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"Arene" redirects here. For other uses, see Arene (disambiguation). An aromatic hydrocarbon (abbreviated as AH) or arene [1] (or sometimes aryl hydrocarbon)[2] 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. 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.[3]
[edit] Benzene ring modelMain 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. The structure is also 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 also represents the equivalent nature of the six carbon-carbon bonds all of bond order ~1.5. This equivalency is well 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:
The circle symbol for aromaticity was introduced by Sir Robert Robinson in 1925 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 pi system in some publications, or only those pi systems that obey Hückel's rule on others. Jensen [4] argues that in line with Robinson's original proposal, the use of the circle symbol should be limited to monocyclic 6 pi-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. [edit] Arene synthesisMany laboratory methods exist for the organic synthesis of arenes from non-arene precursors:
[edit] Arene reactionsThe main arene reactions are
Lesser-known reactions:
[edit] Benzene and derivatives of benzeneBenzene 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):
Examples of other aromatic compounds:
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. [edit] Polyaromatic hydrocarbonsMain article: Polycyclic aromatic hydrocarbon 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.[8] [edit] See also[edit] External links
[edit] References
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