Isocyanic acid

Isocyanic acid is an inorganic compound with the formula HNCO, discovered in 1830 by Liebig and Wöhler. This colourless substance is volatile and poisonous, with a boiling point of 23.5 °C. Isocyanic acid is the simplest stable chemical compound that contains carbon, hydrogen, nitrogen, and oxygen, the four most commonly-found elements in organic chemistry and biology.

Preparation and reactions
Isocyanic acid can be made by protonation of the cyanate anion, such as from salts like potassium cyanate, by either gaseous hydrogen chloride or acids such as oxalic acid.


 * H+ +  NCO-  →  HNCO

HNCO also can be made by the high-temperature thermal decomposition of cyanuric acid, a trimer.

Isocyanic acid hydrolyses to carbon dioxide and ammonia:


 * HNCO + H2O   →   CO2  +   NH3

At sufficiently high concentrations, isocyanic acid oligomerizes to give cyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products. Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.

Isocyanic acid reacts with amines to give ureas:
 * HNCO +  RNH2   →  RNHC(O)NH2.

HNCO adds across electron-rich double bonds, such as vinylethers, to give the corresponding isocyanates.

Isomers
Low-temperature photolysis of solids containing HNCO has been shown to make H-O-CN, cyanic acid, which is a tautomer of isocyanic acid. Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents. Note that sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.

Cyanic and isocyanic acids are isomers of fulminic acid (H-C=N-O), an unstable compound.