Explain `S_N^2` reactions. What is the preference order of different alkyl halides to undergo nucleophilic substitution reaction via SN2 mechanism ?

Explanation

`underset("methy bromide (Aialky halide )")(CH_3 Br) + KOH (aq) overset(S_(N^2))to underset("Methyl alcohol")(CH_3 OH^(+) KBr)`
Mechanism of the reaction : The reaction follows `S_(N^2)` mechanism in case of `1^@` alkyl halides because of least steric hinderance.

Kinetics of the reaction : Rate of reaction `= k[CH_3Br[OH^(-)]` Order of reactivity of alkyl halides `S_(N^2)` mechanism : `3^@ lt 2^@ lt 1^@`