CBSE Class 12-science Answered

Only those molecules that form extremely stable cations undergo S_N1 mechanisms. Normally, only compounds that yield 3° (tertiary) carbonications (or resonance-stabilized carbocations) undergo S_N1 mechanisms rather than S_N2 mechanisms. Carbocations of tertiary alkyl halides not only exhibit stability due to the inductive effect, but the original molecules exhibit steric hindrance of the rear lobe of the bonding orbital, which inhibits S_N2 mechanisms from occurring. Primary alkyl halides, which have little inductive stability of their cations and exhibit no steric hindrance of the rear lobe of the bonding orbital, generally undergo S_N2 mechanisms. Figure 1illustrates the tendencies of alkyl halides toward the two types of substitution mechanisms.