Relative Basicity of Ammonia, Ethylamine & Phenylamine (Cambridge (CIE) A Level Chemistry): Revision Note

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:

• 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

The positive inductive effect of electron donating groups increases the basicity of an amine, while the negative inductive effect of electron withdrawing groups decreases the basicity of an amine