Students should be able to: use thermochemical evidence from enthalpies of…

The idea

The thermochemical argument and the reactivity argument are the same argument. Hydrogenation data show benzene sits roughly 150 kJ mol⁻¹ below where three isolated double bonds would put it, so the delocalised ring is a stable arrangement that a reaction pays to destroy. An addition reaction uses up two of the ring electrons to form a new σ bond, which breaks the delocalisation and gives a product higher in energy than the starting material. A substitution replaces a hydrogen atom, so the ring is regenerated at the end of the mechanism and the stability is kept. That is why benzene does not decolourise bromine water, while an alkene does: the ring cannot polarise a bromine molecule on its own.

What earns the marks

• The predicted enthalpy of hydrogenation calculated as three times the cyclohexene value.
• Comparing that prediction with the measured value and naming the difference as delocalisation energy.
• Stating that addition would break the delocalised ring and lose that stability.
• Substitution restoring the delocalised ring so the product remains low in energy.

Common mistakes and how to put them right

• Explaining the preference for substitution by saying only that benzene is unreactive.

• Claiming benzene cannot undergo addition at all, when it does with hydrogen over nickel and with chlorine in ultraviolet light.

• Using enthalpies of combustion in place of hydrogenation without saying the products are the same.

• Saying benzene decolourises bromine water because it contains double bonds.

Exam tips

• Answer 'why substitution' in terms of the product's energy, not the reagent's strength.
• The bromine water test is the quickest way to show a ring is delocalised rather than a set of alkene bonds; quote the observation as well as the conclusion.