Benzene: structure and aromaticity

Aromatic hydrocarbons (arenes) are named for the pleasant smell of many of them. Those containing a benzene ring are benzenoid; aromatic rings without one are non-benzenoid.

All six H atoms of benzene are equivalent, so it has only one monosubstituted derivative, but three disubstituted ones: ortho (1,2 or 1,6), meta (1,3 or 1,5) and para (1,4). The dimethylbenzenes are o-, m- and p-xylene.

Faraday isolated benzene in 1825. Its formula C₆H₆ suggested heavy unsaturation, and its triozonide pointed to three double bonds, yet it proved unusually stable.

Kekulé (1865) proposed a six-carbon ring with alternating single and double bonds. His model predicted two different 1,2-dibromobenzenes, but only one exists, so he suggested the double bonds oscillate between two positions.

Resonance explains it: benzene is a hybrid of the two Kekulé structures, drawn as a hexagon with a circle for six delocalised π electrons.

Every carbon is sp²: sp²–sp² overlap makes six C–C σ bonds and sp²–1s overlap six C–H σ bonds, all in one plane. Each carbon's leftover p orbital overlaps equally with both neighbours, giving π clouds above and below the ring.

X-ray diffraction shows a planar molecule with all six C–C bonds 139 pm, between C–C (154 pm) and C=C (133 pm). With no true double bond, benzene resists addition, and delocalisation makes it more stable than hypothetical cyclohexatriene.

Aromaticity needs (i) a planar ring, (ii) complete delocalisation of π electrons and (iii) (4n + 2) π electrons, n = 0, 1, 2 … (Hückel rule). Benzene has 6 π electrons (n = 1).

Benzene is obtained commercially from coal tar. In the laboratory it comes from cyclic polymerisation of ethyne, from sodium benzoate heated with soda lime, or from phenol vapour passed over heated zinc dust.