A-Level Chemistry October/November 2024 Q8(a): Explain why phenol is more acidic than water.

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  In phenol, a lone pair of electrons in a p-orbital on the oxygen atom is delocalised into the π-system of the benzene ring.

  This delocalisation stabilises the resulting conjugate base, the phenoxide ion, by spreading the negative charge over the ring. This stabilisation does not occur in the hydroxide ion formed from water, where the charge is localised on the oxygen atom. Therefore, the equilibrium for the dissociation of phenol lies further to the right, making it more acidic than water.

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  How the marks are awarded

  • M1 — Stating that the lone pair of electrons on the oxygen atom is delocalised into the benzene ring, by overlapping with its π-system.
  • M2 — Explaining that this delocalisation stabilises the conjugate base (the phenoxide ion), making it more stable and thus more readily formed than the hydroxide ion from water.

  Common mistakes

  • Simply stating 'phenol has delocalisation' without specifying that it is the lone pair from the oxygen atom that is delocalised into the ring.
  • Confusing the stability of phenol with the stability of its conjugate base. The key to acidity is the stabilisation of the phenoxide ion, not the phenol molecule itself.
  • Incorrectly stating that the O-H bond is stronger in phenol. The delocalisation actually weakens the O-H bond, which is an alternative way to earn the second mark.
  • Failing to make a comparison with water, i.e., not explaining that the hydroxide ion's charge is localised and therefore less stable.

  Examiner tip: When comparing the acidity of different compounds, always analyse and compare the stability of their respective conjugate bases.

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