Why does Cyanide only react in SN2 reactions?

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Answer 1 · 2016-01-21T23:07:55

I think that the premise of your question is incorrect.

Cyanide will also react in $S_{N} 1$ $"S"_"N"1$ reactions.

${CN}^{-}$ $"CN"^-$ is a strong nucleophile.

We expect it to take part in $S_{N} 2$ $"S"_"N"2$ reactions.

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It is also a weak base, so we do not expect either $E2$ $"E2"$ or $E1$ $"E1"$ eliminations.

If an $S_{N} 2$ $S_N2$ reaction is not possible, an $S_{N} 1$ $"S"_"N"1$ reaction might occur.

For example, tert-butyl bromide does not undergo $S_{N} 2$ $"S"_"N"2$ substitution.

Yet, if you heat it with a solution of $KCN$ $"KCN"$ in ethanol, the product is tert-butyl cyanide (2,2-dimethylpropanenitrile).

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The mechanism involves an initial ionization of the alkyl halide:

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followed by a rapid attack by cyanide ion on the tert-butyl carbocation:

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