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Isocyanic acid

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Title: Isocyanic acid  
Author: World Heritage Encyclopedia
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Subject: Hydrogen cyanide, Hydrogen fluoride, Hydrogen chloride, Hydrogen peroxide, Methanol
Collection: Acids, Cyanates, Hydrogen Compounds, Inorganic Carbon Compounds, Isocyanates
Publisher: World Heritage Encyclopedia

Isocyanic acid

Isocyanic acid
IUPAC name
Isocyanic acid
(cyanic acid) N
ChemSpider  Y
Jmol-3D images Image
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)[1]
Boiling point 23.5 °C (74.3 °F; 296.7 K)
Solubility Soluble in benzene, toluene, ether
Main hazards Poisonous
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 N  (: Y/N?)

Isocyanic acid is an organic chemistry and biology.


  • Preparation and reactions 1
  • Isomers: Cyanic acid and fulminic acid 2
  • See also 3
  • References 4
  • Further reading 5
  • External links 6

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.[3]

H+ + NCO- → HNCO

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

C3H3N3O3 → 3 HNCO

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.[4]

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 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.[5]

Isomers: Cyanic acid and fulminic acid

Low-temperature photolysis of solids containing HNCO has been shown to make H-O-C≡N, known as cyanic acid or hydrogen cyanate; it is a tautomer of isocyanic acid.[6] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[4] 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.[7]

See also


  1. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  2. ^ Liebig, J.; Wöhler, F. (1830). "Untersuchungen über die Cyansäuren". Ann. Phys. 20 (11): 394.  
  3. ^ Fischer, G.; Geith, J.; Klapötke, T. M.; Krumm B. (2002). "Synthesis, Properties and Dimerization Study of Isocyanic Acid" (PDF). Z. Naturforschung 57b (1): 19–25. 
  4. ^ 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.
  5. ^ Preidt, Robert. "Chemical in Smoke May Pose Health Risk". MyOptumHealth. AccuWeather. Retrieved 14 September 2011. 
  6. ^ 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.  
  7. ^ Kurzer, Frederick (2000). "Fulminic Acid in the History of Organic Chemistry". Journal of Chemical Education 77 (7): 851–857.  

Further reading

  • Handbook of Chemistry and Physics, 65th. Edition, CRC Press (1984)

External links

  • Walter, Wolfgang (1997). Organic Chemistry: A Comprehensive Degree Text and Source Book. Chichester: Albion Publishing. p. 364.  
  • Cyanic acid from NIST Chemistry WebBook (accessed 2006-09-09)
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