Aromatic hydrocarbon
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An aromatic hydrocarbon (abbreviated as AH) or arene Definition IUPAC Gold Book [Link] is a hydrocarbon, the molecular structure of which 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 aromatic hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic or polycyclic.
Benzene ring model
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 just draw a circle around the inside of the ring to show that there are 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.
Benzene and derivatives of benzene
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 an 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 derivative with alkyl substituents (alkylbenzenes) are:
- Ethylbenzene C6H5-CH2-CH3
- Mesitylene C6H3(-CH3)3
- Toluene C6H5-CH3
- Xylene C6H4(-CH3)2
- Aniline C6H5-NH2
- Acetylsalicylic acid C6H4(-O-C(=O)-CH3)(-COOH)
- Benzoic acid C6H5-COOH
- Biphenyl (C6H5)2
- 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)(-NO2)3
- Salicylic acid C6H4(-OH)(-COOH)
- Trinitrotoluene C6H2(-CH3)(-NO2)3
Polycyclic aromatic hydrocarbons
Some important arenes are the polycyclic aromatic hydrocarbons (PAH); they are also called polynuclear aromatic hydrocarbons. They are composed of more than one aromatic ring. The simplest PAH is benzocyclobutene (C8H6). Common examples are: More exotic examples are:
See also
External links
- [Carcinogenic FAC list] in Portable Document Format.
- [Toxicological profiles of PAH].
- [LIST of PAH].
- [Abiogenic Gas Debate 11:2002 (EXPLORER)]
References
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