Nucleophilic Substitution

Nucleophilic substitution mechanism

• Nucleophile - species that donates an electron pair to form a new covalent bond

• A nucleophile reacts with a polar molecule like a haloalkanes by 'kicking out’ the halogen and taken its place

Steps of nucleophilic substitution

1. Nucleophile approaches the haloalkanes with a partially positive C atom

2. Nucleophile donates its lone pair to the C atom forming a new covalent bond

3. Original bond between the C atom and the halogen breaks heterolytically as the halogen atom takes both shared electrons

4. Halogen is replaced by the nucleophile

Types of nucleophiles

• OH^-(Hydroxide) , CN^-(Cyanide), NH_3. (Ammonia)

Reaction with hydroxide to form alcohols

• Haloalkanes undergo nucleophilic substition with aqueous hydroxide ions from bases like sodium hydroxide when the mixture is warmed

• C₂H₅Br + OH^- → C₂H₅OH + Br^-

  

• This reaction replaces the haloalkane with an alcohol product, is a type of hydrolysis reaction. Water molecule also act as a nucleophile in similar hydrolysis reactions with haloalkanes to generate alcohols. However, the reaction rate is much slower with neutral water molecules than hydroxide ions, which are more nucleophilic

Reaction with cyanide to form nitriles

• Haloalkanes undergo nucleophilic substitution when reacting with ethanolic potassium cyanide. Cyanide ion acts as the nucleophiles displacing a halogen to form a nitrile

• C₂H₅Br + CN^- → C₂H₅CN + Br^-

• This reaction extends the carbon chain length of the original haloalkanes by a carbon atom

Reaction with ammonia to form amines

• When heated under pressure with excess ethanolic ammonia, haloalkanes undergo nucleophilic substitution to form primary amines

• C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br

  

• Initially the ammonia replaces the bromine atom. Subsequently, it abstracts the hydrogen from the intermediate amine, yielding the final amine product alongside ammonium bromide