The nature of leaving groups strongly influences the outcome of a nucleophilic substitution reaction.
In general, in a nucleophilic substitution reaction, a nucleophile displaces a functional group, called the leaving group, from the substrate to give a substituted product. A leaving group departs the substrate molecule through heterolytic cleavage, taking the pair of electrons with it to become a relatively stable weak base in the form of an anion or a neutral molecule.
In a nucleophilic substitution reaction, halide anions are classified as a leaving group. The stability of a leaving group dictates how readily it will depart from the nucleophile, which can influence the rate of the reaction.
The stability of halide anions can be estimated from the pKa values of their corresponding hydrohalic acids. The weakest conjugate base of the strongest hydrohalic acid forms the most stable anion, which acts as the best leaving group. Therefore, among the halides, iodide is a better leaving group than bromide and chloride. Fluoride is a strong conjugate base of the weak acid HF. Hence, it is the least stable anion and a poor leaving group.
Some leaving groups depart from the substrate as a neutral molecule. For example, water departs as a neutral molecule after the heterolytic cleavage of the protonated substrate.
