# Explain why esters generally have lower boiling points than carboxylic acids of comparable molar mass.

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

## Mark scheme (4 marks)

1. Carboxylic acids can form hydrogen bonds between molecules (intermolecular hydrogen bonds), whereas esters cannot form hydrogen bonds between ester molecules.
2. In carboxylic acids, the O–H bond is sufficiently polarised for the hydrogen to act as a hydrogen bond donor; esters lack an O–H group and so cannot donate a hydrogen bond.
3. Hydrogen bonds are stronger/require more energy to break than the van der Waals' (dispersion and dipole–dipole) forces that exist between ester molecules.
4. Therefore, more energy (higher temperature) is needed to separate carboxylic acid molecules from the liquid phase, resulting in a higher boiling point compared with esters of similar molar mass.

## Key terms

- [ester](https://www.gradenine.co.uk/glossary/ester)
- [carboxylic acid](https://www.gradenine.co.uk/glossary/carboxylic-acid)
- [boiling point](https://www.gradenine.co.uk/glossary/boiling-point)

## 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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