Further, greater
the stability of carbocation, greater will be its ease of formation from alkyl halide and faster
will be the rate of reaction In case of alkyl halides, 3
0 alkyl halides undergo SN1 reaction
very fast because of the high stability of 3
0 carbocations We  can sum up the order of reactivity
of alkyl halides towards SN1 and SN2 reactions as follows:
For the same reasons, allylic and benzylic halides show high reactivity towards the SN1
reaction The carbocation thus formed gets stabilised through resonance (Unit 8, Class XI) as
shown below:
In the following pairs of halogen compounds, which would undergo
SN2 reaction faster