Explain the reactivity of `SN^2` mechanism ?

Explanation

The reactivity of alkyl halide towards `SN^2` reaction mechanism has.been found to be in the following order :
Methyl halide> primary halide> Secondary halide> Tertiary halide
This can be easily explained on the basis of formation and the stability of transition state in the reaction. Since the reaction required the approach of the nucleophile to the carbon bearing the leaving group, the presence of the bulky group with a reaction is called stearic hindrance. Therefore the alkyl halides (methyl halide) react most rapidly in eaction because there are only three small hydrogen atom. On the other hand··tertiary alkyl halide are least reactive because of the presence of three bulky gi-oup on carbon. Thus the order of reactivity is
`CH_3 X gt 1^@ gt 2^@ gt 3^@` alkyl halide