Leaving group:

The nature of the leaving group is significant to both the S_N1 and S_N2 reactions - as better the leaving group, as faster the reaction. In the transition states of both of the reactions, the leaving group has gained a partial negative charge and the better which can be stabilized, the much more stable the transition state and the faster the reaction. Hence, the best leaving groups are the ones that make the most stable anions. This is as well related to basicity in the sense that the much more stable the anion, the weaker the base. Iodide and bromide ions are stable ions and weak bases, and show to be good leaving groups. The chloride ion is much less stable, more basic and a poorer leaving group. The fluoride ion is a extremely poor leaving group and as a result alkyl fluorides do not go through nucleophilic substitution. The requirement for a stable leaving group describes why alcohols, ethers, and amines do not go through nucleophilic substitutions because they would involve the loss of a strong base (for example RO^- or R₂N^-).