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 · View as Markdown

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

Model answer (4 marks)

Carboxylic acids can form intermolecular hydrogen bonds because they contain an –OH group; the hydrogen is a good donor and the oxygen an acceptor.

Esters lack an –OH group – they only have an –O–C=O linkage – so they cannot donate hydrogen bonds.

Hydrogen bonds are much stronger than the van der Waals forces (dispersion and dipole–dipole) that hold ester molecules together.

Consequently, more energy (higher temperature) is required to break the hydrogen bonds in carboxylic acids, giving them higher boiling points than esters of similar molar mass.

Examiner tips

  • Show the key difference – presence of –OH in acids vs none in esters. Explain that hydrogen bonds are stronger than van der Waals forces. Link stronger bonds to higher boiling point. Use correct terminology (intermolecular hydrogen bonds, van der Waals forces).

Common mistakes

  • Confusing hydrogen bonding with dipole–dipole only. Forgetting to mention the absence of an –OH group in esters. Using vague terms like "more polar" instead of "hydrogen bonds".

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 in this question

ester · carboxylic acid · boiling point

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