Factors Affecting S_N2 Versus S_N1 Reactions:

S_N1 versus S_N2

There are two other mechanisms included in the nucleophilic substitution of alkyl halides. While polar aprotic solvents are employed, the S_N2 mechanism is chosen. Primary alkyl halides react much more rapid than secondary alkyl halides, with tertiary alkyl halides hardly reacting at all. Within the protic solvent conditions with nonbasic nucleophiles (for example dissolving the alkyl halide in water or alcohol), the S_N1 mechanism is chosen and the order of reactivity is reversed. Tertiary alkyl halides are much more reactive than secondary alkyl halides, and primary alkyl halides do not react.

There are various factors that ascertain whether substitution will be S_N1 or S_N2 and which as well control the rate at which these reactions occur. These involve the nature of the nucleophile and the sort of solvent employed. The reactivity of primary, secondary, and tertiary alkyl halides is controlled via electronic and steric factors.