Question

Why do primary alkyl halides typically undergo SN2 substitution reactions more rapidly?

Answer 1

Consider the shrubbery around `H_3C-CH_2-X` versus `(H_3C)_2CHX` with respect to the ipso carbon.

Explanation:

The bond strength of a `2""^@` alkyl halide must be comparable to a `1""^@` alkyl halide. Enthalpy is thus not really a determinant. In terms of kinetics, the `1""^@` halide is a lot less sterically hindered, and thus substitution reactions should be a lot more favourable (and faster!) with this substrate.