CIE A Level Chemistry复习笔记7.6.5 Relative Basicity of 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

Trends in the basicity of ammonia, ethylamine, and phenylamine