Isocyanic acid

List of interstellar and circumstellar molecules Polyyne Doi (identifier)
Isocyanic acid
Isocyanic acid 3D balls.png
IUPAC name
Isocyanic acid
Other names
3D model (JSmol)
ECHA InfoCard 100.109.068
Molar mass 43.03 g/mol
Appearance Colorless liquid or gas (b.p. near room temperature)
Density 1.14 g/cm3 (20 °C)
Melting point −86 °C (−123 °F; 187 K)[3]
Boiling point 23.5 °C (74.3 °F; 296.6 K)
Solubility Soluble in benzene, toluene, ether
Conjugate acid Oxomethaniminium[2]
Conjugate base Cyanate
Main hazards Poisonous
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Isocyanic acid is a chemical compound with the formula HNCO or H–N=C=O. It is a colourless substance, volatile and poisonous, with a boiling point of 23.5 °C. It is the predominant tautomer of cyanic acid H–O–C≡N.

The derived anion [N=C=O]
is the same as that of cyanic acid, and 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.[4]

Isocyanic acid was discovered in 1830 by Liebig and Wöhler.[5]

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 H–O–C≡N and the elusive fulminic acid H–C≡N+–O[6] and isofulminic acid H–O–N+≡C.[4][7]


Although the electronic structure according to valence bond theory can be written as HN=C=O, as illustrated, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which clearly requires the C≡N bond to be a triple bond.[8][9] In valence bond theory the canonical form H+N≡C−O is the major contributor to the resonance hybrid.



The pure compound has a melting point of −86.8 °C and a boiling point of 23.5 °C, so it is volatile at ambient temperatures.[10][11]


In aqueous solution it is a weak acid.

HNCO ⇌ H+ + NCO;      pKa ≈ 3.7.[12]


Isocyanic acid hydrolyses to carbon dioxide and ammonia:

HNCO + H2O → CO2 + NH3


At sufficiently 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. Cyanuric acid itself decomposes on further heating back to isocyanic acid.[10]

Stability in solution

Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.[13]


Isocyanic acid reacts with amines to give ureas (carbamides):


This reaction is called carbamylation.

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.[14] The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.


The tautomer, known as cyanic acid, HOCN, in which the oxygen atom is protonated, is unstable to decomposition, but in solution it is present 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.[15]

Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H-O-C≡N, also called hydrogen cyanate.[16] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[13] Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.[citation needed]


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.[17]

H+ + NCO- → HNCO

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

C3H3N3O3 → 3 HNCO

In the reverse of the famous synthesis of urea by Friedrich Wöhler,

OC(NH2)2 → HNCO + NH3

isocyanic acid is produced and rapidly trimerizes to cyanuric acid.


Isocyanic acid has been detected in many kinds of interstellar environments.[7]

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.[18]

See also


  1. ^ Cyanamide also has this name, and for which it is more systematically correct
  2. ^ "Oxomethaniminium | CH2NO | ChemSpider". www.chemspider.com. Retrieved 27 January 2019.
  3. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  4. ^ a b William R. Martin and David W. Ball (2019): "Small organic fulminates as high energy materials. Fulminates of acetylene, ethylene, and allene". Journal of Energetic Materials, volume 31, issue 7, pages 70-79. doi:10.1080/07370652.2018.1531089
  5. ^ Liebig, J.; Wöhler, F. (1830). "Untersuchungen über die Cyansäuren". Ann. Phys. 20 (11): 394. Bibcode:1830AnP....96..369L. doi:10.1002/andp.18300961102.
  6. ^ Kurzer, Frederick (2000). "Fulminic Acid in the History of Organic Chemistry". Journal of Chemical Education. 77 (7): 851–857. Bibcode:2000JChEd..77..851K. doi:10.1021/ed077p851.
  7. ^ a b Donghui Quan, Eric Herbst, Yoshihiro Osamura, and Evelyne Roueff (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, volume 725, issue 2, pages doi:10.1088/0004-637X/725/2/2101
  8. ^ Nakamoto, part A, p 190
  9. ^ "The CHNO Isomers". Chemische Berichte. 122: 753–766. doi:10.1002/cber.19891220425.
  10. ^ a b Greenwood, p323
  11. ^ Wells, p 722
  12. ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  13. ^ a b A. S. Narula, K. Ramachandran “Isocyanic Acid” in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, New York. doi:10.1002/047084289X.ri072m Article Online Posting Date: April 15, 2001.
  14. ^ Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
  15. ^ Teles, Joaquim Henrique; Maier, Günther; Andes Hess, B.; Schaad, Lawrence J.; Winnewisser, Manfred; Winnewisser, Brenda P. (1989). "The CHNO Isomers". Chem. Ber. 122 (4): 1099–0682. doi:10.1002/cber.19891220425.
  16. ^ Jacox, M.E.; Milligan, D.E. (1964). "Low-Temperature Infrared Study of Intermediates in the Photolysis of HNCO and DNCO". Journal of Chemical Physics. 40 (9): 2457–2460. Bibcode:1964JChPh..40.2457J. doi:10.1063/1.1725546.
  17. ^ Fischer, G.; Geith, J.; Klapötke, T. M.; Krumm B. (2002). "Synthesis, Properties and Dimerization Study of Isocyanic Acid" (PDF). Z. Naturforsch. 57b (1): 19–25.
  18. ^ Preidt, Robert. "Chemical in Smoke May Pose Health Risk". MyOptumHealth. AccuWeather. Retrieved 14 September 2011.