Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only small amounts of catalyst are required to alter the reaction rate in principle. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst reacts to form a temporary intermediate, which regenerates the original catalyst in a cyclic process. A substance which provides a mechanism with a higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance which does reduce the reaction rate is not considered a catalyst but a reaction inhibitor. Catalysts may be classified as either heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase as the reactant's molecules.
A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations; the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have a lower activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions are typical chemical reactions. The catalyst participates in this slowest step, rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or sublimate in a solid–gas system; the production of most industrially important chemicals involves catalysis. Most biochemically significant processes are catalysed.
Research into catalysis is a major field in applied science and involves many areas of chemistry, notably organometallic chemistry and materials science. Catalysis is relevant to many aspects of environmental science, e.g. the catalytic converter in automobiles and the dynamics of the ozone hole. Catalytic reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated, as opposed to stoichiometric reactions in which all reactants are consumed and more side products are formed. Many transition metals and transition metal complexes are used in catalysis as well. Catalysts called. A catalyst works by providing an alternative reaction pathway to the reaction product; the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates oxygen, as shown below. 2 H2O2 → 2 H2O + O2This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow.
In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms. Upon the addition of a small amount of manganese dioxide, the hydrogen peroxide reacts rapidly; this effect is seen by the effervescence of oxygen. The manganese dioxide is not consumed in the reaction, thus may be recovered unchanged, re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction. Catalytic activity is denoted by the symbol z and measured in mol/s, a unit, called katal and defined the SI unit for catalytic activity since 1999. Catalytic activity is not a kind of reaction rate, but a property of the catalyst under certain conditions, in relation to a specific chemical reaction. Catalytic activity of one katal of a catalyst means one mole of that catalyst will catalyse 1 mole of the reactant to product in one second. A catalyst may and will have different catalytic activity for di
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, can be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur; the substance involved in a chemical reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, they yield one or more products, which have properties different from the reactants. Reactions consist of a sequence of individual sub-steps, the so-called elementary reactions, the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. 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 described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of free 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. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.
The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, reactions between elementary particles, as described by quantum field theory. Chemical reactions such as combustion in fire and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water and earth. In the Middle Ages, chemical transformations were studied by Alchemists, they attempted, in particular, to convert lead into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur. The production of chemical substances that do not occur in nature has long been tried, such as the synthesis of sulfuric and nitric acids attributed to the controversial alchemist Jābir ibn Hayyān; the process involved heating of sulfate and nitrate minerals such as copper sulfate and saltpeter.
In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the lead chamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s, the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, Isaac Newton tried to establish theories of the experimentally observed chemical transformations; the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", contained within combustible bodies and released during combustion; this proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.
Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which resulted in the concepts of stoichiometry and chemical equations. Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically. According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials; this separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, among many discoveries, established the mechanisms of substitution reactions. Chemical equations are used to graphically illustrate chemical reactions, they consist of chemical or structural formulas of the reactants on the left and those of the products on the right.
They are separated by an arrow which indicates the type of the reaction.
In chemistry, a nonmetal is a chemical element that lacks the characteristics of a metal. Physically, a nonmetal tends to have a low melting point, boiling point, density. A nonmetal is brittle when solid and has poor thermal conductivity and electrical conductivity. Chemically, nonmetals tend to have high ionization energy, electron affinity, electronegativity, they share electrons when they react with other elements and chemical compounds. Seventeen elements are classified as nonmetals: most are gases. Metalloids such as boron and germanium are sometimes counted as nonmetals; the nonmetals are divided into two categories reflecting their relative propensity to form chemical compounds: reactive nonmetals and noble gases. The reactive nonmetals vary in their nonmetallic character; the less electronegative of them, such as carbon and sulfur have weak to moderately strong nonmetallic properties and tend to form covalent compounds with metals. The more electronegative of the reactive nonmetals, such as oxygen and fluorine, are characterised by stronger nonmetallic properties and a tendency to form predominantly ionic compounds with metals.
The noble gases are distinguished by their great reluctance to form compounds with other elements. The distinction between categories is not absolute. Boundary overlaps, including with the metalloids, occur as outlying elements in each category show or begin to show less-distinct, hybrid-like, or atypical properties. Although five times more elements are metals than nonmetals, two of the nonmetals—hydrogen and helium—make up over 99 percent of the observable universe. Another nonmetal, makes up half of the Earth's crust and atmosphere. Living organisms are composed entirely of nonmetals: hydrogen, oxygen and nitrogen. Nonmetals form many more compounds than metals. There is no rigorous definition of a nonmetal. Broadly, any element lacking a preponderance of metallic properties can be regarded as a nonmetal; the elements classified as nonmetals include one element in group 1. As there is no agreed definition of a nonmetal, elements in the periodic table vicinity of where the metals meet the nonmetals are inconsistently classified by different authors.
Elements sometimes classified as nonmetals are the metalloids boron, germanium, antimony and astatine. The nonmetal selenium is sometimes instead classified as a metalloid in environmental chemistry. Nonmetals show more variability in their properties; these properties are determined by the interatomic bonding strengths and molecular structures of the nonmetals involved, both of which are subject to variation as the number of valence electrons in each nonmetal varies. Metals, in contrast, have more homogenous structures and their properties are more reconciled. Physically, they exist as diatomic or monatomic gases, with the remainder having more substantial forms, unlike metals, which are nearly all solid and close-packed. If solid, they have a submetallic appearance and are brittle, as opposed to metals, which are lustrous, ductile or malleable. Chemically the nonmetals have high ionisation energies, high electron affinities and high electronegativity values noting that, in general, the higher an element's ionisation energy, electron affinity, electronegativity, the more nonmetallic that element is.
Nonmetals exist as anions or oxyanions in aqueous solution. Complicating the chemistry of the nonmetals is the first row anomaly seen in hydrogen, nitrogen and fluorine; the first row anomaly arises from the electron configurations of the elements concerned. Hydrogen is noted for the different ways, it most forms covalent bonds. It can lose its single valence electron in aqueous solution, leaving behind a bare proton with tremendous polarising power; this subsequently attaches itself to the lone electron pair of an oxygen atom in a water molecule, thereby forming the basis of acid-base chemistry. Under certain conditions a hydrogen atom in a molecule can form a second, bond with an atom or group of atoms in another molecule; such bonding, "helps give snowflakes their hexagonal symmetry, binds DNA into a double helix. From to neon, since the 2p subshell has no inner analogue and experiences no electron repulsion effects it has a small radius, unlike the 3p, 4p and 5p subshells of h
Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, which have a general formula of n−, where X is the heteroatom, M is the addenda atom, O represents oxygen; the structure self-assembles in acidic aqueous solution and is the most stable structure of polyoxometalate catalysts. The first α-Keggin anion, ammonium phosphomolybdate, was first reported by Berzelius in 1826. In 1892, Blomstrand proposed the structure of phosphomolybdic acid and other poly-acids as a chain or ring configuration. Alfred Werner, using the coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid, he assumed 4 − ion, enclosed by four +, where R is a unipositive ion. The + are linked to the central group by primary valences. Two more R2W2O7 groups were linked to the central group by secondary valences; this proposal accounted for the characteristics of most poly-acids, but not all. In 1928, Linus Pauling proposed a structure for α-Keggin anions consisting of a tetrahedral central ion, n−8, caged by twelve WO6 octahedral.
In this proposed structure, three of the oxygen on each of the octahedral shared electrons with three neighboring octahedral. As a result, 18 oxygen atoms were used as bridging atoms between the metal atoms; the remaining oxygen atoms bonded to a proton. This structure explained many characteristics that were observed such as basicities of alkali metal salts and the hydrated of some of the salts; however the structure could not explain the structure of dehydrated acids. James Fargher Keggin with the use of X-ray diffraction experimentally determined the structure of α-Keggin anions in 1934; the Keggin structure accounts for both the hydrated and dehydrated α-Keggin anions without a need for significant structural change. The Keggin structure is the accepted structure for the α-Keggin anions. = + The structure is composed of one heteroatom surrounded by four oxygen atoms to form a tetrahedron. The heteroatom is located centrally and caged by 12 octahedral MO6-units linked to one another by the neighboring oxygen atoms.
There are a total of 24 bridging oxygen atoms. The metal centres in the 12 octahedra are arranged on a sphere equidistant from each other, in four M3O13 units, giving the complete structure an overall tetrahedral symmetry; the bond length between atoms varies depending on the addenda atoms. For the 12–phosphotungstic acid, Keggin determined the bond length between the heteroatom and each the four central oxygen atoms to be 1.5 Å. The bond length form the central oxygen to the addenda atoms is 2.43 Å. The bond length between the addenda atoms and each of the bridging oxygen is 1.9 Å. The remaining 12 oxygen atoms that are each double bonded to an addenda atom have a bond length of 1.70 Å. The octahedra are therefore distorted; this structure allows the molecule to hydrate and dehydrate without significant structural changes and the molecule is thermally stable in the solid state for use in vapor phase reactions at high temperatures. Including the original Keggin structure there are 5 isomers, designated by the prefixes α-, β-, γ-, δ- and ε-.
The original Keggin structure is designated α-. These isomers are sometimes termed Baker, Baker-Figgis or rotational isomers, These involve different rotational orientations of the Mo3O13 units, which lowers the symmetry of the overall structure; the term lacunary is applied to ions which have a fragment missing, sometimes called defect structures. Examples are the n− and n− formed by the removal from the Keggin structure of sufficient Mo and O atoms to eliminate 1 or 3 adjacent MO6 octahedra; the Dawson structure, X2M18O62n−, is made up of two Keggin lacunary fragments with 3 missing octahedra. The cluster cation 7+ has the Keggin structure with a tetrahedral Al atom in the centre of the cluster coordinated to 4 oxygen atoms; the formula can be expressed as 7+. This ion is called the Al13 ion. A Ga13 analogue is known an unusual ionic compound with an Al13 cation and a Keggin polyoxoanion has been characterised. Due to the similar aqueous chemistries of aluminum and iron, it has been long thought that an analogous iron polycation should be isolatable from water.
Moreover, in 2007, the structure of ferrihydrite was determined and shown to be built of iron Keggin ions. This further captured scientists' drive to isolate the iron Keggin ion. In 2015, the iron Keggin ion was isolated from water, but as a polyanion with a −17 charge. Iron-bound water is acidic. However, more important in this synthesis was the bismuth counterions that provided high positive charge to stabilize the high negative charge of the heptadecavalent polyanion; the stability of the Keggin structure allows the metals in the anion to be reduced. Depending on the solvent, acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple-electron steps. For example, silicotungstate anion can be reduced to 20th state; some anions such as silicotungstic acid are strong enough as an acid as sulfuric acid and can be used in its place as an acid catalyst. In general α-Keggin anions are synthesized in acidic solutions. For example, 12-Phosphotungstic acid is formed by condensing phosphate ion with tungstate ions.
The heteropolyacid, formed has the Keggin structure. PO3−4 + 12 WO2−4 + 27 H+ → H3PW
The periodic table known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron configuration, recurring chemical properties. The structure of the table shows periodic trends; the seven rows of the table, called periods have metals on the left and non-metals on the right. The columns, called groups, contain elements with similar chemical behaviours. Six groups have accepted names as well as assigned numbers: for example, group 17 elements are the halogens. Displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals; the organization of the periodic table can be used to derive relationships between the various element properties, to predict chemical properties and behaviours of undiscovered or newly synthesized elements. Russian chemist Dmitri Mendeleev published the first recognizable periodic table in 1869, developed to illustrate periodic trends of the then-known elements.
He predicted some properties of unidentified elements that were expected to fill gaps within the table. Most of his forecasts proved to be correct. Mendeleev's idea has been expanded and refined with the discovery or synthesis of further new elements and the development of new theoretical models to explain chemical behaviour; the modern periodic table now provides a useful framework for analyzing chemical reactions, continues to be used in chemistry, nuclear physics and other sciences. The elements from atomic numbers 1 through 118 have been discovered or synthesized, completing seven full rows of the periodic table; the first 94 elements all occur though some are found only in trace amounts and a few were discovered in nature only after having first been synthesized. Elements 95 to 118 have only been synthesized in nuclear reactors; the synthesis of elements having higher atomic numbers is being pursued: these elements would begin an eighth row, theoretical work has been done to suggest possible candidates for this extension.
Numerous synthetic radionuclides of occurring elements have been produced in laboratories. Each chemical element has a unique atomic number representing the number of protons in its nucleus. Most elements have differing numbers of neutrons among different atoms, with these variants being referred to as isotopes. For example, carbon has three occurring isotopes: all of its atoms have six protons and most have six neutrons as well, but about one per cent have seven neutrons, a small fraction have eight neutrons. Isotopes are never separated in the periodic table. Elements with no stable isotopes have the atomic masses of their most stable isotopes, where such masses are shown, listed in parentheses. In the standard periodic table, the elements are listed in order of increasing atomic number Z. A new row is started. Columns are determined by the electron configuration of the atom. Elements with similar chemical properties fall into the same group in the periodic table, although in the f-block, to some respect in the d-block, the elements in the same period tend to have similar properties, as well.
Thus, it is easy to predict the chemical properties of an element if one knows the properties of the elements around it. Since 2016, the periodic table has 118 confirmed elements, from element 1 to 118. Elements 113, 115, 117 and 118, the most recent discoveries, were confirmed by the International Union of Pure and Applied Chemistry in December 2015, their proposed names, moscovium and oganesson were announced by the IUPAC in June 2016 and made official in November 2016. The first 94 elements occur naturally. Of the 94 occurring elements, 83 are primordial and 11 occur only in decay chains of primordial elements. No element heavier than einsteinium has been observed in macroscopic quantities in its pure form, nor has astatine. A group or family is a vertical column in the periodic table. Groups have more significant periodic trends than periods and blocks, explained below. Modern quantum mechanical theories of atomic structure explain group trends by proposing that elements within the same group have the same electron configurations in their valence shell.
Elements in the same group tend to have a shared chemistry and exhibit a clear trend in properties with increasing atomic number. In some parts of the periodic table, such as the d-block and the f-block, horizontal similarities can be as important as, or more pronounced than, vertical similarities. Under an international naming convention, the groups are numbered numerically from 1 to 18 from the leftmost column to the rightmost column, they were known by roman numerals. In America, the roman numerals were followed by either an "A" if the group was in the s- or p-block, or a "B" if the group was in the d-block; the roman numerals used correspond to the last digit of today's naming convention (e.g. the
The Prins reaction is an organic reaction consisting of an electrophilic addition of an aldehyde or ketone to an alkene or alkyne followed by capture of a nucleophile. The outcome of the reaction depends on reaction conditions. With water and a protic acid such as sulfuric acid as the reaction medium and formaldehyde the reaction product is a 1,3-diol; when water is absent, the cationic intermediate loses a proton to give an allylic alcohol. With an excess of formaldehyde and a low reaction temperature the reaction product is a dioxane; when water is replaced by acetic acid the corresponding esters are formed. The original reactants employed by Dutch chemist Hendrik Jacobus Prins in his 1919 publication were styrene, camphene, eugenol and anethole. Hendrik Jacobus Prins discovered two new organic reactions during his doctoral research in the year of 1911-1912; the first one is the addition of polyhalogen compound to olefins and the second reaction is the acid catalyzed the addition of aldehydes to olefin compounds.
The early studies on Prins reaction are exploratory in nature and did not attract much attention until 1937. The development of petroleum cracking in 1937 increased the production of unsaturated hydrocarbons; as a consequence, commercial availability of lower olefin coupled with an aldehyde produced from oxidation of low boiling paraffin increased the curiosity to study the olefin-aldehyde condensation. On, Prins reaction emerged as a powerful C-O and C-C bond forming technique in the synthesis of various molecules in organic synthesis. In 1937 the reaction was investigated as part of a quest for di-olefins to be used in synthetic rubber; the reaction mechanism for this reaction is depicted in scheme 5. The carbonyl reactant is protonated by a protic acid and for the resulting oxonium ion 3 two resonance structures can be drawn; this electrophile engages in an electrophilic addition with the alkene to the carbocationic intermediate 4. How much positive charge is present on the secondary carbon atom in this intermediate should be determined for each reaction set.
Evidence exists for neighbouring group participation of the hydroxyl oxygen or its neighboring carbon atom. When the overall reaction has a high degree of concertedness, the charge built-up will be modest; the three reaction modes open to this oxo-carbenium intermediate are: in blue: capture of the carbocation by water or any suitable nucleophile through 5 to the 1,3-adduct 6. in black: proton abstraction in an elimination reaction to unsaturated compound 7. When the alkene carries a methylene group and addition can be concerted with transfer of an allyl proton to the carbonyl group which in effect is an ene reaction in scheme 6. in green: capture of the carbocation by additional carbonyl reactant. In this mode the positive charge is dispersed over oxygen and carbon in the resonance structures 8a and 8b. Ring closure leads through intermediate 9 to the dioxane 10. An example is the conversion of styrene to 4-phenyl-m-dioxane. in gray: only in specific reactions and when the carbocation is stable the reaction takes a shortcut to the oxetane 12.
The photochemical Paternò–Büchi reaction between alkenes and aldehydes to oxetanes is more straightforward. Many variations of the Prins reaction exist because it lends itself to cyclization reactions and because it is possible to capture the oxo-carbenium ion with a large array of nucleophiles; the halo-Prins reaction is one such modification with replacement of protic acids and water by lewis acids such as stannic chloride and boron tribromide. The halogen is now the nucleophile recombining with the carbocation; the cyclization of certain allyl pulegones in scheme 7 with titanium tetrachloride in dichloromethane at −78 °C gives access to the decalin skeleton with the hydroxyl group and chlorine group predominantly in cis configuration. This observed cis diastereoselectivity is due to the intermediate formation of a trichlorotitanium alkoxide making possible an easy delivery of chlorine to the carbocation ion from the same face; the trans isomer is preferred when the switch is made to a tin tetrachloride reaction at room temperature.
The Prins-pinacol reaction is a cascade reaction of a pinacol rearrangement. The carbonyl group in the reactant in scheme 8 is masked as a dimethyl acetal and the hydroxyl group is masked as a triisopropylsilyl ether. With lewis acid stannic chloride the oxonium ion is activated and the pinacol rearrangement of the resulting Prins intermediate results in ring contraction and referral of the positive charge to the TIPS ether which forms an aldehyde group in the final product as a mixture of cis and trans isomers with modest diastereoselectivity; the Prins reaction is used in total synthesis of complex natural products, for example, in a key step of that of the synthesis of exiguolide: Heteropoly acid Prins reaction in Alkaloid total synthesis Link Prins reaction @ organic-chemistry.org