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A class of reactions in which a nucleophile attacks a positive or partially positive atom causing the removal of another atom or group of atoms is called nucleophilic substitution.

A nucleophile is a species with an unshared electron pair or negative charge. Ammonia and amine derivatives are nucleophiles.

Nucleophilic substitution can occur by two mechanisms- S_N1 mechanism or S_N2 mechanism.

Amines or ammonia can react with alkyl halides in an S_N2 mechanism. The reaction causes the replacement of hydrogen on a nitrogen atom by an alkyl group.

The reaction contains 2 processes- nucleophilic attack of nitrogen nucleophile forming an ammonium salt and removal of a proton from ammonium salt.

This reaction is used for the preparation of amines from ammonia.

a. When chloropropane reacts with ammonia, the hydrogen on ammonia is replaced by propyl group forming propanamine.

The reaction of chloropropane with ammonia

b. When bromoethane is reacted with cyclohexanamine, the hydrogen on amine is replaced with ethyl group forming N-ethyl cyclohexanamine.

The reaction of cyclohexylamine with bromoethane