Английская Википедия:Isocyanic acid
Шаблон:Short description Шаблон:Redirect Шаблон:Chembox
Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as Шаблон:Chem2. 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) (Шаблон:Chem2).
The derived anion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is Шаблон:Chem2, which is called cyanate. The related functional group Шаблон:Chem2 is isocyanate; it is distinct from cyanate (Шаблон:Chem2), fulminate (Шаблон:Chem2), and nitrile oxide (Шаблон:Chem2).[1]
Isocyanic acid was discovered in 1830 by Justus von Liebig and Friedrich Wöhler.[2]
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, Шаблон:Chem2) and the elusive fulminic acid (Шаблон:Chem2)[3] and isofulminic acid Шаблон:Chem2.[1][4]
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.[5][6] If so, then the canonical form Шаблон:Chem2 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.,[7] as well as the NIST Chemistry WebBook,[8] 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%.Шаблон:CnШаблон:Dubious The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.[9]
Properties
In aqueous solution it is a weak acid, having a pKa of 3.7:[10]
Isocyanic acid hydrolyses to carbon dioxide and ammonia:
Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.[11]
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):
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.[12] 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 Шаблон:Chem2, also called hydrogen cyanate.[13] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[11] Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.Шаблон: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.[14]
- <chem>H+ + NCO- -> HNCO</chem>
HNCO also can be made by the high-temperature thermal decomposition of the trimer cyanuric acid:
- <chem>C3H3N3O3 -> 3 HNCO</chem>
In the reverse of the famous synthesis of urea by Friedrich Wöhler,
- <chem>CO(NH2)2 -> HNCO + NH3</chem>
isocyanic acid is produced and rapidly trimerizes to cyanuric acid.
Occurrence
Isocyanic acid has been detected in many kinds of interstellar environments.[4]
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.[15]
See also
References
External links
- Шаблон:Cite book
- Cyanic acid from NIST Chemistry WebBook (accessed 2006-09-09)
Шаблон:Hydrogen compounds Шаблон:Molecules detected in outer space Шаблон:Cyanates Шаблон:Authority control
- ↑ 1,0 1,1 William R. Martin and David W. Ball (2019): "Small organic fulminates as high energy materials. Fulminates of acetylene, ethylene, and allene". Journal of Energetic Materials, volume 31, issue 7, pages 70-79. Шаблон:Doi
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 4,0 4,1 Donghui Quan, Eric Herbst, Yoshihiro Osamura, and Evelyne Roueff (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, volume 725, issue 2, pages Шаблон:Doi
- ↑ Nakamoto, part A, p 190
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite book
- ↑ Шаблон:Cite web
- ↑ Шаблон:Cite journal
- ↑ IUPAC SC-Database Шаблон:Webarchive A comprehensive database of published data on equilibrium constants of metal complexes and ligands
- ↑ 11,0 11,1 A. S. Narula, K. Ramachandran “Isocyanic Acid” in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, New York. Шаблон:Doi Article Online Posting Date: April 15, 2001.
- ↑ Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite web
