Raschig synthesis

Hydroxylamine can be prepared by a variety of reactions involving the reduction of nitrites, nitric acid or NO, or by the acid hydrolysis of nitroalkanes. In the conventional Raschig synthesis, an aqueous solution of NH4NO2 is reduced with HS04 /S02 at 0° to give the hydroxylamido-A ,A -disulfate anion which is then hydrolysed stepwise to hydroxylammonium sulfate ... 

A modification of the Raschig synthesis uses urea in place of ammonia to synthesize N2 H4 according to (Ref 12) ... 

Although it has long been known that benzaldehyde reacts with hydrazine in alkaline solution to give a yellow, insoluble precipitate of benzalazine1 and that the latter when steam distilled in the presence of mineral acids is hydrolyzed to give the corresponding hydrazine salts,2 no previous attempt has been made to apply these facts to the recovery of hydrazine residues. This reaction is practically quantitative and can be applied not only to the recovery of residual hydrazine left in solution in the Raschig synthesis (No. 31) but to all solutions where the detection and recovery of hydrazine in the presence of ammonia, or hydroxylamine, are of interest. In the process of formation of hydrazine salts from benzalazine, 95 per cent of the benzaldehyde can be recovered. 

One hundred and four grams of crude benzalazine (obtainable from the residual solutions of the Raschig synthesis) is suspended in 400 ml. of water, and 60 ml. of concentrated sulfuric acid added. The solution is steam distilled until no more benzaldehyde comes over and then cooled in an ice-salt mixture to bring about precipitation of the sulfate. Yield 60 g. of N2H4H2SO4 (92 per cent). 

Aqueous solutions of N2H4 have been made by the Raschig synthesis.19 The overall reaction, carried out in aqueous solution, is... 

The Raschig hydrazine synthesis can be extended one further step. Reaction of chloramine with hydrazine in ether yields triazanium chloride, which decomposes instantaneously. This is responsible for hydrazine decomposition during the Raschig synthesis (see Section 5.2.1) ... 

Liberation of nitrogen in these reactions may bear comparison with its formation in a side reaction in the Raschig synthesis of hydrazine ... 

If hydrazine hydrate is not required, hydrazine can be precipitated out from the Raschig synthesis reaction mixture as the sparingly soluble hydrazine sulfate (N2H6 S04 % solubility in water 2.96 g/L). 

The ammonia of Raschig synthesis process is replaced by urea in Schestakov process. 

The only practical methods for preparing hydrazine in quantity are the Raschig synthesis, discovered in the first decade of this century, and a variant thereof. The overall reaction, carried out in aqueous solution, is 2NH3 + NaOCl -> N2H4 + NaCl + H20 Actually, the reaction proceeds in two steps ... 

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