1911 Encyclopædia Britannica/Cyanic Acid and Cyanates

CYANIC ACID AND CYANATES. Cyanic acid, CN·OH, was discovered by F. Wöhler in 1824, and may be obtained by distilling its polymeride, cyanuric acid, in a current of carbon dioxide (F. Wöhler and J. v. Liebig, Berzelius Jahresberichte, 1827, 11, p. 84), the vapours which distil over being condensed in a freezing mixture. It is a very volatile liquid of strong acid reaction, and is only stable below 0° C. It has a smell resembling that of acetic acid. At 0° C. it is rapidly converted into a mixture of cyanuric acid, C₃N₃O₃H₃, and another polymer, cyamelide (CNOH)x; this latter substance is a white amorphous powder, insoluble in water. An aqueous solution of cyanic acid is rapidly hydrolysed (above 0° C.) into a mixture of carbon dioxide and ammonia. Cyanogen chloride, CNCl, may be regarded as the chloride of cyanic acid. It may be prepared by the action of chlorine on hydrocyanic acid or on mercury cyanide. It is a very poisonous volatile liquid, which boils at 15.5° C. It polymerizes readily to cyanuric chloride, C₃N₃Cl₃. Caustic alkalis hydrolyse it readily to the alkaline chloride and cyanate.

The salts of cyanic acid are known as the cyanates, the two most important being potassium cyanate (KOCN) and ammonium cyanate (NH₄OCN). Potassium cyanate may be prepared by heating potassium cyanide with an oxidizing agent, or by heating potassium ferrocyanide with manganese dioxide, potassium carbonate or potassium dichromate (J. v. Liebig, Ann., 1841, 38, p. 108; C. Lea, Jahresb., 1861, p. 789; L. Gattermann, Ber., 1890, 23, p. 1224), the fused mass being extracted with boiling alcohol. It crystallizes in flat plates and is readily soluble in cold water. It is a somewhat important reagent, and has been used by Emil Fischer in various syntheses in the uric acid group (see Purin). Ammonium cyanate possesses considerable theoretical importance since the first synthetical production of an organic from inorganic compounds was accomplished by warming its aqueous solution for some time, urea being formed (F. Wöhler, Berzelius Jahresberichte, 1828, 12, p. 266). J. Walker and J. K. Wood (Jour. Chem. Soc., 1900, 77, p. 24) prepared pure ammonium cyanate by the union of gaseous ammonia and cyanic acid, special precautions being taken to keep the temperature below the point at which the salt is transformed into urea. It crystallizes in fine needles, which melt suddenly at about 80° C., then resolidify, and melt again at about 128° to 130° C. (this temperature being that of the melting point of urea). Substituted ammonias were also made to combine with cyanic acid, and it was found that the substituted ammonium cyanates produced pass much more readily into the corresponding ureas than ammonium cyanate itself. (On the constitution of cyanic acid see F. D. Chattaway and J. M. Wadmore, Jour. Chem. Soc., 1902, 81, p. 191.)

Esters of normal cyanic acid are not known, but those of isocyanic acid (HN·CO) may be prepared by the action of alkyl halides on silver cyanate, or by oxidizing the isonitriles with mercuric oxide. They are volatile liquids which boil without decomposition, and possess a nauseating smell. When hydrolysed with caustic alkalis, they yield primary amines (this reaction determines their constitution). C₂H₅NCO + H₂O = C₂H₅NH₂ + CO₂. When heated with water they yield carbon dioxide and symmetrical dialkyl ureas; with ammonia and amines they form alkyl ureas; and with acid anhydrides they yield tertiary amides.

Ethyl isocyanate, C₂H₅NCO, was first prepared by A. Wurtz (Ann. chim., 1854 (3), 42, p. 43) by distilling a mixture of potassium ethyl sulphate and potassium cyanate. It is a colourless liquid which boils at 60° C.

Cyanuric acid, H₃C₃N₃O₃, was obtained by Wöhler and Liebig by heating urea, and by A. Wurtz by passing chlorine into melting urea. It forms white efflorescent crystals. Treatment with phosphorus pentachloride gives cyanuric chloride, C₃N₃Cl₃, which is also formed by the combination of anhydrous chlorine and prussic acid in the presence of sunlight. These substances contain a ring of three carbon and three nitrogen atoms, i.e. they are symmetrical triazines.