Question

Why does Cyanide only react in SN2 reactions?

Answer 1

I think that the premise of your question is incorrect.

Explanation:

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.

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).

The mechanism involves an initial ionization of the alkyl halide:

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