Isocyanic acid

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Isocyanic acid
Isocyansaure.svg
Isocyanic acid 3D balls.png
Names
IUPAC name
Isocyanic acid
Other names
Carbimide [1]
Carbonic imide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.109.068 OOjs UI icon edit-ltr-progressive.svg
PubChem CID
UNII
  • InChI=1S/CHNO/c2-1-3/h2H Yes check.svgY
    Key: OWIKHYCFFJSOEH-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/CHNO/c2-1-3/h2H
    Key: OWIKHYCFFJSOEH-UHFFFAOYAE
  • Isocyanic acid:O=C=N
  • Cyanic acid:OC#N
Properties
HNCO
Molar mass 43.025 g·mol−1
AppearanceColorless gas (condenses near room temperature)
Density 1.14 g/cm3 [2]
Melting point
  • −86 °C (−123 °F; 187 K) [3]
  • Polymerizes to solid −20 °C (−4 °F; 253 K) [2]
Boiling point 23.5 °C (74.3 °F; 296.6 K)(extrapolated) [2]
Dissolves; polymerizes if concentrated
Solubility Soluble in benzene, 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 their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

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 gas, condensing at 23.5 °C. It is the predominant tautomer and an isomer of cyanic acid (aka. cyanol) (H−O−C≡N), and the monomer of cyanuric acid.

Contents

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−). [6]

Isocyanic acid was discovered in 1830 by Justus von Liebig and Friedrich Wöhler. [7]

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−) [8] and isofulminic acid H−O−N+≡C−. [6] [9]

Structure

Isocyanic acid (HNCO)

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. [10] [11] 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., [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.

Cyanic acid (HOCN)

The tautomer, 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 a triple bond between the nitrogen and carbon atoms. [11]

Properties

In aqueous solution it is a weak acid, having a pKa of 3.7: [4]

HNCO ⇌ H+ + NCO−

Isocyanic acid hydrolyses to carbon dioxide and ammonia:

HNCO + H2O → CO2 + NH3

Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons. [2]

At 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. [2]

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

HNCO + RNH2 → RNHC(O)NH2

This reaction is called carbamylation. Excess isocyanic acid can react with the ureas to give allophanates. [2]

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. [14] The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.

Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H−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 ]

Preparation

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. [16]

H+ + NCO− → HNCO

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

C3H3N3O3 → 3 HNCO

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 of urea by Friedrich Wöhler,

CO(NH2)2 → HNCO + NH3

isocyanic acid is produced and rapidly trimerizes to cyanuric acid.

Occurrence

Isocyanic acid has been detected in many kinds of interstellar environments. [9]

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. [17]

See also

Related Research Articles

<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.

<span class="mw-page-title-main">Organic compound</span> Carbon-containing chemical compound

Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-containing compounds such as alkanes and its derivatives are universally considered organic, but many others are sometimes considered inorganic, such as halides of carbon without carbon-hydrogen and carbon-carbon bonds, and certain compounds of carbon with nitrogen and oxygen.

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 called 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.

An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠‍—‍that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as inorganic chemistry.

<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> Isomers of chemical compounds that interconvert

In chemistry, 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">Cyanate</span> Anion with formula OCN and charge –1

The cyanate ion is an anion with the chemical formula OCN−. It is a resonance of three forms: [O−−C≡N] (61%) ↔ [O=C=N−] (30%) ↔ [O+≡C−N2−] (4%).

<span class="mw-page-title-main">CHNOPS</span> Acronym of the most common elements found in biological life

CHNOPS and CHON are mnemonic acronyms for the most common elements in living organisms. "CHON" stands for carbon, hydrogen, oxygen, and nitrogen, which together make up more than 95 percent of the mass of biological systems. "CHNOPS" adds phosphorus and sulfur.

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

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, the same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism refers to the existence or possibility of isomers.

Sodium cyanate is the inorganic compound with the formula NaOCN. A white solid, it is the sodium salt of the cyanate anion.

<span class="mw-page-title-main">Ammonium cyanate</span> Ionic chemical compound with formula [NH4]+ [OCN]-

Ammoniumcyanate is an inorganic compound with the formula [NH4]+[OCN]−. It is a colorless, solid salt.

<span class="mw-page-title-main">Sulfoxylic acid</span> Chemical compound

Sulfoxylic acid (H2SO2) (also known as hyposulfurous acid or sulfur dihydroxide) is an unstable oxoacid of sulfur in an intermediate oxidation state between hydrogen sulfide and dithionous acid. It consists of two hydroxy groups attached to a sulfur atom. Sulfoxylic acid contains sulfur in an oxidation state of +2. Sulfur monoxide (SO) can be considered as a theoretical anhydride for sulfoxylic acid, but it is not actually known to react with water.

The phosphaethynolate anion, also referred to as PCO, is the phosphorus-containing analogue of the cyanate anion with the chemical formula [PCO]− or [OCP]−. The anion has a linear geometry and is commonly isolated as a salt. When used as a ligand, the phosphaethynolate anion is ambidentate in nature meaning it forms complexes by coordinating via either the phosphorus or oxygen atoms. This versatile character of the anion has allowed it to be incorporated into many transition metal and actinide complexes but now the focus of the research around phosphaethynolate has turned to utilising the anion as a synthetic building block to organophosphanes.

References

  1. ↑ Cyanamide also has this name, and for which it is more systematically correct
  2. 1 2 3 4 5 6 7 8 Narula, Acharan S.; Ramachandran, Kishore (2001). "Isocyanic Acid". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.ri072m. ISBN   0-471-93623-5.
  3. ↑ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN   0-07-049439-8
  4. 1 2 Pettit, Gwyneth; Pettit, Leslie. "SC-Database" (proprietary database). Timble, Yorks: Academic Software — via proprietary executable.
  5. ↑ "Oxomethaniminium | CH2NO | ChemSpider". www.chemspider.com. Retrieved 27 January 2019.
  6. 1 2 Martin, William R.; Ball, David W. (2019). "Small organic fulminates as high energy materials. Fulminates of acetylene, ethylene, and allene". Journal of Energetic Materials. 37 (1): 70–79. Bibcode:2019JEnM...37...70M. doi:10.1080/07370652.2018.1531089.
  7. ↑ Liebig, J.; Wöhler, F. (1830). "Untersuchungen über die Cyansäuren". Annalen der Physik. 20 (11): 394. Bibcode:1830AnP....96..369L. doi:10.1002/andp.18300961102.
  8. ↑ Kurzer, Frederick (2000). "Fulminic Acid in the History of Organic Chemistry". Journal of Chemical Education. 77 (7): 851–857. Bibcode:2000JChEd..77..851K. doi:10.1021/ed077p851.
  9. 1 2 Quan, Donghui; Herbst, Eric; Osamura, Yoshihiro; Roueff, Evelyne (2010). "Gas-Grain Modeling of Isocyanic Acid (Hnco), Cyanic Acid (Hocn), Fulminic Acid (Hcno), and Isofulminic Acid (Honc) in Assorted Interstellar Environments". The Astrophysical Journal. 725 (2): 2101–2109. Bibcode:2010ApJ...725.2101Q. doi:10.1088/0004-637X/725/2/2101. hdl: 1811/47796 .
  10. ↑ Nakamoto, part A, p 190
  11. 1 2 Teles, Joaquim Henrique; Maier, Günther; Andes Hess, B.; Schaad, Lawrence J.; Winnewisser, Manfred; Winnewisser, Brenda P. (1989). "The CHNO Isomers". Chemische Berichte. 122 (4): 753–766. doi:10.1002/cber.19891220425.
  12. ↑ Colthup, Norman B.; Daly, Lawrence H.; Wiberley, Stephen E. (1990). Introduction to Infrared and Raman Spectroscopy. Academic Press (Elsevier). ISBN   978-0-12-182554-6.
  13. ↑ "Isocyanic acid". National Institute of Standards and Technology (U.S. Department of Commerce). Retrieved 2023-04-20.
  14. ↑ Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
  15. ↑ 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. Bibcode:1964JChPh..40.2457J. doi:10.1063/1.1725546.
  16. ↑ Fischer, G.; Geith, J.; Klapötke, T. M.; Krumm B. (2002). "Synthesis, Properties and Dimerization Study of Isocyanic Acid" (PDF). Z. Naturforsch. 57b (1): 19–25. doi:10.1515/znb-2002-0103. S2CID   37461221.
  17. ↑ Preidt, Robert. "Chemical in Smoke May Pose Health Risk". MyOptumHealth. AccuWeather. Retrieved 14 September 2011.
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