3.4.3 Nucleophilic Substitution Mechanism

• 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