A-Level Chemistry October/November 2024 Q2(b)(iv): Explain why alcohols are less acidic than water.

1 answer

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  Alcohols have the general structure R-O-H, where R is an alkyl group.

  1. Alkyl groups are electron-donating and exert a positive inductive effect (+I effect). This means they push electron density towards the oxygen atom in the O-H bond.
  2. The increased electron density on the oxygen atom strengthens the covalent O-H bond. This makes the bond more difficult to break heterolytically to release a proton (H⁺).

  Since acidity is the ability to donate a proton, and the O-H bond in an alcohol is stronger than in water (where H is not electron-donating), alcohols are less acidic than water.

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  How the marks are awarded

  • B1 — The first mark is awarded for correctly stating that alkyl groups are electron-donating or have a positive inductive effect, as written in the first point of the model answer.
  • B1 — The second mark is awarded for explaining the consequence of this effect: the O–H bond is strengthened, making it harder for the H⁺ ion to be donated, as explained in the second point.

  Common mistakes

  • Stating that alkyl groups are electron-withdrawing, which is the opposite of the correct effect.
  • Only stating that alkyl groups have a positive inductive effect without explaining how this affects the O-H bond strength or the ease of proton donation.
  • Confusing the stability of the conjugate base (the alkoxide ion) with the strength of the O-H bond in the acid itself. While related, the direct explanation required here focuses on the O-H bond.
  • Giving a vague answer like 'the alkyl group makes it less acidic' without any reference to electronic effects.

  Examiner tip: Always explain chemical properties by linking a molecule's specific structural features to their electronic effects (like the inductive effect) and then to the resulting reactivity or bond strength.

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