Basicity of Amines (Cambridge (CIE) A Level Chemistry): Revision Note

Basicity of Aqueous Solutions of Amines

• The nitrogen atom in ammonia and amine molecules can accept a proton (H⁺ ion)

• They can therefore act as bases in aqueous solutions by donating their lone pair of electrons to a proton and forming a dative bond

  • For example, ammonia undergoes an acid-base reaction with dilute hydrochloric acid (HCl) to form a salt

NH₃ + HCl → NH₄⁺Cl^–

Ammonia and amines acting as a base

The nitrogen atom in ammonia and amines can donate its lone pair of electrons to form a bond with a proton and therefore act as a base

Strength of ammonia and amines as bases

• The strength of amines depends on the availability of the lone pair of electrons on the nitrogen atom to form a dative covalent bond with a proton

• The more readily this lone pair of electrons is available, the stronger the base is

• Factors that may affect the basicity of amines include:

  • Positive inductive effect - Some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available and therefore increasing the amine’s basicity

  • Delocalisation - The presence of aromatic rings such as the benzene ring causes the lone pair of electrons on the nitrogen atom to be delocalised into the benzene ring

  • The lone pair becomes less available to form a dative covalent bond with ammonia and hence decreases the amine’s basicity

• For example, ethylamine (which has an electron-donating ethyl group) is more basic than phenylamine (which has an electron-withdrawing benzene ring)