Hoffmann Bromamide Degradation Reaction

Hoffmann Bromamide Degradation Reaction

Hoffmann devised a method for making primary amines starting not from a halide or nitrile but from an amide. The amide is treated with bromine in an aqueous or ethanolic solution of sodium hydroxide.

What actually happens: a degradation, not a simple reduction

This is described as a degradation reaction because, along the way, the alkyl or aryl group R migrates from the carbonyl carbon of the amide directly onto the nitrogen atom (via an isocyanate-type intermediate), and the carbonyl carbon itself is lost from the product as carbonate. The nitrogen that ends up bonded to R in the amine is the same nitrogen that was already present in the amide — the amide's carbon is stripped away entirely rather than being retained as part of the amine, which is why this is classed as a migration/degradation mechanism rather than a straightforward reduction like those in Sections 9.4.3 and 9.4.4.

The carbon count: one less than the amide

Because the carbonyl carbon is removed in this way, the amine produced contains one carbon atom fewer than the amide it was made from. So starting from an amide $\text{R}-\text{CONH}_2$ with $n$ carbons (counting the carbonyl carbon), the Hoffmann reaction gives an amine $\text{R}-\text{NH}_2$ with only $n-1$ carbons — R itself is unchanged, but the carbon that used to be the carbonyl carbon is gone. …