Explain why primary amines are weaker bases than ammonia, yet aniline (phenylamine) is a significantly weaker base than both primary aliphatic amines and ammonia.

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).

Amines are organic derivatives of ammonia in which one or more hydrogen atoms are replaced by carbon-containing groups. The base strength of an amine depends on the availability of the lone pair on nitrogen.

Model answer (4 marks)

Primary aliphatic amines are actually stronger bases than ammonia because the alkyl groups donate electron density to the nitrogen by induction, making the lone pair more available for protonation. In aniline the nitrogen lone pair is conjugated with the benzene ring; resonance delocalises the pair into the ring, reducing its basicity. Consequently the equilibrium for protonation of aniline lies further to the left than for aliphatic amines, so aniline is a significantly weaker base than both ammonia and primary aliphatic amines.

Examiner tips

  • Show the inductive donation for aliphatic amines; mention the resonance delocalisation for aniline; link delocalisation to reduced basicity; keep answer concise and use correct terminology (induction, resonance, lone pair).

Common mistakes

  • Confusing the direction of induction (thinking alkyl groups withdraw electrons); forgetting that aniline’s lone pair is delocalised; writing that aniline is stronger rather than weaker than ammonia.

Mark scheme (4 marks)

  1. Primary aliphatic amines are weaker bases than ammonia because the alkyl group is electron-donating (by induction), which increases electron density on nitrogen and should increase base strength — award this mark only if the student correctly identifies that primary aliphatic amines are actually STRONGER bases than ammonia, correcting the stem's apparent misdirection, OR if the student states that alkyl groups donate electrons inductively, making the nitrogen lone pair more available.
  2. In aniline, the lone pair on nitrogen is delocalised into the benzene ring (by conjugation / resonance), reducing its availability to accept a proton.
  3. Because the lone pair is delocalised / less available, aniline is less able to donate the lone pair to a proton (H⁺), so it is a weaker base.
  4. The anilinium ion (protonated aniline) loses the stabilisation by delocalisation because the lone pair is used in bonding to H⁺, so the equilibrium for protonation lies further to the left compared with aliphatic amines, confirming weaker base strength.

Key terms in this question

primary amine · aniline

Related

More Functional groups: classification of organic compounds questions

▶ Try answering this question with AI marking (free) →