Raschig synthesis

The Raschig synthesis , also known as the Raschig process , is a large-scale process for the synthesis of hydrazine . Chlorine , sodium hydroxide and ammonia are used as starting materials . The synthesis is named after its discoverer Fritz Raschig .

The process

The synthesis is divided into several steps. The first step is to produce the required sodium hypochlorite , which takes place according to the following reaction equation:

${\displaystyle \mathrm {2\ NaOH\ +\ Cl_{2}\longrightarrow \ NaOCl\ +\ NaCl\ +\ H_{2}O} }$

Instead of sodium hypochlorite, it is also possible to use other hypochlorite salts, but for reasons of cost it is advisable to use the inexpensive sodium hydroxide.

The hypochlorite obtained is then made to react in the actual reaction with ammonia. Hypochlorous acid is formed from the hypochlorite salt in an aqueous medium . The intermediate product chloramine is then formed according to the following equation :

${\displaystyle \mathrm {(1)\ \ NaOCl\ +\ H_{2}O\longrightarrow \ HClO+\ NaOH} }$

${\displaystyle \mathrm {(2)\ \ HClO\ +\ NH_{3}\longrightarrow \ NH_{2}Cl\ +\ H_{2}O} }$

The nitrogen is oxidized from the oxidation state −3 to −1 . In the next step, the chloramine formed is converted to hydrazine. This happens through the reaction of a molecule of chloramine with a molecule of ammonia according to the following equation:

${\displaystyle \mathrm {ClNH_{2}\ +\ NH_{3}\longrightarrow \ N_{2}H_{4}\ +\ HCl} }$

The net reaction thus results:

${\displaystyle \mathrm {2\ NH_{3}\ +\ OCl^{-}\longrightarrow \ N_{2}H_{4}\ +\ H_{2}O\ +\ Cl^{-}} }$

It is difficult to grasp the hydrazine obtained from this reaction solution , however, since the hydrazine formed oxidizes to molecular nitrogen in a rapid reaction, with ammonium chloride being formed:

${\displaystyle \mathrm {N_{2}H_{4}\ +\ 2\ NH_{2}Cl\longrightarrow \ N_{2}\ +\ 2\ NH_{4}Cl} }$

The situation is also exacerbated by the fact that the formation of hydrazine from chloramine is very slow, while the oxidation of hydrazine to nitrogen is a rapid reaction in relation to. This means that the hydrazine formed continues to react shortly after it is formed. To avoid this, chloramine is first produced at low temperatures and then added to an ammonia solution, whereby there must be a high excess of ammonia. The temperature in the reaction mixture rises due to the exothermic reaction that sets in. Under these conditions the synthesis of hydrazine proceeds rapidly and the chloramine used is completely consumed in a short time. In the absence of chloramine, the oxidation of hydrazine to nitrogen cannot proceed. Following from distillation of the excess ammonia hydrazine can be used as 64% strength hydrazine hydrate are distilled.

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• AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 660.
• F. Raschig: Lecture experiments from the chemistry of inorganic nitrogen compounds , in: Chem. Ber. 1907 , 40 , 4580-4588; doi : 10.1002 / cber.190704004106 .
• E. Abel: On the Raschig synthesis of hydrazine , in: Monatsh. Chem. 1956 , 87 , 164-175; doi : 10.1007 / BF00903601 .