CIE A Level Chemistry复习笔记7.6.3 Basicity 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 its lone pair of electrons to a proton and form a dative bond
  • For example, ammonia undergoes an acid-base reaction with dilute hydrochloric acid (HCl) to form a salt

NH₃ + HCl → NH₄⁺Cl^-

^{base      acid            salt}

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)