Hydrolysis and Reduction Reactions of Cyanides and Isocyanides

Although cyanides and isocyanides are constitutional isomers of each other, their

hydrolysis and reduction chemistry diverges sharply, precisely because the identity of

the atom bonded to $\text{R}$ (carbon in one, nitrogen in the other) fixes which atom

ends up incorporated into the organic product and which is expelled as a small

by-product molecule.

Hydrolysis of a cyanide (nitrile). Complete hydrolysis under aqueous ACIDIC (or

basic) conditions proceeds through an amide intermediate to a carboxylic acid, with the

nitrile's nitrogen expelled as ammonia:

The organic product RETAINS the original carbon skeleton plus the nitrile carbon (now

oxidised to a carboxyl carbon); nitrogen leaves the molecule entirely.

Hydrolysis of an isocyanide. Under aqueous ACIDIC conditions, an isocyanide

hydrolyses very differently: nitrogen (bonded to $\text{R}$) is RETAINED in the organic

product, which is recovered as the primary amine, while the terminal carbon is instead

expelled as methanoic (formic) acid:

This is the mirror image of nitrile hydrolysis: here it is CARBON that leaves the

molecule (as a one-carbon acid) while nitrogen (still bonded to $\text{R}$) stays.

Reduction with $\text{H}_2/\text{Ni}$. A cyanide reduces cleanly to a PRIMARY amine

with one more carbon than $\text{R}$: $\text{R}-\text{CN} \xrightarrow{\text{H}_2/ \text{Ni}} \text{R}-\text{CH}_2-\text{NH}_2$ (this is the same reduction already met in

Section 25.5.1 as a route to primary amines). An isocyanide, by contrast, reduces to a

SECONDARY amine, because the terminal carbon becomes a NEW methyl group bonded to …