Why do primary alkyl halides generally undergo SN2 mechanisms?

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

Steric Hindrance

As more alkyl groups are added to 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 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.

This avoids the formation of the unstable carbocation.