Nucleophilic Substitution Mechanism (Edexcel AS Chemistry): Revision Note

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 the difference in electronegativity between the carbon and halogen atom, the C-X bond is polar

Mechanism with aqueous potassium hydroxide

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

CH₃CH₂Br + OH^- → CH₃CH₂OH + Br^-

• The halogen is replaced by a nucleophile, OH^–

• The mechanism for the reaction is as follows

Mechanism with ammonia

• When ammonia reacts with a primary halogenoalkane 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