AQA A Level Chemistry复习笔记7.5.3 Nucleophilic Substitution

Nucleophilic Substitution


  • The lone pair on the nitrogen atom in amines makes them good nucleophiles, just like ammonia
  • When ammonia reacts with a haloalkane a nucleophilic substitution reaction takes place forming a primary amine
  • For example chloromethane reacts with ammonia in two steps to make methylamine and ammonium chloride

CH3Cl + NH3 → [CH3NH3]+Cl -

[CH3NH3]+Cl - + NH3 → CH3NH2 + NH4+Cl -

  • The methylamine is also a good nucleophile so can undergo further substitution with chloromethane to make dimethylamine

CH3Cl   + CH3NH2 → [(CH3)2NH2]+Cl -

[(CH3)2NH2]+Cl - + NH3 → (CH3)2NH + NH4+Cl -

  • The dimethylamine can further substitute giving a tertiary amine, trimethylamine

CH3Cl + (CH3)2NH → [(CH3)3NH]+Cl -

[(CH3)3NH]+Cl - + NH3 → (CH3)3N + NH4+Cl -

  • The final substitution occurs when the tertiary amine reacts with the chloromethane to make a quaternary ammonium salt

CH3Cl + (CH3)3N → [(CH3)4N]+Cl -

  • Since all these multiple substitutions occur it is not a very efficient way to synthesise amines
  • If you want to just produce the primary amine then a large excess of ammonia is used to ensure it is the dominant nucleophile in the reaction vessel

Mechanism of nucleophilic substitution




The mechanism of nucleophilic substitution between ammonia and a halogenoalkane

  • In the first step the ammonia acts as a nucleophile and in the second step it acts as a base

Exam Tip

Remember that this is the only nucleophilic substitution reaction which needs two moles of the nucleophile. The overall reaction is

RCH2X + 2NH3 → RCH2NH2 + NH4X