GUANIDINE, CN3H5 or HN:C(NH2)2, the amidine of amidocarbonic acid. It occurs in beet juice. It was first prepared in 1861 by A. Strecker, who oxidized guanine with hydrochloric acid and potassium chlorate. It may be obtained synthetically by the action of ammonium iodide on cyanamide, CN.NH2 + NH4I=CN3H5.HI.; by heating ortho-carbonic esters with ammonia to 150 deg. C.; but best by heating ammonium thiocyanate to 180 deg.-190 deg. C., when the thiourea first formed is converted into guanidine thiocyanate, 2CS(NH2)2=HN:C(NH2)2.HCNS+H2S. It is a colourless crystalline solid, readily soluble in water and alcohol; it deliquesces on exposure to air. It has strong basic properties, absorbs carbon dioxide readily, and forms well-defined crystalline salts. Baryta water hydrolyses it to urea. By direct union with glycocoll acid, it yields glycocyamine, NH2.(HN):C.NH.CH2.CO2H, whilst with methyl glycocoll (sarcosine) it forms creatine, NH2.(NH):C.N(CH3).CH2.CO2H. Many derivatives of guanidine were obtained by J. Thiele (_Ann._, 1892, 270, p. 1; 1893, 273, p. 133; _Ber._, 1893, 26, pp. 2598, 2645). By the action of nitric acid on guanidine in the presence of sulphuric acid, nitroguanidine, HN:C(NH2).NH.NO2 (a substance possessing acid properties) is obtained; from which, by reduction with zinc dust, amidoguanidine, HN:C(NH2).NH.NH2, is formed. This amidoguanidine decomposes on hydrolysis with the formation of semicarbazide, NH2.CO.NH.NH2, which, in its turn, breaks down into carbon dioxide, ammonia and hydrazine. Amidoguanidine is a body of hydrazine type, for it reduces gold and silver salts and yields a benzylidine derivative. On oxidation with potassium permanganate, it gives azodicarbondiamidine nitrate, NH2.(HN):C.N:N.C:(NH).NH2.2HNO3, which, when reduced by sulphuretted hydrogen, is converted into the corresponding hydrazodicarbondiamidine, NH2.(HN):C.NH.NH.C:(NH).NH2. By the action of nitrous acid on a nitric acid solution of amidoguanidine, diazoguanidine nitrate, NH2.(HN):C.NH.N2.NO3, is obtained. This diazo compound is decomposed by caustic alkalis with the formation of cyanamide and hydrazoic acid, CH4N5.NO3=N3H+CN.NH2+HNO3, whilst acetates and carbonates convert it into amidotetrazotic acid, N--N. // H2N.C ||. \\ NH--N Amidotetrazotic acid yields addition compounds with amines, and by the further action of nitrous acid yields a very explosive derivative, diazotetrazol, CN6. By fusing guanidine with urea, dicyandiamidine H2N.(HN):C.NH.CO.NH2, is formed.