Carbylamine Reaction

Carbylamine Reaction (Isocyanide Test)

When a primary amine is warmed with chloroform and ethanolic (alcoholic) potassium hydroxide, it is converted into an isocyanide (also called a carbylamine). The defining feature of these products is their intensely unpleasant, penetrating smell, which is why the reaction is universally remembered as the "carbylamine reaction."

For an aromatic primary amine such as aniline, the same transformation gives the corresponding aryl isocyanide:

Mechanistically, the alcoholic KOH first generates dichlorocarbene ($:\text{CCl}_2$) from chloroform by successive removal of HCl; this highly reactive carbene is then attacked by the lone pair on the amine nitrogen, and loss of a further two molecules of HCl (again mediated by KOH) delivers the terminal isocyanide, which carries a carbon–nitrogen triple-bond-like functional group ($-\overset{+}{\text{N}}\!\!\equiv\!\overset{-}{\text{C}}$).

Why Only Primary Amines Respond

The reaction absolutely requires an amine nitrogen that still carries two N–H hydrogens, because both of them must be removed (along with the halogens of chloroform) to build the isocyanide's terminal carbon. A primary amine, $\text{R–NH}_2$, satisfies this condition, whether R is an alkyl group or an aryl ring.

A secondary amine has only one N–H hydrogen, and a tertiary amine has none at all — in both cases the nitrogen cannot supply the two hydrogens the mechanism needs, so neither class reacts with chloroform/alcoholic KOH at all. No isocyanide, and hence no foul smell, is produced. …