Nucleophilic Substitution (AQA A Level Chemistry): Revision Note

Mechanism: Nucleophilic Substitution

• A nucleophile is an electron-rich species that can donate a pair of electrons

  • ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species

  • Nucleophilic refers to reactions that involve a nucleophile

• Various species can behave as nucleophiles, and some make better nucleophiles than others

• A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom that 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)

• Halogenoalkanes undergo substitution reactions with the nucleophiles OH^–, CN^–, and NH₃

Mechanism with OH^-

• For example, in the following reaction, a halogenoalkane reacts with aqueous alkali to form an alcohol

• The mechanism for the reaction is as follows

Mechanism with CN^-

• The nucleophile in this reaction is the cyanide, CN^- ion

• Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane

• The product is a nitrile

  • For example, bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile

  • The halogen is replaced by a nucleophile, :CN^– 

CH₃CH₂Br + :CN^– → CH₃CH₂CN + :Br^–

• The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain

• This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material

• The mechanism for the reaction is as follows

Mechanism with NH₃

• 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 

Related topics

• Reactions of Halogenoalkanes

• Elimination