1.
Chemical formula
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These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, the simplest types of chemical formulas are called empirical formulas, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulas indicate the numbers of each type of atom in a molecule. For example, the formula for glucose is CH2O, while its molecular formula is C6H12O6. This is possible if the relevant bonding is easy to show in one dimension, an example is the condensed molecular/chemical formula for ethanol, which is CH3-CH2-OH or CH3CH2OH. For reasons of structural complexity, there is no condensed chemical formula that specifies glucose, chemical formulas may be used in chemical equations to describe chemical reactions and other chemical transformations, such as the dissolving of ionic compounds into solution. A chemical formula identifies each constituent element by its chemical symbol, in empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, as ratios to the key element. For molecular compounds, these numbers can all be expressed as whole numbers. For example, the formula of ethanol may be written C2H6O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of compounds, however, cannot be written with entirely whole-number empirical formulas. An example is boron carbide, whose formula of CBn is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the consists of simple molecules. These types of formulas are known as molecular formulas and condensed formulas. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the formula for glucose is C6H12O6 rather than the glucose empirical formula. However, except for very simple substances, molecular chemical formulas lack needed structural information, for simple molecules, a condensed formula is a type of chemical formula that may fully imply a correct structural formula. For example, ethanol may be represented by the chemical formula CH3CH2OH
2.
Hydrocarbon
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In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon, and thus are group 14 hydrides. Hydrocarbons from which one atom has been removed are functional groups. Aromatic hydrocarbons, alkanes, alkenes, cycloalkanes and alkyne-based compounds are different types of hydrocarbons, the classifications for hydrocarbons, defined by IUPAC nomenclature of organic chemistry are as follows, Saturated hydrocarbons are the simplest of the hydrocarbon species. They are composed entirely of single bonds and are saturated with hydrogen, the formula for acyclic saturated hydrocarbons is CnH2n+2. The most general form of saturated hydrocarbons is CnH2n+2, where r is the number of rings and those with exactly one ring are the cycloalkanes. Saturated hydrocarbons are the basis of petroleum fuels and are found as linear or branched species. Substitution reaction is their characteristics property, hydrocarbons with the same molecular formula but different structural formulae are called structural isomers. As given in the example of 3-methylhexane and its higher homologues, chiral saturated hydrocarbons constitute the side chains of biomolecules such as chlorophyll and tocopherol. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms and those with double bond are called alkenes. Those with one double bond have the formula CnH2n and those containing triple bonds are called alkyne. Those with one triple bond have the formula CnH2n−2, aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring. Hydrocarbons can be gases, liquids, waxes or low melting solids or polymers, in terms of shells, carbon consists of an incomplete outer shell, which comprises 4 electrons, and thus has 4 electrons available for covalent or dative bonding. Some hydrocarbons also are abundant in the solar system, lakes of liquid methane and ethane have been found on Titan, Saturns largest moon, confirmed by the Cassini-Huygens Mission. Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon compounds, hydrocarbons are a primary energy source for current civilizations. The predominant use of hydrocarbons is as a fuel source. In their solid form, hydrocarbons take the form of asphalt, mixtures of volatile hydrocarbons are now used in preference to the chlorofluorocarbons as a propellant for aerosol sprays, due to chlorofluorocarbons impact on the ozone layer. Methane and ethane are gaseous at ambient temperatures and cannot be liquefied by pressure alone. Propane is however easily liquefied, and exists in propane bottles mostly as a liquid, butane is so easily liquefied that it provides a safe, volatile fuel for small pocket lighters
3.
Homoaromaticity
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Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a cycle of π electrons that is responsible for this preserved chemical stability. The concept of homoaromaticity was pioneered by Saul Winstein in 1959, to date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common. The homotropylium cation is perhaps the best studied example of a homoaromatic compound, the term homoaromaticity derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugated alkenes previously observed in the literature. The IUPAC Gold Book requires that Bis-, Tris-, etc. prefixes be used to describe compounds in which two, three, etc. sp3 centers separately interrupt conjugation of the aromatic system. The concept of homoaromaticity has its origins in the debate over the non-classical carbonium ions that occurred in the 1950s, Saul Winstein, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclohexyl cation. In a series of experiments, Winstein et al. observed that the solvolysis reaction occurred empirically faster when the tosyl leaving group was in the equatorial position. The group ascribed this difference in rates to the anchimeric assistance invoked by the cis isomer. This result thus supported a non-classical structure for the cation, Winstein subsequently observed that this non-classical model of the 3-bicyclohexyl cation is analogous to the previously well-studied aromatic cyclopropenyl cation. Like the cyclopropenyl cation, positive charge is delocalized over three equivalent carbons containing two π electrons and this electronic configuration thus satisfies Huckels rule for aromaticity. The group thus proposed the name tris-homocyclopropenyl—the tris-homo counterpart to the cyclopropenyl cation, the criterion for aromaticity has evolved as new developments and inisights continue to contribute to our understanding of these remarkably stable organic molecules. The required characteristics of these molecules has remained the subject of some controversy. Classically, aromatic compounds were defined as planar molecules that possess a cyclically delocalized system of π electrons, most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability relative to predictions based on localized resonance structures. Consequently, the criterion for homoaromatic delocalization remains similarly ambiguous and somewhat controversial, after initial reports of a homoaromatic structure for the tris-homocyclopropenyl cation were published by Winstein, many groups began to report observations of similar compounds. Much of the evidence for homoaromaticity comes from observations of unusual NMR properties associated with this molecule. From this observation, Pettit, et al. concluded that the structure of the cyclooctatrienyl cation must be incorrect. Upon further consideration, Pettit was inclined to represent the compound as the homotropylium ion and this structure shows how delocalization is cyclic and involves 6 π electrons, consistent with Huckels rule for aromaticity. The magnetic field of the NMR could thus induce a current in the ion
4.
Cyclodecapentaene
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Cyclodecapentaene or annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and it is not aromatic, however, because various types of ring strain destabilize an all-planar geometry. The all-cis isomer, a fully convex decagon, would have bond angles of 144°, instead, the all-cis isomer can adopt a planar boat-like conformation to relieve the angle strain. This is still unstable compared to the next planar trans, cis, trans, yet even this isomer is also unstable, suffering from steric repulsion between the two internal hydrogen atoms. The nonplanar trans, cis, cis, cis, cis isomer is the most stable of all the possible isomers, the annulene compound can be obtained by photolysis of cis-9, 10-dihydronaphthalene as a mixture of isomers. Because of their lack in stability even at low temperatures the reaction products revert to the original dihydronaphthalene, aromaticity can be induced in compounds having a annulene-type core by fixation of the planar geometries. There exist two methods to bring it about, one way is to replace two hydrogen atoms by a methylene bridge gives the planar bicyclic 1, 6-methanoannulene. This compound is aromatic as indicated by from lack in bond length alternation seen in its X-ray structure, another way to restore planarity, and therefore aromaticity, in annulene rings is incorporation of a methine bridge to a tricyclicannulene core structure. When deprotonated to form the anion this type of compound is even more stabilized, the central carbanion makes the molecule even more planar and the number of resonance structures that can be drawn is extended to 7 included two resonance forms with a complete benzene ring.4. Azulene is also a 10 π-electron system in which aromaticity is maintained by direct transannular bonding to form a fused 7–5 bicyclic molecule
annulene.svg)
