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| Names | |
|---|---|
| IUPAC name Isocyanic acid | |
| Other names Carbimide[1] Carbonic imide | |
| Identifiers | |
3D model (JSmol) |
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| ChEBI | |
| ChemSpider |
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| ECHA InfoCard | 100.109.068 |
| UNII |
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| Properties | |
| HNCO | |
| Molar mass | 43.025 g·mol−1 |
| Appearance | Colorless gas (condenses near room temperature) |
| Density | 1.14 g/cm3[2] |
| Melting point | |
| Boiling point | 23.5 °C (74.3 °F; 296.6 K) (extrapolated)[2] |
| Dissolves; polymerizes if concentrated | |
| Solubility | Soluble inbenzene,toluene,diethyl ether |
| Acidity (pKa) | 3.7[4] |
| Conjugate acid | Oxomethaniminium[5] |
| Conjugate base | Cyanate |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Poisonous |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Isocyanic acid is achemical compound with thestructural formula HNCO, which is often written asH−N=C=O. It is a colourless,volatile andpoisonous gas,condensing at 23.5 °C. It is the predominanttautomer and anisomer of cyanic acid(aka. cyanol) (H−O−C≡N), and the monomer ofcyanuric acid.
The derivedanion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is[N=C=O]−, which is calledcyanate. The relatedfunctional group−N=C=O isisocyanate; it is distinct fromcyanate (−O−C≡N),fulminate (−O−N+≡C−), andnitrile oxide (−C≡N+−O−).[6]
Isocyanic acid was discovered in 1830 byJustus von Liebig andFriedrich Wöhler.[7]
Isocyanic acid is the simplest stablechemical compound that containscarbon,hydrogen,nitrogen, andoxygen, thefour most commonly found elements inorganic chemistry and biology. It is the only fairly stable one of the four linear isomers withmolecular formula HOCN that have been synthesized, the others being cyanic acid (cyanol,H−O−C≡N) and the elusivefulminic acid (H−C≡N+−O−)[8] andisofulminic acidH−O−N+≡C−.[6][9]
Although the electronic structure according tovalence bond theory can be written as H−N=C=O, thevibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which some say indicates a carbon–nitrogen triple bond.[10][11] If so, then the canonical formH−N+≡C−O− is the majorresonance 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.,[12] as well as the NIST Chemistry WebBook,[13] 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.
Thetautomer, known as cyanic acid, HOCN, in which the oxygen atom is protonated exists in equilibrium with isocyanic acid to the extent of about 3%.[citation needed][dubious –discuss] The vibrational spectrum is indicative of the presence of atriple bond between the nitrogen and carbon atoms.[11]
In aqueous solution it is aweak acid, having apKa of 3.7:[4]
Isocyanic acidhydrolyses tocarbon dioxide andammonia:
Dilute solutions of isocyanic acid are stable in inertsolvents, e.g.ether andchlorinated hydrocarbons.[2]
At high concentrations, isocyanic acid oligomerizes to give thetrimercyanuric acid andcyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products.[2]
Isocyanic acid reacts withamines to giveureas (carbamides):
This reaction is calledcarbamylation. Excess isocyanic acid can react with the ureas to giveallophanates.[2]
HNCO adds across electron-rich double bonds, such asvinylethers, to give the corresponding isocyanates.
Isocyanic acid, HNCO, is aLewis acid whosefree energy,enthalpy andentropy changes for its 1:1 association with a number of bases incarbon tetrachloride solution at 25 °C have been reported.[14] The acceptor properties of HNCO are compared with other Lewis acid in theECW model.
Low-temperaturephotolysis of solids containing HNCO creates the tautomer cyanic acidH−O−C≡N, also called hydrogen cyanate.[15] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[2] Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.[citation needed]
Isocyanic acid can be made by protonation of thecyanate anion, such as from salts likepotassium cyanate, by either gaseoushydrogen chloride or acids such asoxalic acid.[16]
HNCO also can be made by the high-temperature thermal decomposition of the trimercyanuric 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.[2]
In the reverse of the famous synthesis ofurea byFriedrich Wöhler,
isocyanic acid is produced and rapidly trimerizes to cyanuric acid.
Isocyanic acid has been detected in many kinds ofinterstellar environments.[9]
Isocyanic acid is also present in various forms ofsmoke, includingsmog andcigarette smoke. It was detected usingmass spectrometry, and easilydissolves in water, posing a health risk to the lungs.[17]
