Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

134 SBMICAEBAZIDB
The esters of carbamic acid, the urethanes, which are formed by
combination of alcohols with compounds of the cyanic acid series,
are stable substances. The reaction by which they are formed is,
likewise, capable of undergoing many variations. It may be recalled
that a second method of synthesising them consists in acting on the
esters of chloroformic acid with ammonia and amines.
(c) SEMICAEBAZIDE 1
Hydrazine sulphate (52 g.) and 21 g. of anhydrous sodium carbonate
are dissolved in 200 c.c. of boiling water. The solution is
cooled to 50°, a solution of 35 g. of potassium cyanate in 100 c.c. of
water is added, and the whole is allowed to stand over night. After
small amounts of hydrazodicarbonamide, produced in the reaction
H2N.CO.NH.NH2 + O=C=NH -> H2N.CO.NH.NH.CO.NH2, have
been removed by filtration, 60 c.c. of acetone are added to the
solution, which is again allowed to stand, with frequent shaking,
for twenty-four hours. The acetone semicarbazone which has then
crystallised out is filtered dry at the pump, washed with a little
water, and dried on a porous plate or in vacuo.
The mother liquor is evaporated to dryness on the water bath,
and the powdered residue is extracted with alcohol in an extraction
apparatus. Semicarbazone crystallises out in the flask of the apparatus.
If a sample of the main portion of the material leaves a considerable
amount of ash when ignited on platinum foil, this portion
also should be extracted in the same way.
To decompose the semicarbazone it is gently warmed with concentrated
hydrochloric acid (8 c.c. for each 10 g. of material) until
dissolution is just complete. On cooling the solution semicarbazide
hydrochloride sets to a thick crystalline mass which is filtered dry
at the pump and washed, first with a little cold hydrochloric acid
(1 : 1), then twice with 3 to 5 c.c. portions of ice-cold alcohol. The
salt is dried in a desiccator. Yield 22-25 g.
In order to obtain the free semicarbazide, 5-5 g. of the hydrochloride
are ground in a small mortar with 4-5 g. of anhydrous sodium
acetate (see p. 127). The paste produced by the formation of free
acetic acid is transferred with a spatula to a 100 c.c. conical flask;
the last portions are washed in with absolute alcohol and the contents
of the flask are then boiled on the water bath with (altogether) 50 c.c.
of absolute alcohol. During the boiling the flask is frequently
1 Thiele and Stange, Ber., 1894, 27, 31; H. Biltz, Annalen, 1905, 339, 250.