CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.6.3 Relative Basicity of Ammonia, Ethylamine & Phenylamine

Relative Basicity of Aqueous Ammonia, Ethylamine & Phenylamine

  • Ammonia and amines act as bases as they can donate their lone pair of electrons to form a dative covalent bond with a proton
  • The basicity of the amines depends on how readily available their lone pair of electrons is
  • Electron-donating groups (such as alkyl groups) increase the electron density on the nitrogen atom and cause the lone pair of electrons to become more available for dative covalent bonding
    • The amine becomes more basic
  • Delocalisation of the lone pair of electrons into an aromatic ring (such as a benzene ring) causes the lone pair of electrons to become less available for dative covalent bonding
    • The amine becomes less basic

Comparing basicity of ammonia, ethylamine & phenylamine

  • The order of basicity of ammonia, ethylamine and phenylamine is as follows:

 Ethylamine > ammonia > phenylamine

     STRONGEST BASE                                                       WEAKEST BASE

  • This trend can be explained by looking at the groups attached to the amine (-NH2) group
  • In ethylamine, the electron-donating alkyl group donates electron density to the nitrogen atom causing its lone pair to become more available to form a dative covalent bond with a proton
  • Ammonia lacks an electron-donating group hence it is less basic than ethylamine however it is more basic than phenylamine as the lone pair on the nitrogen is not delocalised
  • In phenylamine the lone pair of electrons overlap with the conjugated system on the benzene ring and become delocalised; As a result, the lone pair of electrons become less readily available to form a bond with a proton

Nitrogen Compounds - Strength of Bases, downloadable AS & A Level Chemistry revision notes

Trends in the basicity of ammonia, ethylamine, and phenylamine

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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