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]
Although the electronic structure according to valence bond theory can be written as H−N=C=O, the vibrational spectrum has a band at 2268.8cm−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 2268.8cm−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 1327cm−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]
At high concentrations, isocyanic acid oligomerizes to give the trimercyanuric 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]
12Martin, 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.
↑"Isocyanic acid". National Institute of Standards and Technology (U.S. Department of Commerce). Retrieved 2023-04-20.
↑Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
↑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.
