# 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

## 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

- [hydroxyl group](https://www.gradenine.co.uk/glossary/hydroxyl-group)
- [carbonyl group](https://www.gradenine.co.uk/glossary/carbonyl-group)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-presence-of-both-4e013772) · Published by Druglandscape Ltd.