Explain how the presence of both a hydroxyl group and a carbonyl group within the same molecule can give rise to two distinct classes of organic compound, and outline one chemical property that distinguishes these two classes from each other.

IB DP Chemistry Higher Level (2023 syllabus) — S3.2 Functional groups: classification of organic compounds · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A hydroxyl group attached to the same carbon as a carbonyl gives a carboxyl group (–COOH), i.e. a carboxylic acid.
When the –OH and C=O are on different carbons the molecule contains both an alcohol and an aldehyde/ketone functional group (a hydroxy‑carbonyl compound).
Carboxylic acids are Brønsted–Lowry acids: they donate a proton from the –OH to a base (e.g. Na₂CO₃ gives CO₂).
Hydroxy‑carbonyl compounds do not donate a proton under the same mild conditions.

Examiner tips

  • Use the exact terms – carboxylic acid, hydroxy‑carbonyl compound; state the proton‑donating property; keep answer to 4 points

Common mistakes

  • Confusing the position of the –OH with the carbonyl; writing that the –OH is on the carbonyl carbon when it is not; claiming hydroxy‑carbonyl compounds are acids

Mark scheme (4 marks)

  1. A hydroxyl (–OH) and carbonyl (C=O) on the same carbon atom gives an aldehyde or ketone — correction: when –OH is on the same carbon as C=O they form a carboxylic acid (–COOH); identifying that a carboxyl group (–COOH) is formed when the hydroxyl and carbonyl are on the same carbon (i.e. the –OH is bonded to the carbonyl carbon)
  2. When the hydroxyl group and carbonyl group are on different carbons within the same molecule, the compound is simultaneously an alcohol and an aldehyde/ketone (hydroxy-carbonyl compound), or can cyclise to form a hemiacetal/hemiketal
  3. Carboxylic acids are Brønsted–Lowry acids and donate a proton from the –OH to a base (e.g. react with Na₂CO₃ to release CO₂), whereas hydroxy-carbonyl compounds (alcohols + aldehydes/ketones) do not donate a proton under the same mild conditions
  4. The increased acidity of carboxylic acids relative to alcohols is attributed to resonance / delocalisation of negative charge over both oxygen atoms in the carboxylate anion, stabilising the conjugate base

Key terms in this question

hydroxyl group · carbonyl group

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