Isocyanic acid#Tautomerism

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.Nakamoto, part A, p 190{{cite journal | doi = 10.1002/cber.19891220425 | volume=122 | title=The CHNO Isomers | journal=Chemische Berichte | year=1989 | pages=753–766| last1=Teles | first1=Joaquim Henrique | last2=Maier | first2=Günther | last3=Andes Hess | first3=B. | last4=Schaad | first4=Lawrence J. | last5=Winnewisser | first5=Manfred | last6=Winnewisser | first6=Brenda P. | issue=4 }} If so, then the canonical form {{chem2|H\sN+\tC\sO−}} 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.,{{cite book

| last1 = Colthup

| first1 = Norman B.

| last2 = Daly

| first2 = Lawrence H.

| last3 = Wiberley

| first3 = Stephen E.

| date = 1990

| title = Introduction to Infrared and Raman Spectroscopy

| url = https://www.sciencedirect.com/book/9780121825546/introduction-to-infrared-and-raman-spectroscopy

| publisher = Academic Press (Elsevier)

| page =

| isbn = 978-0-12-182554-6

}}

as well as the NIST Chemistry WebBook,

{{cite web

| title = Isocyanic acid

| url = https://webbook.nist.gov/cgi/cbook.cgi?ID=C75138&Mask=800#Electronic-Spec

| publisher = National Institute of Standards and Technology (U.S. Department of Commerce)

| access-date = 2023-04-20

}}

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.

{{Anchor|cyanicacid2025-03-05}}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|date=January 2024}}{{dubious|reason=Teles, Maier, Andes Hess et al (1989) states that this is a common misconception.|date=January 2024}} The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.

In aqueous solution it is a weak acid, having a pK_a of 3.7:Pettit, Gwyneth; Pettit, Leslie. [http://www.acadsoft.co.uk/scdbase/scdbase.htm "SC-Database" (proprietary database)]. Timble, Yorks: Academic Software — via proprietary executable.

:{{chem2|HNCO <-> H+ + NCO-}}

Isocyanic acid hydrolyses to carbon dioxide and ammonia:

:{{chem2|HNCO + H2O -> CO2 + NH3}}

Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.{{cite book | doi = 10.1002/047084289X.ri072m | chapter = Isocyanic Acid | title = Encyclopedia of Reagents for Organic Synthesis | date = 2001 | last1 = Narula | first1 = Acharan S. | last2 = Ramachandran | first2 = Kishore | isbn = 0-471-93623-5 }}

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.

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

:{{chem2|HNCO + RNH2 -> RNHC(O)NH2}}

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

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.Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120. 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 {{chem2|H\sO\sC\tN}}, also called hydrogen cyanate.{{cite journal | author = Jacox, M.E. |author2=Milligan, D.E. | title = Low-Temperature Infrared Study of Intermediates in the Photolysis of HNCO and DNCO | journal = Journal of Chemical Physics | year = 1964 | volume = 40 | issue = 9 | pages = 2457–2460 | doi = 10.1063/1.1725546|bibcode = 1964JChPh..40.2457J }} 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.{{Citation needed|reason=No source has been provided to suggest that this is true.|date=July 2017}}

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.{{cite journal | title = Synthesis, Properties and Dimerization Study of Isocyanic Acid | author1 = Fischer, G. | author2 = Geith, J. | author3 = Klapötke, T. M. | author4 = Krumm B. | journal = Z. Naturforsch. | year = 2002 | volume = 57b | issue = 1 | pages = 19–25 | doi = 10.1515/znb-2002-0103 | s2cid = 37461221 | url = http://www.znaturforsch.com/ab/v57b/s57b0019.pdf}}

:{{chem2 | H+ + NCO- -> HNCO }}

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

:{{chem2 | 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.

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

:{{chem2 | CO(NH2)2 -> HNCO + NH3 }}

isocyanic acid is produced and rapidly trimerizes to cyanuric acid.

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

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.{{cite web|last=Preidt|first=Robert|title=Chemical in Smoke May Pose Health Risk|url=http://www.accuweather.com/blogs/health/respiratory/50653/chemical-in-smoke-may-pose-hea.asp|work=MyOptumHealth|publisher=AccuWeather|access-date=14 September 2011}}

• Cyanate
• Thiocyanic acid

{{reflist}}

• {{cite book | author = Walter, Wolfgang | title = Organic Chemistry: A Comprehensive Degree Text and Source Book | year = 1997 | publisher = Albion Publishing | location = Chichester | url = https://books.google.com/books?id=RXX2RaCplmAC&q=isocyanic+acid+melting+point&pg=PA364 | access-date = 2008-06-21 | page = 364 | isbn = 978-1-898563-37-2}}
• [http://webbook.nist.gov/cgi/cbook.cgi?Name=cyanic+acid&Units=SI Cyanic acid] from NIST Chemistry WebBook (accessed 2006-09-09)

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Category:Acids

Category:Cyanates

Category:Hydrogen compounds

Category:Inorganic carbon compounds

Category:Isocyanates