Revise Phenol for Cambridge 9701 (32.2)

Unlike aliphatic alcohols, which are neutral, phenol is a weak acid. It donates its proton from the hydroxyl group to form the phenoxide ion and a proton. While it is acidic, it is significantly weaker than carboxylic acids.

$C₆H₅OH(aq) ⇌ C₆H₅O⁻(aq) + H⁺(aq) \quad K_a \approx 1 \times 10^{-10} \text{ mol dm}^{-3}$

The acidity of phenol arises from the stability of its conjugate base, the phenoxide ion. The lone pair of electrons on the oxygen atom can be delocalised into the delocalised π-electron system of the benzene ring. This spreads the negative charge over the entire ion, making it more stable than the localised charge on an alkoxide ion (like ethoxide, C₂H₅O⁻). The more stable the conjugate base, the stronger the acid.

Being a weak acid, phenol reacts with strong bases like sodium hydroxide to form a salt (sodium phenoxide) and water. This is a standard acid-base neutralisation reaction. Sodium phenoxide is soluble in water.

$C₆H₅OH(aq) + NaOH(aq) \rightarrow C₆H₅O⁻Na⁺(aq) + H₂O(l)$

However, phenol is not acidic enough to react with weak bases like sodium hydrogencarbonate (NaHCO₃). It will react, albeit slowly, with the slightly stronger base sodium carbonate (Na₂CO₃). This provides a crucial method for distinguishing between phenols and carboxylic acids, as carboxylic acids are strong enough to react with both Na₂CO₃ and NaHCO₃, producing effervescence.

The hydroxyl group has a powerful effect on the benzene ring's reactivity. The lone pair of electrons on the oxygen atom is drawn into the ring's π-system, increasing the electron density. This makes the ring 'activated' and much more susceptible to attack by electrophiles compared to benzene itself. Consequently, reactions occur under much milder conditions.

Bromination: When aqueous bromine is added to phenol at room temperature, a rapid reaction occurs. The orange-brown bromine water is decolourised, and a white precipitate of 2,4,6-tribromophenol is formed. No halogen carrier catalyst is needed due to the high activation of the ring.

$C₆H₅OH(aq) + 3Br₂(aq) \rightarrow C₆H₂Br₃OH(s) + 3HBr(aq)$

Nitration: Phenol reacts with dilute nitric acid at room temperature to produce a mixture of 2-nitrophenol and 4-nitrophenol. This contrasts sharply with the nitration of benzene, which requires a harsh mixture of concentrated nitric and sulfuric acids (the nitrating mixture).