Reduction of Nitriles

Reduction of Nitriles

Nitriles ($\text{R}-\text{C}\!\equiv\!\text{N}$) can be reduced to give primary amines. Two reducing systems accomplish this: lithium aluminium hydride ($\text{LiAlH}_4$), or catalytic hydrogenation (hydrogen gas over a metal catalyst such as nickel, or sodium amalgam in ethanol).

The triple-bonded nitrile carbon is reduced all the way to a $-\text{CH}_2-\text{NH}_2$ group, so the resulting primary amine carries one carbon atom more than would be obtained by directly ammonolysing the corresponding halide.

Why this route matters: ascent of the amine series

This extra carbon is exactly the point of the method. A nitrile is itself typically made from an alkyl halide by displacement with cyanide ion, so the two-step sequence

takes a halide with $n$ carbons and delivers an amine with $n+1$ carbons. This deliberate lengthening of the carbon chain is described as an ascent of the amine series — it is the standard way to build a primary amine that has one carbon atom more than the halide (or amine) you started with, something the direct ammonolysis route in Section 9.4.2 cannot achieve on its own. …