Edexcel A Level Chemistry:复习笔记3.4.3 Nucleophilic Substitution Mechanism

Nucleophilic Substitution: Mechanisms


  • A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
  • An atom that has a partial negative charge is replaced by the nucleophile
  • Halogenoalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)


Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

Mechanism with aqueous potassium hydroxide

  • In the following reaction a halogenoalkane reacts with aqueous alkali to form an alcohol


The halogen is replaced by a nucleophile, OH–

  • The mechanism for the reaction is as follows


Nucleophilic substitution reaction of bromoethane and aqueous alkali (e.g. NaOH)


Mechanism with 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-

  • Excess ammonia is used to prevent further substitution and favour the formation of a primary amine


The mechanism of nucleophilic substitution between ammonia and a halogenoalkane