Isocyanic acid

Isocyanic acid

Names
IUPAC name Isocyanic acid
Identifiers
CAS Number	75-13-8 Y 420-05-3 (cyanic acid) N
ChEBI	CHEBI:29202 Y
ChemSpider	6107 Y
Jmol 3D model	Interactive image
PubChem	6347
InChI InChI=1S/CHNO/c2-1-3/h2H Y Key: OWIKHYCFFJSOEH-UHFFFAOYSA-N Y InChI=1/CHNO/c2-1-3/h2H Key: OWIKHYCFFJSOEH-UHFFFAOYAE
SMILES O=C=[N@H]
Properties
Chemical formula	HNCO
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)
Boiling point	23.5 °C (74.3 °F; 296.6 K)
Solubility in water	Dissolves
Solubility	Soluble in benzene, toluene, ether
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Isocyanic acid is an organic compound with the formula HNCO, discovered in 1830 by Liebig and Wöhler. This colourless substance is volatile and poisonous, with a boiling point of 23.5 °C. 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.

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.

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.

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

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 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.

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. Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents. 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.

See also

• Cyanate
• Thiocyanic acid

Further reading

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

External links

• Cyanic acid from NIST Chemistry WebBook (accessed 2006-09-09)