Nucleophilic Substitution:

 Nucleophilic substitutions are reactions that include the substitution of one nucleophile for other. Alkyl halides, carboxylic acids, and carboxylic acid derivatives go through nucleophilic substitution. Though, the mechanisms included for alkyl halides are little bit different from those included for carboxylic acids and their derivatives. The reaction of a methoxide ion along with ethanoyl chloride is an instance of nucleophilic substitution, in which one nucleophile (the methoxide ion) substitutes other nucleophile (Cl).

Figure:   Nucleophilic substitution.