A-Level Chemistry May/June 2025 Q9(b): Describe the relative basicities of C6H5NH2, CH3CH2CH2NH2 and NH3. Explain your answer.

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  The order of basicity, from most basic to least basic, is:

  CH₃CH₂CH₂NH₂ > NH₃ > C₆H₅NH₂

  Explanation:

  • The basicity of these compounds depends on the availability of the lone pair of electrons on the nitrogen atom to accept a proton (H⁺).

  • In propylamine (CH₃CH₂CH₂NH₂), the propyl group is an alkyl group which is electron-donating. It has a positive inductive effect, pushing electron density onto the nitrogen atom. This increases the electron density on the nitrogen, making the lone pair more available to accept a proton. Therefore, propylamine is a stronger base than ammonia.

  • In phenylamine (C₆H₅NH₂), the lone pair of electrons on the nitrogen atom is delocalised into the π-electron system of the benzene ring. This makes the lone pair less available to accept a proton. Therefore, phenylamine is a weaker base than ammonia.

  • Ammonia (NH₃) serves as a reference. It is more basic than phenylamine because its lone pair is not delocalised, and less basic than propylamine because it lacks an electron-donating alkyl group.

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  How the marks are awarded

  • M1 — Correctly stating the order of basicity as propylamine being the strongest, followed by ammonia, and then phenylamine as the weakest.
  • M2 — Defining a base in this context by stating that the lone pair on the nitrogen atom is responsible for accepting a proton (H⁺).
  • M3 — Explaining phenylamine's low basicity by stating that the lone pair on the nitrogen is delocalised into the benzene ring's π-system.
  • M4 — Explaining propylamine's high basicity by stating that the alkyl (propyl) group is electron-donating, increasing electron density on the nitrogen.

  Common mistakes

  • Stating the incorrect order of basicity, often placing ammonia as the strongest or weakest base.
  • Failing to explain why the electronic effects change basicity, e.g., just saying 'propylamine has an electron-donating group' without linking it to the increased availability of the lone pair.
  • Incorrectly describing the electronic effect in phenylamine, for example, by stating the benzene ring 'pulls the nitrogen atom' instead of specifically mentioning the delocalisation of the nitrogen's lone pair.
  • Confusing the positive inductive effect of alkyl groups with the delocalisation effect of the benzene ring.

  Examiner tip: Always explain trends in basicity by clearly linking the electronic effects of attached groups (inductive or delocalisation) to the availability of the lone pair on the nitrogen atom.

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