AQA A Level Chemistry复习笔记7.5.3 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