Hydrazine salts, dehydration

Several procedures are available for the preparation of anhydrous hydrazine. Of these, the dehydration of NgH H3O andthe cleavage of hydrazine salts with NH3 are of special Interest. The first method yields anhydrous N3H4, if an efficient column with about 15 theoretical plates is used. The distillation is carried out in a stream of N3, using a mixture of 100 g. of 78.5% hydrazine hydrate and 140 g. of NaOH. The N3H4 obtained is at least 99.5% pure. 

Protection for primary amines. Sheehan and Guziec1 have introduced this reagent for replacement of both hydrogens of a primary amine function by incorporation into the very stable 4,5-diphenyl-4-oxazoline-2-one ring system (3).2 These Ox-protected derivatives are obtained by treatment of a primary amine or amino acid tetramethyl-ammonium salt with (1) in DMF. After stirring for 0.5 hr., the mixture is acidified (HC1) and extracted with ethyl acetate to obtain a diastereomeric mixture of hydroxy-oxazolidinoncs (2). The mixture is dehydrated to the Ox derivative (3) by trifluoroacetic acid (TFA). Ox derivatives are stable to aqueous base, refluxing ethanolic hydrazine,... 

Pyrylium salts also react with hydrazine and its derivatives to give the N-aminoazonium salts (Eq. 3), although the reaction takes other courses in some cases.50,70,80-93 The reaction involves the formation of intermediate hydrazones by nucleophilic attack at the a-position of pyrylium rings. The hydrazones undergo ring closure followed by dehydration. This method has been utilized for the preparation of highly substituted pyridine AT-imines. 

The use of microwave has recently been used to provide 5-amino-imidazoles from 2-aminopyridines. At a higher temperature and longer reaction times, dehydration occurs to form the imidazo-pyridinium salts. Reaction with hydrazine adds to the pyrimidinium salt, which then undergoes rearrangement to open the ring. The unstabilized diene undergoes hydrolysis to provide the 2-amino-4,5-di-substituted 1/f-imidazole. Chemically, PPA can be used to form the pyrimidinium salt also. ... 

Hydrazine is in the liquid state associated through hydrogen bonding and is monomolecular in the vapour phase . It forms an azeotropic mixture with water, which has approximately the same boiling point as anhydrous hydrazine. Barium oxide is useful as a dehydrating agent and the anhydrous solvent may also be obtained by reacting hydrazinium salts with liquid ammonia i 2 due to the insolubility of ammonium sulphate in this solvent. 

The schematic of the process is shown in Fig. 28.29. The manufacturing procedure is divided into five major steps the synthesis of azine, the removal of low boilers, high boilers separation, hydrolysis of azine, and the dehydration of hydrazine. The synthesis of azine requires a residence time of 7 hours. Ammonia, hydrogen peroxide, and methyl ethyl ketone (MEK) are fed into the reactors along with methanol and acetonitrile at about 50°C. Small amounts of the disodium salt of ethylenediamine tetracetic acid are added to the reactants. In the low-boilers removal step, the unreacted ammonia, MeOH, MEK, and acetonitrile are separated and recycled to the reactors. The high-boilers solution contains water, MEK-azine, and acetamide. The acetamide is formed in the following reaction ... 

Figure 2.5 shows the TG and DTA curves of hydrazinium picrate hemihydrate (N2H5CgH2(NO2)3O-0.5H2O) [10]. The endotherm at 148 °C in the DTA curve may be attributed to dehydration, since the corresponding TG curve shows a 2.9% weight loss against 3.3% for the loss of half a molecule of water. The exotherm at 167°C is due to decomposition of the salt, probably to carbonaceous matter, which is further oxidized in air. The small exotherm at 223 °C may well be attributed to this oxidation. During the decomposition at 167°C, part of the hydrazinium picrate sublimes, as is seen from a yellow deposit of the salt on the sides of the lid in the furnace, and the hydrazine content present... 

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