Phenol — common mistakes

Examiners frequently ask for the reagents and conditions for the substitution reactions of phenol. Remember: aqueous bromine for bromination and dilute nitric acid for nitration. Emphasise the mild conditions compared to benzene to gain full marks.

Phenol is historically known as carbolic acid. It was famously used by Joseph Lister as an antiseptic in surgery in the 19th century. Although it is a weak acid, the name 'carbolic acid' has persisted.

This is a common point of confusion. The -OH group has two opposing effects: 1) The inductive effect: Oxygen is very electronegative and pulls electron density from the C-O bond, weakening the O-H bond. 2) The resonance effect: The oxygen lone pair delocalises into the ring. For ring activation, the resonance effect dominates. For acidity, it's the stability of the resulting phenoxide ion that matters. The resonance effect strongly stabilises the phenoxide ion, making the initial proton loss more favourable. So, the resonance effect is key to both properties, but it manifests differently.

Phenol is very easily oxidised, much more so than primary or secondary alcohols. On exposure to air and light, it slowly oxidises to form coloured impurities, often appearing as a pink or brown liquid. Strong oxidising agents will break open the stable benzene ring, so it does not undergo the simple oxidation to an aldehyde or ketone that aliphatic alcohols do.

The -OH group is so strongly activating that with a very reactive electrophile/reagent like aqueous bromine, substitution happens at all three activated positions (2, 4, and 6). Nitration uses dilute nitric acid, which is a less aggressive reagent system than aqueous bromine. Therefore, under these milder conditions, only monosubstitution tends to occur, leading to a mixture of the 2- and 4- isomers.