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14-September-2008 18:38:39 - Aromatic hydrocarbon Arene redirects here. For other uses, see Arene disambiguation. An aromatic hydrocarbon abbreviated as AH or arene 1 is a hydrocarbon, of which the molecular structure incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. This sweet scent actually came from impurities in the compounds which are not actually aromatic in the sense initially described. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible such hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic or polycyclic. Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule, are also aromatic compounds. In these compounds, at least one carbon atom is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes an oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.2 Contents 1 Benzene ring model 2 Arene synthesis 3 Arene reactions 4 Benzene and derivatives of benzene 5 Polyaromatic hydrocarbons 6 See also 7 External links 8 References Benzene ring model Benzene Benzene Main article: aromaticity Benzene, C6H6, is the simplest AH and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Friedrich August Kekulé von Stradonitz in the 19th century. Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons. This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon. Many chemists draw a circle around the inside of the ring to show six electrons floating around in delocalized molecular orbitals the size of the ring itself. This also accurately represents the equivalent nature of the six bonds all of bond order ~1.5. This equivalency is well explained by resonance forms. The electrons float above and below the ring, and the electromagnetic fields they generate keep the ring flat. General properties: Display aromaticity. The Carbon-Hydrogen ratio is very large. They burn with a sooty yellow flame because of the high carbon-hydrogen ratio. They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions. Arene synthesis Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors: Alkyne trimerization, 2+2+2 cyclization of three alkynes Dötz reaction Diels-Alder reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide. Aromatization of cyclohexanes and other aliphatic rings: reagents are, catalysts used in hydrogenation such as platinum, palladium and nickel reverse hydrogenation, quinones and the elements sulfur and selenium 3. Bergman cyclization, enyne plus hydrogen donor Arene reactions The main arene reactions are Electrophilic aromatic substitution Nucleophilic aromatic substitution Many coupling reactions to biraryls Hydrogenation to saturated rings Lesser-known reactions: Unusual thermal Diels-Alder reactivity of arenes can be found in the Wagner-Jauregg reaction Other photochemical cycloaddition reactions with alkenes through excimers. Benzene and derivatives of benzene Toluene Toluene Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol, which carries a hydroxyl group and toluene with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns ortho, meta, and para are devised. For example, three isomers exist for cresol because the methyl group and the hydroxyl group can be placed next to each other ortho, one position removed from each other meta, or two positions removed from each other para. Xylenol has two methyl groups in addition to the hydroxyl group, and, for this structure, 6 isomers exist. Examples of benzene derivatives with alkyl substituents alkylbenzenes: Ethylbenzene C6H5-CH2-CH3 Toluene C6H5-CH3 Xylene, m-Xylene, p-Xylene C6H4-CH32 Mesitylene, Pseudocumene, Hemimellitene C6H3-CH33 Prehnitene, Isodurene, Durene C6H2-CH34 Pentamethylbenzene C6H-CH35 Mellitene C6-CH36 Examples of other aromatic compounds: Aniline C6H5-NH2 Acetylsalicylic acid C6H4-O-C=O-CH3-COOH Benzoic acid C6H5-COOH Biphenyl C6H52 Chlorobenzene C6H5-Cl Nitrobenzene C6H5-NO2 Paracetamol C6H4-NH-C=O-CH3-OH Phenacetin C6H4-NH-C=O-CH3-O-CH2-CH3 Phenol C6H5-OH Picric acid C6H2-OH-NO23 Salicylic acid C6H4-OH-COOH Trinitrotoluene C6H2-CH3-NO23 The arene ring has an ability to stabilize charges. This is seen in, for example, phenol C6H5-OH, which is acidic at the hydroxyl OH, since a charge on this oxygen alkoxide -O- is partially delocalized into the benzene ring. Polyaromatic hydrocarbons Main article: Polycyclic aromatic hydrocarbon Naphthalene Naphthalene Some important arenes are the polyaromatic hydrocarbons PAH; they are also called polycyclic aromatic hydrocarbons and polynuclear aromatic hydrocarbons. They are composed of more than one aromatic ring. The simplest PAHs are benzocyclopropene C7H6, benzocyclopropane C7H8, benzocyclobutadiene C8H6, and benzocyclobutene C8H8. A simple synthesis of benzocyclopropene is published 1. Common examples are naphthalene with two fused rings, anthracene with three, tetracene with four, and pentacene with five linearly fused rings. Phenanthrene and triphenylene are examples of non-linear connections. More exotic examples are helicenes and corannulene. These compounds are one of the most widespread organic pollutants, remaining on beaches and marine environmentals for a long time after an oil spill. Recent investigations have concluced that their toxicity is up to 100 times worse than first assumed. 4 See also Asphaltene Simple aromatic rings External links R. H. Logan's homepage: Aromaticity And Aromatic Hydrocarbons Carcinogenic FAC list in Portable Document Format. Toxicological profiles of PAH. LIST of PAH. Abiogenic Gas Debate 11:2002 EXPLORER References ^ Definition IUPAC Gold Book Link ^ HighBeam Encyclopedia: aromatic compound ^ Jerry March Advanced Organic Chemistry 3Ed., ISBN 0-471-85472-7 ^ Sound battles back, but threats linger. NOAA Fisheries. Retrieved on 2008-02-02. Retrieved from http://en..org/wiki/Aromatic_hydrocarbon Categories: Aromatic hydrocarbons | Hydrocarbons Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Bosanski БългарÑ?ки Català Česky Deutsch Ελληνικά Eesti Español Esperanto Français í•œêµì–´ Hrvatski Italiano עברית LatvieÅ¡u Nederlands 日本語 Polski Português Română РуÑ?Ñ?кий SlovenÄ?ina Suomi Svenska தமிழà¯? УкраїнÑ?ька ä¸æ–‡ This page was last modified on 27 August 2008, at 10:20
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