Preparation of hydrazones, semicarbazones, etc

Hydrazine and, more so, its 1,1-substitution products form with aldehydes and ketones derivatives that are often valuable from an analytical or preparative point of view. 

Hydrazones derived from unsubstituted hydrazine are mostly rather unstable and have only specialized preparative interest, e.g., in Wolff-Kishner reduction897 and synthesis of heterocycles. In many cases it is difficult to prepare them because hydrazine tends to react with both amino groups, thereby yielding azines an excess of hydrazine is usually used so as to avoid this azine formation. The condensing agent may be triethylamine, barium oxide, or sodium hydroxide, sometimes in alcohol. Diethyl phosphonate898 has been recommended as an excellent solvent that is said also to catalyse formation of the hydrazone. 

Hydrazine sulfate may replace the hydrate as reactant the sulfuric acid is buffered with sodium acetate, and the resulting sodium sulfate is precipitated by ethanol. The condensation occurs at room temperature or on gentle warming. Ketones require a somewhat longer reaction time than aldehydes. 

Acetone hydrazone 556,899 Acetone (15 g) is added to hydrazine hydrate (15 g) containing a few pieces of barium oxide, whereby considerable heat is generated. The mixture is set aside for a few days, then filtered and fractionated, giving a yield of about 31 % of the hydrazone, b.p. 124-125°. The hydrazone decomposes slowly when kept, generating ammonia and nitrogen. 

To avoid azine formation it has also been proposed900 that the carbonyl compound should first be converted into a 1,1-dialkylhydrazone and that this be then treated with hydrazine. 

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