Benzene and aromaticity

Benzene, C₆H₆, was discovered by Faraday in 1825. It seemed very unsaturated, yet it behaved strangely: it formed a triozonide (as if it had three double bonds), but gave only one monosubstituted product, so all six hydrogens must be equivalent.

Kekulé proposed a ring of six carbons with alternating single and double bonds, switching rapidly between two arrangements. Evidence: ozonolysis of o-xylene gives products from both Kekulé forms.

Modern measurements show that all six C–C bonds in benzene are the same length, 139 pm: between a C–C single bond (154 pm) and a C=C double bond (133 pm). So there are no separate single and double bonds. The six π electrons are delocalised evenly round the ring, above and below its plane. Benzene is a resonance hybrid of the Kekulé structures, and this makes it extra stable: it prefers to keep its ring intact.

A ring is aromatic if it meets three conditions:

• (i) it is planar;
• (ii) its π electrons are completely delocalised round the ring;
• (iii) it has (4n + 2) π electrons, where n = 0, 1, 2, … (Hückel's rule). Benzene has 6 (n = 1).

Cyclopentadiene is not aromatic: one of its carbons is sp³, which breaks the delocalisation, and it has only 4 π electrons. Cyclo-octatetraene, with 8 π electrons, is not planar and not aromatic either.

Preparing benzene:

• Cyclic polymerisation of ethyne (red-hot iron tube).
• Decarboxylation of sodium benzoate by heating with soda lime.
• Reduction of phenol by heating with zinc dust.

Commercially, benzene is obtained from coal tar. It can also be made from ethene (via ethyne) or from hexane by aromatisation.