Isocyanic acid

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Isocyanic acid
Isocyanic acid 3D balls.png
IUPAC name
Isocyanic acid
Other names
Carbimide [1]
3D model (JSmol)
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PubChem CID
  • InChI=1S/CHNO/c2-1-3/h2H Yes check.svgY
  • InChI=1/CHNO/c2-1-3/h2H
  • Isocyanic acid:O=C=N
  • Cyanic acid:OC#N
Molar mass 43.025 g·mol−1
AppearanceColorless liquid or gas (boiling point near room temperature)
Density 1.14 g/cm3 (20 °C)
Melting point −86 °C (−123 °F; 187 K) [2]
Boiling point 23.5 °C (74.3 °F; 296.6 K)
Solubility Soluble in benzene, toluene, diethyl ether
Conjugate acid Oxomethaniminium [3]
Conjugate base Cyanate
Occupational safety and health (OHS/OSH):
Main hazards
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as H−N=C=O. It is a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C. It is the predominant tautomer and an isomer of cyanic acid (aka. cyanol) (H−O−C≡N).


The derived anion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is [N=C=O], which 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 Justus von Liebig and Friedrich 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 (cyanol, 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 H−N=C=O, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which some say indicates a carbon–nitrogen triple bond. [8] [9] If so, then the canonical form H−N+≡C−O is the major resonance 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. [10] , as well as the NIST Chemistry WebBook, [11] 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.



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. [12] [13]


In aqueous solution it is a weak acid, having a pKa of 3.7: [14]


Isocyanic acid hydrolyses to carbon dioxide and ammonia:


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. [12]

Stability in solution

Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons. [15]


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. [16] 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. [17]

Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H−O−C≡N, also called hydrogen cyanate. [18] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents. [15] 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. [19]

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

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

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. [20]

See also

Related Research Articles

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

<span class="mw-page-title-main">Friedrich Wöhler</span> German chemist (1800–1882)

Friedrich Wöhler FRS(For) HonFRSE was a German chemist known for his work in both organic and inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the first to prepare several inorganic compounds, including silane and silicon nitride.

Carbon compounds are defined as chemical substances containing carbon. More compounds of carbon exist than any other chemical element except for hydrogen. Organic carbon compounds are far more numerous than inorganic carbon compounds. In general bonds of carbon with other elements are covalent bonds. Carbon is tetravalent but carbon free radicals and carbenes occur as short-lived intermediates. Ions of carbon are carbocations and carbanions are also short-lived. An important carbon property is catenation as the ability to form long carbon chains and rings.

In chemistry, a structural isomer of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metamer was formerly used for the same concept.

Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two amino groups joined by a carbonyl functional group. It is thus the simplest amide of carbamic acid.

<span class="mw-page-title-main">Isocyanate</span> Chemical group (–N=C=O)

In organic chemistry, isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.

<span class="mw-page-title-main">Mercury(II) fulminate</span> Chemical compound

Mercury(II) fulminate, or Hg(CNO)2, is a primary explosive. It is highly sensitive to friction, heat and shock and is mainly used as a trigger for other explosives in percussion caps and detonators. Mercury(II) cyanate, though its chemical formula is identical, has a different atomic arrangement; the cyanate and fulminate anions are isomers.

In chemistry, azide is a linear, polyatomic anion with the formula N−3 and structure N=N+=N. It is the conjugate base of hydrazoic acid HN3. Organic azides are organic compounds with the formula RN3, containing the azide functional group. The dominant application of azides is as a propellant in air bags.

In organic chemistry, a nitrile is any organic compound that has a −C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

<span class="mw-page-title-main">Fulminic acid</span> Chemical compound (H−C≡N−O)

Fulminic acid is an acid with the formula HCNO, more specifically H−C≡N+−O. It is an isomer of isocyanic acid and of its elusive tautomer, cyanic acid, and also of isofulminic acid.

<span class="mw-page-title-main">Tautomer</span> Structural isomers of chemical compounds that readily interconvert

Tautomers are structural isomers of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydrogen atom within the compound. The phenomenon of tautomerization is called tautomerism, also called desmotropism. Tautomerism is for example relevant to the behavior of amino acids and nucleic acids, two of the fundamental building blocks of life.

In chemistry, a trimer is a molecule or polyatomic anion formed by combination or association of three molecules or ions of the same substance. In technical jargon, a trimer is a kind of oligomer derived from three identical precursors often in competition with polymerization.

<span class="mw-page-title-main">Cyanuric acid</span> Chemical compound belonging to the class of triazine

Cyanuric acid or 1,3,5-triazine-2,4,6-triol is a chemical compound with the formula (CNOH)3. Like many industrially useful chemicals, this triazine has many synonyms. This white, odorless solid finds use as a precursor or a component of bleaches, disinfectants, and herbicides. In 1997, worldwide production was 160 000 tonnes.

<span class="mw-page-title-main">Fulminate</span> Chemical compounds containing an –O–N≡C group

Fulminates are chemical compounds which include the fulminate ion. The fulminate ion is a pseudohalic ion because its charge and reactivity are similar to those of the halogens. Due to the instability of the ion, fulminate salts are friction-sensitive explosives. The best known is mercury(II) fulminate, which has been used as a primary explosive in detonators. Fulminates can be formed from metals, such as silver and mercury, dissolved in nitric acid and reacted with ethanol. The weak single nitrogen-oxygen bond is responsible for their instability. Nitrogen very easily forms a stable triple bond to another nitrogen atom, forming nitrogen gas.

<span class="mw-page-title-main">Cyanate</span> Anion with formula OCN and charge –1

Cyanate is an anion with the structural formula [O=C=N], usually written OCN. It also refers to any salt containing it, such as ammonium cyanate.

Potassium cyanate is an inorganic compound with the formula KOCN. It is a colourless solid. It is used to prepare many other compounds including useful herbicide. Worldwide production of the potassium and sodium salts was 20,000 tons in 2006.

The Wöhler synthesis is the conversion of ammonium cyanate into urea. This chemical reaction was described in 1828 by Friedrich Wöhler. It is often cited as the starting point of modern organic chemistry. Although the Wöhler reaction concerns the conversion of ammonium cyanate, this salt appears only as an (unstable) intermediate. Wöhler demonstrated the reaction in his original publication with different sets of reactants: a combination of cyanic acid and ammonia, a combination of silver cyanate and ammonium chloride, a combination of lead cyanate and ammonia and finally from a combination of mercury cyanate and cyanatic ammonia.

Cyanogen bromide is the inorganic compound with the formula (CN)Br or BrCN. It is a colorless solid that is widely used to modify biopolymers, fragment proteins and peptides, and synthesize other compounds. The compound is classified as a pseudohalogen.

<span class="mw-page-title-main">Isomer</span> Chemical compounds with the same molecular formula but different atomic arrangements

In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.

<span class="mw-page-title-main">Silver cyanate</span> Chemical compound

Silver cyanate is the cyanate salt of silver. It can be made by the reaction of potassium cyanate with silver nitrate in aqueous solution, from which it precipitates as a solid.


  1. Cyanamide also has this name, and for which it is more systematically correct
  2. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN   0-07-049439-8
  3. "Oxomethaniminium | CH2NO | ChemSpider". Retrieved 27 January 2019.
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  13. Wells, p 722
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