# 1911 Encyclopædia Britannica/Hydrazine

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​HYDRAZINE (Diamidogen), N$_2$H$_4$ or H$_2$ N·NH$_2$, a compound of hydrogen and nitrogen, first prepared by Th. Curtius in 1887 from diazo-acetic ester, N$_2$CH·CO$_2$C$_2$H$_5$. This ester, which is obtained by the action of potassium nitrate on the hydrochloride of amidoacetic ester, yields on hydrolysis with hot concentrated potassium hydroxide an acid, which Curtius regarded as C$_3$H$_3$N$_6$(CO$_2$H)$_3$, but which A. Hantzsch and O. Silberrad (Ber., 1900, 33, p. 58) showed to be C$_2$H$_2$N$_4$(CO$_2$H)$_2$, bisdiazoacetic acid. On digestion of its warm aqueous solution with warm dilute sulphuric acid, hydrazine sulphate and oxalic acid are obtained. C. A. Lobry de Bruyn (Ber., 1895, 28, p. 3085) prepared free hydrazine by dissolving its hydrochloride in methyl alcohol and adding sodium methylate; sodium chloride was precipitated and the residual liquid afterwards fractionated under reduced pressure. It can also be prepared by reducing potassium dinitrososulphonate in ice cold water by means of sodium amalgam:—
| KSO$_3$ | N⋅NO→ | KSO$_3$ | N⋅NH$_2$→K$_2$SO$_4$+N$_2$H$_4$. |
| KO | N⋅NO→ | H | N⋅NH$_2$→K$_2$SO$_4$+N$_2$H$_4$. |
​P. J. Schestakov (J. Russ. Phys. Chem. Soc., 1905, 37, p. 1) obtained hydrazine by oxidizing urea with sodium hypochlorite in the presence of benzaldehyde, which, by combining with the hydrazine, protected it from oxidation. F. Raschig (German Patent 198307, 1908) obtained good yields by oxidizing ammonia with sodium hypochlorite in solutions made viscous with glue. Free hydrazine is a colourless liquid which boils at 113.5° C., and solidifies about 0° C. to colourless crystals; it is heavier than water, in which it dissolves with rise of temperature. It is rapidly oxidized on exposure, is a strong reducing agent, and reacts vigorously with the halogens. Under certain conditions it may be oxidized to azoimide (A. W. Browne and F. F. Shetterly, J. Amer. C.S., 1908, p. 53). By fractional distillation of its aqueous solution hydrazine hydrate N$_2$H$_4$·H$_2$O (or perhaps H$_2$N·NH$_3$OH), a strong base, is obtained, which precipitates the metals from solutions of copper and silver salts at ordinary temperatures. It dissociates completely in a vacuum at 143°, and when heated under atmospheric pressure to 183° it decomposes into ammonia and nitrogen (A. Scott, J. Chem. Soc., 1904, 85, p. 913). The sulphate N$_2$H$_4$·H$_2$SO$_4$, crystallizes in tables which are slightly soluble in cold water and readily soluble in hot water; it is decomposed by heating above 250° C. with explosive evolution of gas and liberation of sulphur. By the addition of barium chloride to the sulphate, a solution of the hydrochloride is obtained, from which the crystallized salt may be obtained on evaporation.
Many organic derivatives of hydrazine are known, the most important being phenylhydrazine, which was discovered by Emil Fischer in 1877. It can be best prepared by V. Meyer and Lecco’s method (Ber., 1883, 16, p. 2976), which consists in reducing phenyldiazonium chloride in concentrated hydrochloric acid solution with stannous chloride also dissolved in concentrated hydrochloric acid. Phenylhydrazine is liberated from the hydrochloride so obtained by adding sodium hydroxide, the solution being then extracted with ether, the ether distilled off, and the residual oil purified by distillation under reduced pressure. Another method is due to E. Bamberger. The diazonium chloride, by the addition of an alkaline sulphite, is converted into a diazosulphonate, which is then reduced by zinc dust and acetic acid to phenylhydrazine potassium sulphite. This salt is then hydrolysed by heating it with hydrochloric acid—
* C$_6$H$_5$N$_2$Cl + K$_2$SO$_3$ = KCl + C$_6$H$_5$N$_2$·SO$_3$K,
* C$_6$H$_5$N$_2$·SO$_3$K + 2H = C$_6$H$_5$·NH·NH·SO$_3$K,
* C$_6$H$_5$NH·NH·SO$_3$K + HCl + H$_2$O = C$_6$H$_5$·NH·NH$_2$·HCl + KHSO$_4$.
Phenylhydrazine is a colourless oily liquid which turns brown on exposure. It boils at 241° C., and melts at 17.5° C. It is slightly soluble in water, and is strongly basic, forming well-defined salts with acids. For the detection of substances containing the carbonyl group (such for example as aldehydes and ketones) phenylhydrazine is a very important reagent, since it combines with them with elimination of water and the formation of well-defined hydrazones (see Aldehydes, Ketones and Sugars). It is a strong reducing agent; it precipitates cuprous oxide when heated with Fehling’s solution, nitrogen and benzene being formed at the same time—C$_6$H$_5$·NH·NH$_2$ + 2CuO = Cu$_2$O + N$_2$ + H$_2$O + C$_6$H$_5$. By energetic reduction of phenylhydrazine (e.g. by use of zinc dust and hydrochloric acid), ammonia and aniline are produced—C$_6$H$_5$NH·NH$_2$ + 2H = C$_6$H$_5$NH$_2$ + NH$_3$. It is also a most important synthetic reagent. It combines with aceto-acetic ester to form phenylmethylpyrazolone, from which antipyrine (q.v.) may be obtained. Indoles (q.v.) are formed by heating certain hydrazones with anhydrous zinc chloride; while semicarbazides, pyrrols (q.v.) and many other types of organic compounds may be synthesized by the use of suitable phenylhydrazine derivatives.