The reaction in which a nucleophile replaces already existing nucleophile in a molecule is called nucleophilic substitution reaction. Haloalkanes are substrate in these reactions. In this type of reaction, a nucleophile reacts with haloalkane (the
substrate) having a partial positive charge on the carbon atom bonded to halogen. A substitution reaction takes place and halogen atom, called leaving group departs as halide ion. Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction.

It is one of the most useful classes of organic reactions of alkyl halides in which halogen is bonded to sp³ hybridized carbon.

What is the final product when ethyl bromide reacts with potassium phthalimide followed by acid hydrolysis?

Phtahlic acid + Ethyl amine 

Ethyl bromide reacts with potassium phthalimide followed by acid hydrolysis to produce Phtahlic acid and ethyl amine