Question

Why do primary alkyl halides generally undergo SN2 mechanisms?

Answer 1

Primary alkyl halides undergo `"S"_N2` mechanisms because (a) 1° substrates have little steric hindrance to nucleophilic attack and (b) 1° carbocations are relatively unstable.

Explanation:

Steric Hindrance

As you add more alkyl groups o the α carbon atom, the substrate becomes less susceptible to `"S"_N2` attack

A 1° alkyl is sterically unhindered, so an `"S"_N2` reaction is likely.

Instability of 1° Carbocations

Alkyl groups release electrons by inductive and hyperconjugation effects so that they can stabilize a positive charge on the α carbon.

A 1° alkyl halide has only one alkyl group, so it is relatively unstable. It is unlikely to form a 1° carbocation in an `"S"_N1` reaction.

Instead, it will take the lower-energy `"S"_N2` path, in which the nucleophile "kicks out" the halide leaving group, and void the formation of the unstable carbocation.