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 element ...

with the

structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are connected to one another. The chemical bonding within the molecule is al ...

HNCO, which is often written as . It is a colourless, volatile and

poisonous A poison is any chemical substance that is harmful or lethal to living organisms. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figurati ...

gas, condensing at 23.5 °C. It is the predominant

tautomer In chemistry, 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 reloca ...

and an

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

of cyanic acid ''(aka. cyanol)'' (), and the monomer of

cyanuric acid Cyanuric acid or 1,3,5-triazine-2,4,6-triol is a chemical compound with the chemical formula, formula (CNOH)3. Like many industrially useful chemicals, this triazine has many synonyms. This white, odorless solid finds use as a precursor or a com ...

. 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 conven ...

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

cyanate The cyanate ion is an anion with the chemical formula . It is a resonance of three forms: (61%) ↔ (30%) ↔ (4%). Cyanate is the derived anion of isocyanic acid, H−N=C=O, and its lesser tautomer cyanic acid (a.k.a. cyanol), H−O−C� ...

. The related

functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

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 Fulminates are chemical compounds which include the fulminate ion (). The fulminate ion is a pseudohalic ion because its charge and reactivity are similar to those of the halogens. The name is derived from the Latin ''fulminātus'', meaning to ...

(), and

nitrile oxide In organic chemistry, a nitrile is any organic compound that has a functional group. The name of the compound is composed of a base, which includes the carbon of the , suffixed with "nitrile", so for example is called " propionitrile" (or pr ...

(). Isocyanic acid was discovered in 1830 by

Justus von Liebig Justus ''Freiherr'' von Liebig (12 May 1803 – 18 April 1873) was a Germans, German scientist who made major contributions to the theory, practice, and pedagogy of chemistry, as well as to agricultural and biology, biological chemistry; he is ...

and

Friedrich Wöhler Friedrich Wöhler Royal Society of London, FRS(For) HonFRSE (; 31 July 180023 September 1882) was a German chemist known for his work in both organic chemistry, organic and inorganic chemistry, being the first to isolate the chemical elements be ...

. Isocyanic acid is the simplest stable

that contains

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

, and

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

, 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 matter, organic materials, i.e., matter in its various forms that contain ...

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

molecular formula 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, such as paren ...

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

fulminic acid Fulminic acid is an acid with the formula HCNO, more specifically . It is an isomer of isocyanic acid () and of its elusive tautomer, cyanic acid (), and also of isofulminic acid (). Fulminate is the anion of any of its salts. For historic ...

() and

isofulminic acid Fulminic acid is an acid with the formula HCNO, more specifically . It is an isomer of isocyanic acid () and of its elusive tautomer, cyanic acid (), and also of isofulminic acid (). Fulminate is the anion of any of its salts. For historical r ...

Structure

Isocyanic acid (HNCO)

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 some say indicates a carbon–nitrogen triple bond. If so, then the canonical form is the major

resonance structure In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

. However, classic vibrational analysis would indicate that the 2268.8 cm⁻¹ is the asymmetric N=C=O stretch, as per Colthup et al., as well as the NIST Chemistry WebBook, 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

, known as cyanic acid, HOCN, in which the oxygen atom is protonated exists 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 Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

between the nitrogen and carbon atoms.

Properties

In aqueous solution it is a

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a proton, , and an anion, . The dissociation or ionization of a strong acid in solution is effectively complete, except in its most concentrated s ...

, having a p''K''_a of 3.7:Pettit, Gwyneth; Pettit, Leslie.
"SC-Database" (proprietary database)
Timble, Yorks: Academic Software — via proprietary executable. : Isocyanic acid

hydrolyses Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

and

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

: : Dilute solutions of isocyanic acid are stable in inert

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s, e.g.

ether In organic chemistry, ethers are a class of compounds that contain an ether group, a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group (e.g., alkyl or aryl). They have the general formula , where R and R� ...

and chlorinated hydrocarbons. At high concentrations, isocyanic acid oligomerizes to give the trimer

and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products. Isocyanic acid reacts with

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s to give

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

s (carbamides): : 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 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 ...

whose free energy,

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

and

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

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

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...

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

ECW model In chemistry, the ECW model is a semi-quantitative model that describes and predicts the strength of Lewis acid–Lewis base interactions. Many chemical reactions can be described as acid–base reactions, so models for such interactions are of pot ...

. Low-temperature

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

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, such as from salts like

potassium cyanate Potassium cyanate is an inorganic compound with the chemical formula, formula KOCN (sometimes denoted KCNO). It is a colourless solid. It is used to prepare many other compounds including useful herbicide. Worldwide production of the potassium an ...

, by either gaseous

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

or acids such as

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...

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

: : 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

, : 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 an aerosol (a suspension of airborne particulates and gases) emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwante ...

, 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 odour. The word was then inte ...

and cigarette smoke. 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 used ...

, 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 s ...

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