Nucleophilic substitution

• When does nucleophilic substitution occur?


  When halogenoalkanes react with KOH/KCN/NH3.
• What happens in nucleophilic substitution?


  A nucleophile is a positive seeking species replacing another positive seeking species.
• Why does nucleophilic substitution occur?


  Polar bond in the reagent is attacked by the lone pair on the nucleophile.
• Nucleophilic substitution mechanism?

  
• Why is ammonia different in nucleophilic substitution?


  As it is a neutral nucleophile.

  Amine product can act as a nucleophile, further substitution can occur.

  Excess NH3 favours primary amine product.