What is the main factor for alkyl halides and aryl halides that determines wheather they will procede through Sn1 or Sn2 mechanism? Please also explain this in terms of stability of intermediate or any other proper way.
Only those molecules that form extremely stable cations undergo SN1 mechanisms. Normally, only compounds that yield 3° (tertiary) carbonications (or resonance-stabilized carbocations) undergo SN1 mechanisms rather than SN2 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 SN2 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 SN2 mechanisms. Figure 1illustrates the tendencies of alkyl halides toward the two types of substitution mechanisms.