(1) Nature of substrate : SN2 : The transition state (T.S.) of SN2 mechanism + is pentacoordinate, it is crowded. As a result SN2 mechanism is favoured in primary halides and least favoured in tertiary halides.
SN1 : A planar carbocation intermediate is formed in SN1 reaction. Bulky alkyl groups can be easily accommodated in planar carbocation and it has no steric crowding.
As a result SN1 mechanism is favoured in tertiary halides and least favoured in primary halides.
The carbocation intermediate is stabilized by + effect of alkyl substituents and also by hyperconjugation y effect of alkyl substituents containing a-hydrogens.
As a result,
SN1 mechanism is favoured in tertiary halides and east favoured in primary halides.
Thus,
Tertiary alkyl halides undergo nucleophilic substitution by SN1 mechanism while primary halides follow SN2 mechanism.
(2) Nucleophilicity of the reagent : A strong nucleophile attacks the substrate faster and favours SN2 mechanism. The rate of SN1 mechanism is independent of the nature of nucleophile.
Nucleophile does not react in the 1st step (slow step) of SN1. Nucleophile reacts fast after the carbocation intermediate is formed.
(3) Solvent polarity : (1) SN1 reaction proceeds more rapidly in polar protic solvents than in aprotic solvent.
Polar protic solvent decreases the rate of SN2 reaction.
(2) In SN2 mechanism, rate depends on substrate as well as nucleophile.
A polar solvent stabilizes nucleophile by solvation.
Thus,
Solvent deactivates the nucleophile by stabilizing it.
Hence,
aprotic solvents or solvent of low polarity will favour SN2 mechanism.
