CYANOGEN AND ITS COMPOUNDS. The compound which in its chemical behaviour most closely resembles inorganic substances, and which forms as it were a connecting link between these and organic bodies, is the radicle cyanogen. Before proceeding, therefore, to the systematic consideration of the great organic families, cyanogen and its compounds may be conveniently treated of.

Cyanogen,1 CN or Cy, in the free state Cy2, is generally prepared by heating the cyanide of some heavy metal (usually mercury) : HgCy2 = Hg + Cy,. A brown substance, most probably a polymeride of cyanogen, known as paracyanogen, is always formed in this reaction.

Cyanogen is a colourless gas, having a pungent odour resembling that of bitter almond oil. It burns in air with a purple flame, and is extremely poisonous. The gas is condensable into a liquid under a pressure of about 4 atmospheres. The liquid boils at – 21°C., and solidifies at – 34° C. Water absorbs about 4 volumes of the gas in the cold. The aqueous solution decomposei on standing, ammonium oxalate being the chief product of the reaction : ,NH4)2C204 • At the same time small quantities of urea, ammonium carbonate, and cyanide are formed. The addition of a mineral acid to the solution greatly retards the decomposition, oxamide being then produced : 02N2 20H2 = C202(NH2)2 . Conversely, when ox- amide or ammonium oxalate is heated cyanogen is produced : (NH4)2C204 – 40112= C2N2; C202(N112)2– 20112 = 02N2.

In its chemical relationships cyanogen is the exact analogue of the halogen elements. Its compounds with metals or positive radicles are called cyanides.

Compounds of cyanogen with Cl, Br, I, S, OH, and NH, are known, and are remarkable for their polymeric modifications. The following is a list of the more important compounds :- Cyanogen chloride, CNCI Cyanic acid, CNOH Cyanuric chloride, C3N3C23 Cyanuric acid, CEN303H3 Cyanogen bromide, CN Br I Sulphocyanie acid, CNSH Cyanuric bromide, C3N01.34 Cyanamide, CN(NH2) Cyanogen iodide, CN I I Dicyanamide, C2N,(NH,), Cyanogen sulphide, C,N,S Cyauuric amide, CsN,(NI[2)3 Cyanogen sclenide, C2N5Se Ammelide, C3N,(NII2)(011)2 Ammeline, CsNs(NH2)2OI1 Hydrogen Cyanide, or Hydrocyanic or Prussic Acid, HCN or HCy. - This compound is formed synthetically by passing electric sparks through a mixture of nitrogen and ethine gases : 02112+ N2 = 2H0N . In practice benzene vapour may be used instead of pure ethine, as it is partially resolved into the latter substance by the action of the spark. The anhydrous acid is also obtained by passing dry hydrogen sulphide over mercuric cyanide. The aqueous solution of the acid is prepared by the action of acids upon metallic cyanides : 1101 + KCN = KCI + HCN ; by the action of ammonia on chloroform : NH3 + CHC13 = HCN + 31101; and also (most conveniently) by heating a mixture of 5 parts of potassium ferrocyanide with 3 parts of sulphuric acid and 4 parts of water.

The pure acid is a colourless liquid, having an odour of bitter almonds ; it is a most violent poison. Its boiling-point is 26°•5 C. and its point of solidification – 15° C. The pure acid and its strong aqueous solution are both inflanr mable, burning in air with a violet flame. Both the anhy drous and aqueous acids are very unstable, the formei decomposing into ammonia and a brown substance, and the latter undergoing the same decomposition with the additional formation of ammonium formate. Small quantities of formic or of a mineral acid prevent this decomposition, but on mixture with strong acids, a complete decomposition into formic acid ensues : - HCN + 20112 + Hel = II.COOH + N114C1 Ifydroc•anIc Water. ydroch I ori Formic Ammonium acid. acid. acid. chloride.

Alkalies induce a similar change, alkaline formate and free ammonia being produced. When ammonium formate is heated, the inverse reaction takes place : - HCO(ON114) 20H2 = HCN Ammonium hydrocyanicformate.

formate. acid.

A polymeride, 11303N3, is known.

Metallic Cyanides. - Cyanogen being a monad radicle forms, like 01, Br, and I, a series of salts typified by the formulae M'Cy, M"0y2, 111"'20376, WCy,,. Of these the most important is potassium cyanide, which can be formed by passing nitrogen over a mixture of red hot carbon and potassium carbonate : -