- simplest aromatic hydrocarbon is benzene, C6H6
- sweetish-smelling liquid first isolated in 1825 by Michael Faraday from the oily residue that had collected in gas lines
- this formula has so few hydrogen atoms for the number of carbon atoms that an unsaturated molecule would seem likely
- Kekule proposed a ring structure involving alternating single and double bonds, years later Dewar and Ladenburg proposed very different molecules for the benzene molecule
- benzene should behave as if it was an alkane, if so it should react with bromine to give an addition product and with potassium permanganate to give an oxidation product
- in fact when bromine and benzene react together a substitution product is formed
- benzene and potassium permanganate do not react at all
- from enthalpy measurements, we find that benzene is about 150kJ . mol-1 more stable than a compound having the Kekule structure should be
- substitution reactions dominate the chemistry of benzene
- eg the reaction between bromine and benzene in the presence of an iron catalyst to produce bromobenzene
- despite its preference for substitutions, under extreme conditions benzene can be made to undergo addition reactions
- eg at pressures of 20 to 30 MPa, benzene can be hydrogenated to cyclohexane
- next simplest aromatic hydrocarbon is toluene (methylbenzene), is almost as good as a solvent as benzene and is comparatively less harmful
- it is possible to have compounds containing connected benzene rings
- Naphthalene, a compound in which two benzene rings share a common side, is used as a moth repellent
- The carcinogenic compound 3,4-benzopyrene is produced in significant quantities in any burning or charring process, thus both cigarette smoke and char broiled foods contain this hazardous chemical
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