Acid hydrazides, Curtius rearrangement, acyl azides from

A closely related reaction of general applicability is the Curtius rearrangement222 of acyl azides. The rearranging species in the Schmidt reaction (see p. 898) is in fact also a protonated acyl azide these azides are readily prepared by the action of nitrous acid on acyl hydrazides which are themselves formed from esters and hydrazine (Section 9.6.17, p. 1269). On heating in aprotic solvents the acyl azides decompose to yield the corresponding isocyanates in good yield. 

As described earlier (Section 4.4.1.1), the intermediates of the Curtius reaction are acyl azides, which themially rearrange to isocyanates. One of the classical procedures for the preparation of acyl azides consists of the formation of hydrazides from esters and hydrazine, followed by treatment of the hydrazides with nitrous acid, generated from sodium nitrite and acetic, hydrochloric or sulfuric acid. Acyl azides are commonly used in the crude state or in solution since they are thermally unstable and potentially explosive. 

---