Describe the structure of benzene.

State one piece of chemical evidence proving benzene does not contain alternate single and double bonds.

planar and six-membered/hexagonal ring;

Accept suitable diagram showing either ring structure with circle representing delocalization or a Kekulé-type structure.

Allow flat for planar.

all carbon-carbon bond lengths equal/0.140 nm / all carbon-carbon bond lengths between single/0.154 nm and double/0.134 nm / all carbon-carbon bonds have same strength;

all bond angles 120° /equivalent;

Allow “all carbons sp² (hybridized)”.

delocalization of electrons / OWTTE;

Allow “p orbital/\(\pi \) electrons extend over all carbon atoms”.

benzene does not (readily) undergo addition reactions / benzene more likely to undergo substitution reactions / benzene does not decolourize bromine water;

only one isomer of 1,2-disubstituted benzene (eg, 1,2-dibromobenzene) exists (if there were alternate single and double bonds these would be two);

there are three isomers of type \({{\text{C}}_6}{{\text{H}}_4}{{\text{X}}_2}\), so if there were alternate single and double bonds there would be four;

benzene not hydrogenated by hydrogen (and a platinum catalyst) under usual conditions that hydrogenate an alkene;

Do not award this mark if high pressure is stated.

benzene not oxidized by potassium manganate(VII)/potassium permanganate/\({\text{KMn}}{{\text{O}}_4}\);

Allow other suitable named oxidizing agent.

Accept appropriate thermochemical evidence.

Although a straightforward question, most candidates were only able to score two out of the three possible marks in describing the structure of benzene.

Well answered but some candidates gave physical instead of chemical evidence failing to gain the mark. The most popular answer was the tendency to undergo substitution reactions.