Cyanic Acid

Potassium cyanide is formed, together with cyanide, when cyanogen reacts with potash:

(CN)₂ + 2KOH = KNCO + KCN + H₂O.

The same salt is produced by the oxidation of fused cyanide by the air or a reducible oxide such as red lead; also by heating potassium dichromate with anhydrous potassium ferrocyanide, and extracting the melt with ethyl alcohol containing a little methyl alcohol. Sodium cyanate is formed, together with other products, when sodium nitrite is heated with carbon. Cyanic acid cannot be isolated by liberation from potassium cyanate, because it is unstable in aqueous solution, taking up the elements of water to form ammonium hydrogen carbonate: CONH + 2H₂O = NH₄HCO₃. Consequently a cyanate effervesces with dilute hydrochloric acid like a carbonate.

Cyanic acid may, however, be obtained by heating cyanuric acid in. a current of carbon dioxide and condensing the evolved vapour in a freezing mixture. Thus obtained, at low temperature, cyanic acid is found to be a colourless, volatile liquid, which has a density of 1.1558 at -20° C. and 1.140 at 0° C. It has a pungent smell resembling that of acetic acid. Its vapour density at 440° C. is 1.50 (air = l), which supports the monomolecular formula, CONH. The heat of formation of cyanic acid from its elements (C as diamond) is.about 37,000 calories. At atmospheric temperature cyanic acid passes, with evolution of heat, into the solid polymer cyamelide, whence cyanic acid is recovered by vaporisation. Aqueous cyanic acid is subject to three types of change:

1. Polymerisation of the non-ionised acid in concentrated solution: 3HCNO = (HCNO)₃.
2. Decomposition in presence of an acid:
   
   CNO' + H₂O + 2H^• = NH₄^• + CO₂.
3. Spontaneous decomposition of dilute solution:
   
   2H^• + 2CNO' + H₂O = CO(NH₂)₂ + CO₂.

Since the isomeric fulminic acid is C=N-OH, there remain two possible formulae for cyanic acid: N≡C-OH and O=C=NH, the former of which might be called normal and the latter iso-cyanic acid. The normal acid appears, however, not to exist either in the free state or in the form of its salts or esters. Cyanic acid and the cyanates, therefore, are iso-compounds, and are indeed carbonimide and its derivatives. Nevertheless the name cyanic, rather than isocyanic, acid is usually employed for this acid. Michael and Hibbert support the conclusion that the cyanic acid both in the state of vapour and in solution is carbonimide, CO:NH.

The relation of cyanic acid, CONH, to its two polymers, cyanuric acid, (CONH)₃, and cyamelide, (CONH)_n, is an interesting one, and forms a valuable subject for study from the standpoint of the phase rule. The relationship between the vapour of cyanic acid and its two solid forms has been investigated by van't Hoff, who has found that the triple point at which these three phases are in equilibrium is at 150° C. under 50 mm. pressure. As previously stated, it is only when cyanic acid vapour is rapidly cooled in a freezing mixture that the unstable liquid cyanic acid is produced.

Cyanic acid forms an additive compound with hydrochloric acid, and is reduced by nascent hydrogen to formamide, HCONH₂.

Ammonium cyanate is an interesting salt, because Wohler in 1828 observed its transformation into urea, and thus showed that an "organic" compound could be obtained from an inorganic source without the intervention of "vital force." If a solution of potassium cyanate, a salt obtained by the oxidation of potassium cyanide by fusion with red lead, is evaporated to dryness with an equivalent quantity of ammonium sulphate, urea is formed from the resulting ammonium cyanate, and may be extracted with alcohol from the residue, leaving potassium sulphate. The reaction, which is bi-molecular, since it takes place between cyanate and ammonium ions, may be represented thus:

CON' + NH₄^• ⇔ CO(NH₂)₂.

The dynamics of this reaction has been studied by Walker, Walker and Appleyard, Fawsitt, and Walker and Kay. The transformation of ammonium eyanate, in the state of its ions, into urea is accompanied by a heat evolution of 5000 calories per gram molecule.

By the action of hydrochloric acid urea undergoes the following reaction:

CO(NH₂)₂ + 2HCl + H₂O = 2NH₄Cl + CO₂, which is made up of these stages:

1. CO(NH₂)₂ ⇔ CON(NH₄)
2. CON(NH₄) + HCl = CONH + NH₄Cl
3. CONH + H₂O + HCl = CO₂ + NH₄Cl.

Metallic cyanates are hydrolysed by hot water into carbonates and urea according to the reaction:

M(CNO)₂ + 2H₂O = MCO₃ + CO(NH₂)₂.

The reaction takes place in two stages: (i) a slow hydrolysis of CNO' ions yielding carbonate and NH₄^• ions, (ii) interaction of NH₄^• and CNO' to form urea. The cyanates of sodium and potassium are hydrolysed by water according to the equation:

4MCNO + 6H₂O = 2M₂CO₃ + (NH₄)₂CO₃ + CO(NH₂)₂.

Bromine reacts with potassium cyanate according to the equation:

4KCNO + 4H₂O + 3Br₂ = 4KBr + 2NH₄Br + N₂ + 4CO₂.

Silver, lead, mercurous, and cupric cyanates are white precipitates practically insoluble in water.