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An aromatic hydrocarbon or arene〔Definition IUPAC Gold Book (Link )〕 (or sometimes aryl hydrocarbon)〔(Mechanisms of Activation of the Aryl Hydrocarbon Receptor ) by Maria Backlund, Institute of Environmental Medicine, Karolinska Institutet〕 is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming rings. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered; the term was coined as such simply because many of the compounds have a sweet or pleasant odour. 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'' (MAH) or ''polycyclic'' (PAH). Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule (for monocyclic rings: when the number of its π-electrons equals 4n+2, where n=0,1,2,3.......), are also called 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 a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.〔(HighBeam Encyclopedia: aromatic compound )〕 == Benzene ring model == (詳細はBenzene, C6H6, is the simplest aromatic hydrocarbon, and it was the first one named as such. The nature of its bonding was first recognized by August Kekulé 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 neighbouring carbons. This leaves one electron to share with one of the same two neighbouring carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon. The structure is alternatively illustrated as 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 depiction represents the equivalent nature of the six carbon-carbon bonds all of bond order ~1.5; the equivalency is explained by resonance forms. The electrons are visualized as floating above and below the ring with the electromagnetic fields they generate acting to keep the ring flat. General properties of aromatic hydrocarbons: # They display aromaticity # The carbon-hydrogen ratio is high # They burn with a sooty yellow flame because of the high carbon-hydrogen ratio # They undergo electrophilic substitution reactions and nucleophilic aromatic substitutions The circle symbol for aromaticity was introduced by Sir Robert Robinson and his student James Armit in 1925〔James Wilkins Armit and Robert Robinson (1925) "Polynuclear heterocyclic aromatic types. Part II. Some anhydronium bases," ''Journal of the Chemical Society, Transactions'', 127: 1604-1618.〕 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry. The proper use of the symbol is debated; it is used to describe any cyclic π system in some publications, or only those π systems that obey Hückel's rule in others. Jensen〔William B. Jensen (April 2009) ("The circle symbol for aromaticity," ) ''Journal of Chemical Education'', 86(4): 423-424.〕 argues that, in line with Robinson's original proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. In this way the circle symbol for a 6c–6e bond can be compared to the Y symbol for a 3c–2e bond. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aromatic hydrocarbon」の詳細全文を読む スポンサード リンク
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