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.

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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