Greater solvation of attacking nucleophile & poor solvation of transition state promotes the reaction by S_N2 mechanism.

Increase in the no. of phenyl ring at C of C–X causes a shift in mechanism from S_N2 to S_N1.

Major part of energy needed for the heterolysis of C–X bond in ${\mathrm{S}}_{{\mathrm{N}}^1}$ reaction mechanism is obtained by the solvation of X– by the polar protic solvent.

Allyl & benzyl halides undergo S_N1 reactions as they give resonance stabilized carbocation.