Why do primary alkyl halides generally undergo SN2 mechanisms?

Question

Ryan Andrews · Accepted Answer

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.

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.

Ruby Allen · Answer

undefined Primary alkyl halides generally undergo SN2 mechanisms because of the less steric hindrance, allowing nucleophiles to approach and replace the leaving group more easily compared to SN1 reactions.