates of higher angular momentum become thermally populated with rising temperatures, typical rotational excitation energies are on the order of a few cm−1
29.
Sulfur
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Sulfur or sulphur is a chemical element with symbol S and atomic number 16. It is abundant, multivalent, and nonmetallic, under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a yellow crystalline solid at room temperature. Chemically, sulfur reacts with all elements except for gold, platinum, iridium, tellurium, though sometimes found in pure, native form, sulfur usually occurs as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in ancient times, being mentioned for its uses in ancient India, ancient Greece, China, in the Bible, sulfur is called brimstone. Today, almost all elemental sulfur is produced as a byproduct of removing sulfur-containing contaminants from natural gas, the greatest commercial use of the element is the production of sulfuric acid for sulfate and phosphate fertilizers, and other chemical processes. The element sulfur is used in matches, insecticides, and fungicides, many sulfur compounds are odoriferous, and the smells of odorized natural gas, skunk scent, grapefruit, and garlic are due to organosulfur compounds. Hydrogen sulfide gives the characteristic odor to rotting eggs and other biological processes, sulfur is an essential element for all life, but almost always in the form of organosulfur compounds or metal sulfides. Three amino acids and two vitamins are organosulfur compounds, many cofactors also contain sulfur including glutathione and thioredoxin and iron–sulfur proteins. Disulfides, S–S bonds, confer mechanical strength and insolubility of the keratin, found in outer skin, hair. Sulfur is one of the chemical elements needed for biochemical functioning and is an elemental macronutrient for all organisms. Sulfur is derived from the Latin word sulpur, which was Hellenized to sulphur, the spelling sulfur appears toward the end of the Classical period. In 12th-century Anglo-French, it was sulfre, in the 14th century the Latin ph was restored, for sulphre, the parallel f~ph spellings continued in Britain until the 19th century, when the word was standardized as sulphur. Sulfur was the form chosen in the United States, whereas Canada uses both, the IUPAC adopted the spelling sulfur in 1990, as did the Nomenclature Committee of the Royal Society of Chemistry in 1992, restoring the spelling sulfur to Britain. Sulfur forms polyatomic molecules with different chemical formulas, the best-known allotrope being octasulfur, cyclo-S8. The point group of cyclo-S8 is D4d and its dipole moment is 0 D. Octasulfur is a soft, bright-yellow solid that is odorless and it melts at 115.21 °C, boils at 444.6 °C and sublimes easily. At 95.2 °C, below its melting temperature, cyclo-octasulfur changes from α-octasulfur to the β-polymorph, the structure of the S8 ring is virtually unchanged by this phase change, which affects the intermolecular interactions. At higher temperatures, the viscosity decreases as depolymerization occurs, molten sulfur assumes a dark red color above 200 °C
30.
Phosphorus
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Phosphorus is a chemical element with symbol P and atomic number 15. As an element, phosphorus exists in two major forms—white phosphorus and red phosphorus—but because it is reactive, phosphorus is never found as a free element on Earth. At 0. 099%, phosphorus is the most abundant pnictogen in the Earths crust, with few exceptions, minerals containing phosphorus are in the maximally oxidised state as inorganic phosphate rocks. The glow of phosphorus itself originates from oxidation of the white phosphorus — a process now termed chemiluminescence, together with nitrogen, arsenic, antimony, and bismuth, phosphorus is classified as a pnictogen. Phosphates are a component of DNA, RNA, ATP, and the phospholipids, demonstrating the link between phosphorus and life, elemental phosphorus was first isolated from human urine, and bone ash was an important early phosphate source. Phosphate mines contain fossils, especially marine fossils, because phosphate is present in the deposits of animal remains. Low phosphate levels are an important limit to growth in aquatic systems. The vast majority of compounds produced are consumed as fertilisers. Phosphate is needed to replace the phosphorus that plants remove from the soil, other applications include the role of organophosphorus compounds in detergents, pesticides, and nerve agents. Phosphorus exists as several forms that exhibit different properties. The two most common allotropes are white phosphorus and red phosphorus, from the perspective of applications and chemical literature, the most important form of elemental phosphorus is white phosphorus, often abbreviated as WP. It is a soft and waxy solid consists of tetrahedral P4 molecules. This P4 tetrahedron is also present in liquid and gaseous phosphorus up to the temperature of 800 °C when it starts decomposing to P2 molecules, White phosphorus exists in two crystalline forms, α and β. At room temperature, the α-form is stable, which is common and it has cubic crystal structure and at 195.2 K, it transforms into β-form. These forms differ in terms of the orientations of the constituent P4 tetrahedra. White phosphorus is the least stable, the most reactive, the most volatile, the least dense, White phosphorus gradually changes to red phosphorus. This transformation is accelerated by light and heat, and samples of white phosphorus almost always some red phosphorus. For this reason, white phosphorus that is aged or otherwise impure is also called yellow phosphorus, when exposed to oxygen, white phosphorus glows in the dark with a very faint tinge of green and blue
31.
Resonance (chemistry)
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In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. A molecule or ion with such delocalized electrons is represented by several contributing structures, each contributing structure can be represented by a Lewis structure, with only an integer number of covalent bonds between each pair of atoms within the structure. Several Lewis structures are used collectively to describe the molecular structure. Electron delocalization lowers the energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy. An isomer is a molecule with the chemical formula but with different arrangements of atoms in space. Resonance contributors of a molecule, on the contrary, can differ by the arrangements of electrons. Therefore, the resonance hybrid cannot be represented by a combination of isomers, benzene undergoes substitution reactions, rather than addition reactions as typical for alkenes. He proposed that the bond in benzene is intermediate of a single and double bond. The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in 1926 in a discussion of the states of the helium atom. He compared the structure of the atom with the classical system of resonating coupled harmonic oscillators. Linus Pauling used this mechanism to explain the partial valence of molecules in 1928, the alternative term mesomerism popular in German and French publications with the same meaning was introduced by C. K. Ingold in 1938, but did not catch on in the English literature. The current concept of effect has taken on a related. The double headed arrow was introduced by the German chemist Fritz Arndt who preferred the German phrase zwischenstufe or intermediate stage, the real structure is an intermediate of these structures represented by a resonance hybrid. The contributing structures are not isomers and they differ only in the position of electrons, not in the position of nuclei. Each Lewis formula must have the number of valence electrons. Bonds that have different bond orders in different contributing structures do not have typical bond lengths, the real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be and it is a common misconception that resonance structures are actual transient states of the molecule, with the molecule oscillating between them or existing as an equilibrium between them
32.
Delocalized electron
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In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond. The term is general and can have different meanings in different fields. In organic chemistry, this refers to resonance in conjugated systems, in solid-state physics, this refers to free electrons that facilitate electrical conduction. In quantum chemistry, this refers to molecular orbitals that extend over several adjacent atoms, in the simple aromatic ring of benzene the delocalization of six π electrons over the C6 ring is often graphically indicated by a circle. The fact that the six C-C bonds are equidistant is one indication of this delocalization, in valence bond theory, delocalization in benzene is represented by resonance structures. Delocalized electrons also exist in the structure of solid metals, metallic structure consists of aligned positive ions in a sea of delocalized electrons. This means that the electrons are free to move throughout the structure, in diamond all four outer electrons of each carbon atom are localized between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current, in graphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a system of electrons that is also a part of the chemical bonding. The delocalized electrons are free to move throughout the plane, for this reason, graphite conducts electricity along the planes of carbon atoms, but does not conduct in a direction at right angles to the plane. Standard ab initio quantum chemistry methods lead to delocalized orbitals that, in general, localized orbitals may then be found as linear combinations of the delocalized orbitals, given by an appropriate unitary transformation. In the methane molecule for example, ab initio calculations show bonding character in four molecular orbitals, there are two orbital levels, a bonding molecular orbital formed from the 2s orbital on carbon and triply degenerate bonding molecular orbitals from each of the 2p orbitals on carbon. The localized sp3 orbitals corresponding to each individual bond in valence bond theory can be obtained from a combination of the four molecular orbitals
33.
Bond dipole moment
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The bond dipole moment uses the idea of electric dipole moment to measure the polarity of a chemical bond within a molecule. It occurs whenever there is a separation of positive and negative charges, the bond dipole μ is given by, μ = δ d. The bond dipole is modeled as +δ — δ- with a distance d between the partial charges +δ and δ- and it is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional for electric dipole moment vectors. The SI unit for electric dipole moment is the coulomb-meter and this is too large to be practical on the molecular scale. Bond dipole moments are measured in debyes, represented by the symbol D, which is obtained by measuring the charge δ in units of 10−10 statcoulomb. Note that 10−10 statcoulomb is 0.208 units of charge, so 1.0 debye results from an electron. Another useful conversion factor is 1 C m =2. 9979×1029 D, for diatomic molecules there is only one bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D.5 D. For polyatomic molecules there is more than one bond, and the molecular dipole moment may be approximated as the vector sum of individual bond dipole moments. Often bond dipoles are obtained by the process, a known total dipole of a molecule can be decomposed into bond dipoles. This is done to transfer bond dipole moments to molecules that have the same bonds, the vector sum of the transferred bond dipoles gives an estimate for the total dipole of the molecule
34.
Diazomethane
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Diazomethane is the chemical compound CH2N2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound, in the pure form at room temperature, it is an extremely sensitive explosive yellow gas, thus, it is almost universally used as a solution in diethyl ether. The compound is a methylating agent in the laboratory. For safety and convenience diazomethane is always prepared as needed as a solution in ether and it converts carboxylic acids into their methyl esters or into their homologues. In the Büchner–Curtius–Schlotterbeck reaction diazomethane reacts with an aldehyde to form ketones, when diazomethane is reacted with alcohols or phenols in presence of boron trifluoride, methyl ethers are obtained. Diazomethane is also used as a carbene source. It readily takes part in 1, 3-dipolar cycloadditions, diazomethane is prepared by hydrolysis of an ethereal solution of an N-methyl nitrosamide with aqueous base. CH2N2 reacts with basic solutions of D2O to give the deuterated derivative CD2N2, the concentration of CH2N2 can be determined in either of two convenient ways. It can be treated with an excess of acid in cold Et2O. Unreacted benzoic acid is then back-titrated with standard NaOH, alternatively, the concentration of CH2N2 in Et2O can be determined spectrophotometrically at 410 nm where its extinction coefficient, ε, is 7.2. The gas-phase concentration of diazomethane can be determined using photoacoustic spectroscopy, many substituted derivatives of diazomethane have been prepared, The very stable 2CN2, Ph2CN2. 3SiCHN2, which is available as a solution and is as effective as CH2N2 for methylation. PhCN2, a red liquid b. p. <25 °C at 0.1 mm Hg, diazomethane is toxic by inhalation or by contact with the skin or eyes. Symptoms include chest discomfort, headache, weakness and, in severe cases, deaths from diazomethane poisoning have been reported. In one instance a laboratory worker consumed a hamburger near a fumehood where he was generating a large quantity of diazomethane, like any other alkylating agent it is expected to be carcinogenic, but such concerns are overshadowed by its serious acute toxicity. CH2N2 may explode in contact with sharp edges, such as ground-glass joints, glassware should be inspected before use and preparation should take place behind a blast shield. Specialized kits to prepare diazomethane with flame-polished joints are commercially available, the compound explodes when heated beyond 100 °C, exposed to intense light, alkali metals, or calcium sulfate. Use of a blast shield is recommended while using this compound
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Hydrazoic acid
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Hydrazoic acid, also known as hydrogen azide or azoimide, is a compound with the chemical formula HN3. It is a colorless, volatile, and explosive liquid at room temperature and pressure and it is a compound of nitrogen and hydrogen, and is therefore a pnictogen hydride. It was first isolated in 1890 by Theodor Curtius, the acid has few applications, but its conjugate base, the azide ion, is useful in specialized processes. Hydrazoic acid is soluble in water, undiluted hydrazoic acid is dangerously explosive with a standard enthalpy of formation ΔfHo = +264 kJmol−1). When dilute, the gas and aqueous solutions can be safely handled, the acid is usually formed by acidification of an azide salt like sodium azide. The pure acid may be obtained by fractional distillation as an extremely explosive colorless liquid with an unpleasant smell. NaN3 + HCl → HN3 + NaCl Its aqueous solution can also be prepared by treatment of barium azide solution with dilute sulfuric acid and it was originally prepared by the reaction of aqueous hydrazine with nitrous acid. N2H5+ + HNO2 → HN3 + H+ +2 H2O Other oxidizing agents, such as hydrogen peroxide, NOCl, NCl3 or nitric acid, in its properties hydrazoic acid shows some analogy to the halogen acids, since it forms poorly soluble lead, silver and mercury salts. The metallic salts all crystallize in the form and decompose on heating, leaving a residue of the pure metal. It is a weak acid Its heavy metal salts are explosive, azides of heavier alkali metals or alkaline earth metals are not explosive, but decompose in a more controlled way upon heating, releasing spectroscopically-pure N2 gas. Solutions of hydrazoic acid dissolve many metals with liberation of hydrogen and formation of salts, dissolution in the strongest acids produces explosive salts containing the H 2N=N=N+ ion, for example, HN=N=N + HSbCl 6 → +− The ion H 2N=N=N+ is isoelectronic to diazomethane. The decomposition of hydrazoic acid, triggered by shock, friction, spark, etc. goes as follows, 2HN3 → H2 + 3N2 Hydrazoic acid is volatile and it has a pungent smell and its vapor can cause violent headaches. The compound acts as a non-cumulative poison
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Nucleophile
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A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases, Nucleophilic describes the affinity of a nucleophile to the nuclei. Nucleophilicity, sometimes referred to as strength, refers to a substances nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis, nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge. The terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1933, the word nucleophile is derived from nucleus and the Greek word φιλος, philos for love. In general, in a row across the table, the more basic the ion the more reactive it is as a nucleophile. g. The iodide ion is more nucleophilic than the fluoride ion, many schemes attempting to quantify relative nucleophilic strength have been devised. The following empirical data have been obtained by measuring reaction rates for a number of reactions involving a large number of nucleophiles and electrophiles. Nucleophiles displaying the so-called alpha effect are usually omitted in this type of treatment. This treatment results in the values for typical nucleophilic anions, acetate 2.7, chloride 3.0, azide 4.0, hydroxide 4.2, aniline 4.5, iodide 5.0. Typical substrate constants are 0.66 for ethyl tosylate,0.77 for β-propiolactone,1.00 for 2, 3-epoxypropanol,0.87 for benzyl chloride, and 1.43 for benzoyl chloride. The equation predicts that, in a nucleophilic displacement on benzyl chloride, the Ritchie equation, derived in 1972, is another free-energy relationship, log 10 = N + where N+ is the nucleophile dependent parameter and k0 the reaction rate constant for water. In this equation, a substrate-dependent parameter like s in the Swain–Scott equation is absent, the equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, which is in violation of the Reactivity–selectivity principle. For this reason this equation is called the constant selectivity relationship. Many other reaction types have since been described. Typical Ritchie N+ values are,0.5 for methanol,5.9 for the anion,7.5 for the methoxide anion,8.5 for the azide anion. The values for the relative cation reactivities are -0.4 for the malachite green cation, +2.6 for the benzenediazonium cation, the constant s is defined as 1 with 2-methyl-1-pentene as the nucleophile
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Electrophile
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In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to a rich centre. It participates in a reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids, most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons. The electrophiles are attacked by the most electron-populated part of one nucleophile and these occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of water to titrate against a sample to deduce the number of double bonds present. Forming of a bromonium ion The alkene is working as an electron donor. The three-membered bromonium ion 2 consisted with two atoms and a bromine atom forms with a release of Br−. Attacking of bromide ion The bromonium ion is opened by the attack of Br− from the back side and this yields the vicinal dibromide with an antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack 2 to give an alcohol or an ether and this process is called AdE2 mechanism. Iodine, chlorine, sulfenyl ion, mercury cation, and dichlorocarbene also react through similar pathways, the direct conversion of 1 to 3 will appear when the Br− is large excess in the reaction medium. A β-bromo carbenium ion intermediate may be predominant instead of 3 if the alkene has a cation-stabilizing substituent like phenyl group, there is an example of the isolation of the bromonium ion 2. Hydrogen halides such as hydrogen chloride adds to alkenes to give alkyl halides in hydrohalogenation, for example, the reaction of Hcl with ethylene furnishes chloroethane. The reaction proceeds with an intermediate, being different from the above halogen addition. An example is shown below, Proton adds to one of the atoms on the alkene to form cation 1. Chloride ion combines with the cation 1 to form the adducts 2 and 3, in this manner, the stereoselectivity of the product, that is, from which side Cl− will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H+ is usually decided by Markovnikovs rule, thus, H+ attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation will form. This process is called A-SE2 mechanism, hydrogen fluoride and hydrogen iodide react with alkenes in a similar manner, and Markovnikov-type products will be given
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Frontier molecular orbital theory
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In chemistry, frontier molecular orbital theory is an application of MO theory describing HOMO / LUMO interactions. In 1952, Kenichi Fukui published a paper in the Journal of Chemical Physics titled A molecular theory of reactivity in aromatic hydrocarbons, though widely criticized at the time, he later shared the Nobel Prize in Chemistry with Roald Hoffmann for his work on reaction mechanisms. He used these interactions to better understand the conclusions of the Woodward–Hoffmann rules, Fukui realized that a good approximation for reactivity could be found by looking at the frontier orbitals. This was based on three observations of molecular orbital theory as two molecules interact, The occupied orbitals of different molecules repel each other. Positive charges of one molecule attract the negative charges of the other, the occupied orbitals of one molecule and the unoccupied orbitals of the other interact with each other causing attraction. From these observations, frontier molecular orbital theory simplifies reactivity to interactions between the HOMO of one species and the LUMO of the other and it can be shown that if the total number of these systems is odd then the reaction is thermally allowed. A cycloaddition is a reaction that forms at least two new bonds, and in doing so, converts two or more open-chain molecules into rings. The transition states for these reactions involves the electrons of the molecules moving in continuous rings. These reactions can be predicted by the Woodward–Hoffmann rules and thus are closely approximated by FMO Theory, thus, there is one s component and no a component, which means the reaction is allowed thermally. FMO theory also finds that this reaction is allowed and goes further by predicting its stereoselectivity. Since this is a, the reaction can be simplified by considering the reaction between butadiene and ethene, the maleic anhydride is an electron-withdrawing species that makes the dieneophile electron deficient, forcing the regular Diels–Alder reaction. Thus, only the reaction between the HOMO of cyclopentadiene and the LUMO of maleic anhydride is allowed, since the exo-product has primary orbital interactions it can still form, but since the endo-product forms faster it is the major product. *Note, The HOMO of ethene and the LUMO of butadiene are both symmetric, meaning the reaction between these species is allowed as well and this is referred to as the inverse electron demand Diels–Alder. A sigmatropic rearrangement is a reaction in which a bond moves across a conjugated pi system with a concomitant shift in the pi bonds. The shift in the bond may be antarafacial or suprafacial. For an antarafacial shift, the reaction is not allowed and these results can be predicted with FMO theory by observing the interaction between the HOMO and LUMO of the species. To use FMO theory, the reaction should be considered as two ideas, whether or not the reaction is allowed, and which mechanism the reaction proceeds though. In the case of a shift on pentadiene, the HOMO of the sigma bond, assuming the reaction happens suprafacially, the shift results with the HOMO of butadiene on the 4 carbons that are not involved in the sigma bond of the product
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HOMO/LUMO
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In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for highest occupied molecular orbital and lowest unoccupied molecular orbital, the energy difference between the HOMO and LUMO is termed the HOMO–LUMO gap. HOMO and LUMO are sometimes called frontier orbitals in frontier molecular orbital theory, the difference in energy between these two frontier orbitals can be used to predict the strength and stability of transition metal complexes, as well as the colors they produce in solution. Roughly, the HOMO level is to organic semiconductors what the valence band maximum is to inorganic semiconductors, the same analogy exists between the LUMO level and the conduction band minimum. In organometallic chemistry, the size of the LUMO lobe can help predict where addition to pi ligands will occur, a SOMO is a singly occupied molecular orbital such as half-filled HOMO of a radical. This abbreviation may also be extended to semi occupied molecular orbital, frontier molecular orbital theory Diels–Alder reaction Electron configuration Koopmans theorem Ligand Organic semiconductor OrbiMol Molecular orbital database
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Heteroatom
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In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure, typical heteroatoms are nitrogen, oxygen, sulfur, phosphorus, chlorine, bromine, and iodine. In the context of zeolites, the term refers to partial isomorphous substitution of the typical framework atoms by other elements such as beryllium, vanadium. The goal is usually to adjust properties of the material to optimize the material for a certain application
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Carbonyl group
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In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom, C=O. It is common to several classes of compounds, as part of many larger functional groups. A compound containing a group is often referred to as a carbonyl compound. The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex, the remainder of this article concerns itself with the organic chemistry definition of carbonyl, where carbon and oxygen share a double bond. A carbonyl group characterizes the types of compounds, Note that the most specific labels are usually employed. For example, ROR structures are known as acid anhydride rather than the more generic ester, other organic carbonyls are urea and the carbamates, the derivatives of acyl chlorides chloroformates and phosgene, carbonate esters, thioesters, lactones, lactams, hydroxamates, and isocyanates. Examples of inorganic compounds are carbon dioxide and carbonyl sulfide. A special group of compounds are 1, 3-dicarbonyl compounds that have acidic protons in the central methylene unit. Examples are Meldrums acid, diethyl malonate and acetylacetone, because oxygen is more electronegative than carbon, carbonyl compounds often have resonance structures which affect their reactivity. This relative electronegativity draws electron density away from carbon, increasing the bonds polarity, carbon can then be attacked by nucleophiles or a negatively charged part of another molecule. During the reaction, the double bond is broken. This reaction is known as addition-elimination or condensation, the electronegative oxygen also can react with an electrophile, for example a proton in an acidic solution or with Lewis acids to form an oxocarbenium ion. The polarity of oxygen also makes the alpha hydrogens of carbonyl compounds much more acidic than typical sp3 C-H bonds, for example, the pKa values of acetaldehyde and acetone are 16.7 and 19 respectively, while the pKa value of methane is extrapolated to be approximately 50. This is because a carbonyl is in resonance with an enol. The deprotonation of the enol with a base produces an enolate. Amides are the most stable of the carbonyl couplings due to their high resonance stabilization between the nitrogen-carbon and carbon-oxygen bonds, carbonyl groups can be reduced by reaction with hydride reagents such as NaBH4 and LiAlH4, with bakers yeast, or by catalytic hydrogenation. Ketones give secondary alcohols while aldehydes, esters and carboxylic acids give primary alcohols, carbonyls can be alkylated in nucleophilic addition reactions using organometallic compounds such as organolithium reagents, Grignard reagents, or acetylides. Carbonyls also may be alkylated by enolates as in aldol reactions, carbonyls are also the prototypical groups with vinylogous reactivity