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 $"S"_"N"1$ reactions.

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

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

www.chemguide.co.uk

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

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

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

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

www.chemguide.co.uk

The mechanism involves an initial ionization of the alkyl halide:

www.chemguide.co.uk

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

www.chemguide.co.uk