Isocyanic acid is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

with the structural formula HNCO, which is often written as . It is a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C. It is the predominant

tautomer Tautomers () are structural isomers (constitutional 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 hyd ...

of cyanic acid (). The derived

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is , which is called

cyanate Cyanate is an anion with the structural formula , usually written . It also refers to any salt containing it, such as ammonium cyanate. It is an isomer of the much less stable fulminate anion .William R. Martin and David W. Ball (2019): "Sm ...

. The related

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the res ...

isocyanate In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

; it is distinct from

(), fulminate (), and nitrile oxide (). Isocyanic acid was discovered in 1830 by

Justus von Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biology, biological chemistry, and is considered one of the principal founders of organic chemistry. As a profess ...

and

Friedrich Wöhler Friedrich Wöhler () FRS(For) HonFRSE (31 July 180023 September 1882) was a German chemist known for his work in inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the first ...

. Isocyanic acid is the simplest stable

that contains

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

, and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

, the four most commonly found elements in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

and biology. It is the only fairly stable one of the four linear isomers with

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

HOCN that have been synthesized, the others being cyanic acid (cyanol, ) and the elusive fulminic acid () and isofulminic acid .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. 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

Structure

Although the electronic structure according to

valence bond theory In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of ...

can be written as H−N=C=O, the vibrational spectrum has a band at 2268.8 cm⁻¹ in the gas phase, which clearly indicates a carbon–nitrogen triple bond. Thus the canonical form is the major resonance structure.

Properties

Physical

The pure compound has a

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of −86.8 °C and a boiling point of 23.5 °C, so it is volatile at ambient temperatures.Greenwood, p323

Acidity

In aqueous solution it is a weak acid, having a p''K''_a of 3.7:IUPAC SC-Database
A comprehensive database of published data on equilibrium constants of metal complexes and ligands :HNCO <=> H+ + NCO-

Decomposition

Isocyanic acid hydrolyses to

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

: :HNCO + H2O -> CO2 + NH3

Oligomerization

At sufficiently high concentrations, isocyanic acid oligomerizes to give the trimer cyanuric acid and

cyamelide Cyamelide is an amorphous white solid with the approximate formula (HNCO)x. It is the product of the polymerisation of cyanic acid together with its cyclic trimer cyanuric acid. It is a porcelain-like white substance which is insoluble in wa ...

, 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.

Stability in solution

Dilute solutions of isocyanic acid are stable in inert

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

s, e.g.

ether In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again b ...

and

chlorinated hydrocarbons An organochloride, organochlorine compound, chlorocarbon, or chlorinated hydrocarbon is an organic compound containing at least one covalently bonded atom of chlorine. The chloroalkane class (alkanes with one or more hydrogens substituted by ch ...

.A. S. Narula, K. Ramachandran “Isocyanic Acid” in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, New York. Article Online Posting Date: April 15, 2001.

Reactions

Isocyanic acid reacts with

amine 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 su ...

s to give ureas (carbamides): :HNCO + RNH2 -> RNHC(O)NH2 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 A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

whose free energy,

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

and

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...

changes for its 1:1 association with a number of bases in

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...

solution at 25 °C have been reported. The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.

Tautomerism

The

, 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 A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond orde ...

between the nitrogen and carbon atoms. Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid , also called hydrogen cyanate. 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.

Preparation

Isocyanic acid can be made by protonation of the

cyanate anion Cyanate is an anion with the structural formula , usually written . It also refers to any salt containing it, such as ammonium cyanate. It is an isomer of the much less stable fulminate anion .William R. Martin and David W. Ball (2019): "Small ...

, such as from salts like potassium cyanate, by either gaseous hydrogen chloride or acids such as

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...

. :H+ + NCO- -> HNCO HNCO also can be made by the high-temperature thermal decomposition of the trimer cyanuric acid: :C3H3N3O3 -> 3 HNCO In the reverse of the famous synthesis of urea by

, :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. Isocyanic acid is also present in various forms of

smoke Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrainment (engineering), entrained or otherwise mixed into the mass. It is commo ...

, including

smog Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words '' smoke'' and ''fog'' to refer to smoky fog due to its opacity, and odor. The word was then in ...

and

cigarette smoke Tobacco smoke is a sooty aerosol produced by the incomplete combustion of tobacco during the smoking of cigarettes and other tobacco products. Temperatures in burning cigarettes range from about 400 °C between puffs to about 900 °C d ...

. It was detected using

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...

, and easily dissolves in water, posing a health risk to the lungs.

References

External links

*
Cyanic acid
from

NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...

Chemistry WebBook (accessed 2006-09-09) {{Authority control Acids Cyanates Hydrogen compounds Inorganic carbon compounds Isocyanates