Why do primary alkyl halides generally undergo SN2 mechanisms?

1 Answer
Oct 26, 2014

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.

Explanation:

Steric Hindrance

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

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

Instability of 1° Carbocations

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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.