is formed in a multitude of chemical reactions. Only a few processes have asquired commercial importance. These all oxidize ammonia or , an ammonia derivative, to hydrazine. hypochlorite or hydrogen peroxide is used as the oxidizing agent. Certain processes (Bayer-, H₂O₂-processes) operate in the presence of ketones.
 

Raschig Process

In the Raschig process, ammonia is oxidized with sodium hypochlorite:

Sodium hypochlorite is obtained as a ca. 4.7 mol/L solution by mixing chlorine and sodium hydroxide with cooling in a molar ratio of 1:2 (see Fig, 1.4.-3). This is diluted to ca. 1 mol/L and reacted with an aqueous ammonia solution (ca. 15%) at temperatures around 0 °C (with cooling) forming chloramine and sodium hydroxide. The yield is almost quantitative.

The alkaline chloramine solution is then reacted, at ca. 130°C under pressure, with a 20- to 30-fold molar excess of anhydrous ammonia. The excess ammonia then separated from the reaction mixture, is recycled. and the hydrazine-water azeotrope (b.p. 120.5 °C) are distilled off leaving solid sodium chloride. The aqueous hydrazine solution obtained is finally concentrated by distillation. Ca. 70% of the theoretical yield is obtained. Important side reactions are:
(1)reaction of chloramine with the hydrazine formed:



This reaction is particularly catalyzed by copper. A large excess of ammonia and the addition of complexing agents such as ethylenediaminetetra-acetic acid () are used as countermeasures.

(2) decomposition of hydrazine during its evaporation from solid sodium chloride

If hydrazine hydrate is not required, hydrazine can be precipitated out from the Raschig synthesis reaction mixture as the sparingly soluble hydrazine sulfate (N₂H₆²⁺SO₄^2- , solubility in water: 2.96 g/L).

Urea Process

In this process a mixture of urea, sodium hypochlorite and sodium hydroxide is converted into hydrazine, sodium chloride and sodium carbonate.

The reaction components are mixed cold in the above molar ratios and the mixture rapidly heated to 100°C. The working up is the same as with the Raschig process, giving 60 to 70% of the theoretical yield of hydrazine. The advantages of this process are the avoidance of a large excess of ammonia and working at atmospheric pressure, which must be set against the serious disadvantage of using two moles of sodium hydroxide per mole of hydrazine. Furthermore, the production of a sodium chloride-sodium carbonate-mixture poses an additional ecological problem.

Bayer Process

Of a number of hydrazine syntheses based on the oxidation of ammonia with sodium hypochlorite in the presence of ketones (acetone, methyl ethyl ketone) (see Fig. 1.4-4), only that of Bayer A.G. appears to have graduated to a commercial process.


 
The two main reactions are the formation and hydrolysis of acetone azine:

The formation of azine is not a means of trapping the hydrazine formed in the Raschig process with acetone, the reaction proceeding by way of two distinct intermediates:
dimethyl oxazirane and acetone hydrazone, as follows:



Sodium hypochlorite solution (ca. 1.5 mol/L), ammonia and acetone in a molar ratio of 1: 15 to 20:2 are reacted together at 35°C. A solution results consisting of 5 to 7% by weight of acetone azine together with sodium chloride and excess ammonia. This ammonia is distilled off and returned to the reaction. Next, the acetone azine-water-azeotrope (b.p. 95°C) is distilled off leaving the sodium chloride solution. Herein lies the ssential difference from the Raschig process in which the hydrazine water mixture has to be separated from solid sodium chloride.

Finally, the acetone azine is hydrolyzed with water in a reaction distillation tower into acetone (head product) and a 10% aqueous hydrazine solution (sump product) at temperatures up to 180°C and pressures of 8 to 12 bar. The hydrazine solution is concentrated to its azeotrope composition of 64% by weight of hydrazine. The hydrazine yield is 80 to 90%, based on the hypochlorite utilized.

H₂O₂ Process

This process is similar to the Bayer process except that hydrogen peroxide is used as the oxidizing agent and the ketone used is methyl ethyl ketone:

Since the reaction with hydrogen peroxide is too slow, anactivator is added as a catalyst, which in the process variant published by ATOCHEM is a mixture of acetamide, amrnonium acetate and sodium hydrogen phosphate. Other catalysts can also be used. The presumed reaction mechanism is similar to that of the Bayer process:

The methyl ethyl ketone azine formed, which is poorly soluble in water, is separated and hydrolyzed to hydrazine and ketone. The catalyst-containing aqueous solution is returned to the synthesis.

This process is operated commercially by ATOCHEM in France and in a very similar process by Mitsubishi Gas Chemicals in Japan. The advantage of this process over the Bayer and Raschig processes is that sodium chloride is not formed as a byproduct.