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|Molar mass||43.025 g·mol−1|
|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)|
|Boiling point||23.5 °C (74.3 °F; 296.6 K)|
|Solubility||Soluble in benzene, toluene, ether|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as 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−.
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− and isofulminic acid H–O–N+≡C−.
Although the electronic structure according to valence bond theory can be written as HN=C=O, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which clearly indicates a carbon–nitrogen triple bond. Thus the canonical form H≡C−O− is the major resonance structure.
- HNCO ⇌ H+ + NCO−
- 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.
Stability in solution
- HNCO + RNH2 → RNHC(O)NH2.
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. 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.
Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H-O-C≡N, 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.
- H+ + NCO- → HNCO
HNCO also can be made by the high-temperature thermal decomposition of the trimer cyanuric acid:
- C3H3N3O3 → 3 HNCO
- OC(NH2)2 → HNCO + NH3
isocyanic acid is produced and rapidly trimerizes to cyanuric acid.
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.
- Cyanamide also has this name, and for which it is more systematically correct
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