Isocyanic acid
Isocyanic acid
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Isocyanic acid

Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as H−N=C=O. It is a colourless, volatile and poisonous gas, condensing at 23.5 °C. It is the predominant tautomer and an isomer of cyanic acid (aka. cyanol) (H−O−C≡N), and the monomer of cyanuric acid.

The derived anion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is [N=C=O], which is called cyanate. The related functional group −N=C=O is isocyanate; it is distinct from cyanate (−O−C≡N), fulminate (−O−N+≡C), and nitrile oxide (−C≡N+−O).

Isocyanic acid was discovered in 1830 by Justus von Liebig and Friedrich Wöhler.

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. It is the only fairly stable one of the four linear isomers with molecular formula HOCN that have been synthesized, the others being cyanic acid (cyanol, H−O−C≡N) and the elusive fulminic acid (H−C≡N+−O) and isofulminic acid H−O−N+≡C.

Although the electronic structure according to valence bond theory can be written as H−N=C=O, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which some say indicates a carbon–nitrogen triple bond. If so, then the canonical form H−N+≡C−O is the major resonance structure.

However, classic vibrational analysis would indicate that the 2268.8 cm−1 is the asymmetric N=C=O stretch, as per Colthup et al., as well as the NIST Chemistry WebBook, which also reports the corresponding symmetric N=C=O stretch (weak in infrared, but strong in Raman) to be 1327 cm−1. Based on these classic assignments, there is no need to invoke a full charged state for the N and O atoms, to explain the vibrational spectral data.

The tautomer, known as cyanic acid, HOCN, in which the oxygen atom is protonated exists in equilibrium with isocyanic acid to the extent of about 3%.[citation needed][dubiousdiscuss] The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.

In aqueous solution it is a weak acid, having a pKa of 3.7:

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