Isocyanic acid

Structure

Isocyanic acid (HNCO)

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 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.,{{cite book as well as the NIST Chemistry WebBook, | access-date = 2023-04-20 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.

Cyanic acid (HOCN)

cyanicacid2025-03-05The 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%. The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.

Properties

In aqueous solution it is a weak acid, having a pKa of 3.7: : Isocyanic acid hydrolyses to carbon dioxide and ammonia: : Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.

At high concentrations, isocyanic acid oligomerizes to give the trimer cyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products.

Isocyanic acid reacts with amines to give ureas (carbamides): : This reaction is called carbamylation. Excess isocyanic acid can react with the ureas to give allophanates.

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

Isocyanic acid, HNCO, is a Lewis acid whose free energy, enthalpy and entropy changes for its 1:1 association with a number of bases in carbon tetrachloride solution at 25 °C have been reported. The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.

Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid , also called hydrogen cyanate. Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents. Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.

Preparation

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. : HNCO also can be made by the high-temperature thermal decomposition of the trimer cyanuric acid: : In all cases the resulting acid must be kept very cold or in gaseous form, as concentrated isocyanic acid polymerizes rapidly above −20 °C.

In the reverse of the famous synthesis of urea by Friedrich Wöhler, : isocyanic acid is produced and rapidly trimerizes to cyanuric acid.

Occurrence

Isocyanic acid has been detected in many kinds of interstellar environments.

Isocyanic acid is also present in various forms of smoke, including smog and cigarette smoke. It was detected using mass spectrometry, and easily dissolves in water, posing a health risk to the lungs.

References

References

1. [[Cyanamide]] also has this name, and for which it is more systematically correct
2. Pradyot Patnaik. ''Handbook of Inorganic Chemicals''. McGraw-Hill, 2002, {{ISBN. 0-07-049439-8
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4. Liebig, J.. (1830). "Untersuchungen über die Cyansäuren". Annalen der Physik.
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7. (2010). "Gas-Grain Modeling of Isocyanic Acid (Hnco), Cyanic Acid (Hocn), Fulminic Acid (Hcno), and Isofulminic Acid (Honc) in Assorted Interstellar Environments". The Astrophysical Journal.
8. Nakamoto, part A, p 190
9. (1989). "The CHNO Isomers". Chemische Berichte.
10. Pettit, Gwyneth; Pettit, Leslie. [http://www.acadsoft.co.uk/scdbase/scdbase.htm "SC-Database" (proprietary database)]. Timble, Yorks: Academic Software — via proprietary executable.
11. (2001). "Encyclopedia of Reagents for Organic Synthesis".
12. Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
13. Jacox, M.E.. (1964). "Low-Temperature Infrared Study of Intermediates in the Photolysis of HNCO and DNCO". Journal of Chemical Physics.
14. (2002). "Synthesis, Properties and Dimerization Study of Isocyanic Acid". Z. Naturforsch..
15. Preidt, Robert. "Chemical in Smoke May Pose Health Risk". AccuWeather.