Azoimide, Or Hydrazoic Acid, NH, a compound of nitrogen and hydrogen, first isolated in 1890 by Th. Curtius (Berichte, 1890, 23, p. 3023). It is the hydrogen compound corresponding to P. Greiss' diazoimino benzene, CHN, which is prepared by the addition of ammonia to diazobenzene perbromide.
Curtius found that benzoyl glycollic acid gave benzoyl hydrazine with hydrazine hydrate:
CHOCO·CHCOOH + 2NH·HO = HO + CHCONH·NH + NH·NH·CH·COOH.
(Ethyl benzoate may be employed instead of benzoyl glycollic acid for this reaction.) This compound gave a nitroso compound with nitrous acid, which changed spontaneously into benzoylazoimide by loss of water:
CHCO·NH·NH + HONO = HO + CHCO·N(NO)·NH.
CHCO·N(NO)·NH = HO + CHCO·N.
The resulting benzoylazoimide is easily hydrolysed by boiling with alcoholic solutions of caustic alkalis, a benzoate of the alkali metal and an alkali salt of the new acid being obtained; the latter is precipitated in crystalline condition on standing.
An improved method of preparation was found in the use of hippuric acid, which reacts with hydrazine hydrate to form hippuryl hydrazine, CHCONH·CHCONH·NH, and this substance is converted by nitrous acid into diazo-hippuramide, CHCONH·CH·CO·NH·N·OH, which is hydrolysed by the action of caustic alkalis with the production of salts of hydrazoic acid. To obtain the free acid it is best to dissolve the diazo-hippuramide in dilute soda, warm the solution to ensure the formation of the sodium salt, and distil the resulting liquid with dilute sulphuric acid. The pure acid may be obtained by fractional distillation as a colourless liquid of very unpleasant smell, boiling at 30° C., and extremely explosive. It is soluble in water, and the solution dissolves many metals (zinc, iron, etc.) with liberation of hydrogen and formation of salts (azoimides, azides or hydrazoates). All the salts are explosive and readily interact with the alkyl iodides. In its properties it shows some analogy to the halogen acids, since it forms difficultly soluble lead, silver and mercurous salts. The metallic salts all crystallize in the anhydrous condition and decompose on heating, leaving a residue of the pure metal.
The acid is a "weak" acid, being ionized only to a very slight extent in dilute aqueous solution.
E. Noelting and E. Grandmougin (Berichte, 1891, 24, p. 2546) obtained azoimide from dinitraniline, CH(NO)·NH, by diazotization and conversion of the diazo compound into the perbromide, (NO)CH·N·Br. This compound is then decomposed by ammonia, dinitrophenylhydrazoate being formed, which on hydrolysis with alcoholic potash gives potassium hydrazoate (azide) and dinitrophenol. The solution is then acidified and distilled, when azoimide passes over. Somewhat later, they found that it could be prepared from diazobenzene imide, provided a nitro group were present in the ortho or para position to the diazo group. The para-nitro compound is dropped slowly into a cold solution of one part of caustic potash in ten parts of absolute alcohol; the solution becomes dark red in colour and is then warmed for two days on the water bath. After the greater portion of the alcohol has distilled off, the solution is acidified with sulphuric acid and the azoimide distilled over. The yield obtained is only about 40% of that required by theory, on account of secondary reactions taking place.
Ortho-nitro-diazobenzene imide only yields 30%.
W. Wislicenus (Berichte, 1892, 25, p. 2084) has prepared the sodium salt by passing nitrous oxide over sodamide at high temperatures. The acid can also be obtained by the action of nitrous acid on hydrazine sulphate; by the oxidation of hydrazine by hydrogen peroxide and sulphuric acid (A. W. Browne, J. Amer. Chem. Soc., 1905, 25, p. 251), or by ammonium metavanadate (A. W. Browne and F. F. Shetterly, Abst. J.C.S., 1907, ii. p. 863).
Ammonium azoimide, N·NH, may be prepared by boiling diazohippuramide with alcoholic ammonia, until no more ammonia escapes, the following reaction taking place:
CHCO·NHCHCONH·N·OH + 2NH = N·NH + HO + CHCO·NH·CH·CO·NH.
The liquid is then allowed to stand for twelve hours, and the clear alcoholic solution is decanted from the precipitated hippuramide. To the alcoholic solution, four times its volume of ether is added, when the ammonium salt is precipitated. It is then filtered, washed with ether, and air-dried. The salt is readily soluble in water, and is only feebly alkaline. It is extremely explosive. Hydrazine azoimide, NH, is also known.
Chloroazoimide, Cl·N, the chloride corresponding to azoimide, was obtained by F. Raschig (Ber., 1908, 41, p. 4194) as a highly explosive colourless gas on acidifying a mixture of sodium azide and hypochlorite with acetic or boric acid.