Last updated
IUPAC name
Systematic IUPAC name
Oxidanimine [1]
Other names
Hydrogen oxonitrate(I)

Nitronous oxide
Nitrosyl hydride

Hyponitrous acid monomer
3D model (JSmol)
MeSH Nitroxyl
PubChem CID
  • InChI=1S/HNO/c1-2/h1H Yes check.svgY
  • N=O
Molar mass 31.014 g·mol−1
log P 0.74
33.88 J K−1 mol−1
Std molar
220.91 J K−1 mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Nitroxyl (common name) or azanone (IUPAC name) [2] is the chemical compound HNO. It is well known in the gas phase. [3] [4] Nitroxyl can be formed as a short-lived intermediate in the solution phase. The conjugate base, NO, nitroxide anion, is the reduced form of nitric oxide (NO) and is isoelectronic with dioxygen. The bond dissociation energy of H−NO is 49.5 kcal/mol (207 kJ/mol), which is unusually weak for a bond to the hydrogen atom.



Nitroxyl is produced from the reagents Angeli's salt (Na2N2O3) and Piloty's acid (PhSO2NHOH). [5] Other notable studies on the production of HNO exploit cycloadducts of acyl nitroso species, which are known to decompose via hydrolysis to HNO and acyl acid. Upon photolysis these compounds release the acyl nitroso species which then further decompose. [6] HNO is generated via organic oxidation of cyclohexanone oxime with lead tetraacetate to form 1-nitrosocyclohexyl acetate: [7]

Nitrosocyclohexyl acetate.png

This compound can be hydrolyzed under basic conditions in a phosphate buffer to HNO, acetic acid, and cyclohexanone.

Dichloramine reacts with the hydroxide ion, which is always present in water, to yield nitroxyl and the chloride ion. [8]


Nitroxyl is a weak acid, with pKa of about 11, the conjugate base being the triplet state of NO, sometimes called nitroxide. Nitroxyl itself, however, is a singlet ground state. Thus, deprotonation of nitroxyl uniquely involves the forbidden spin crossing from the singlet state starting material to triplet state product:

1HNO + B3NO + BH

Due to the spin-forbidden nature of deprotonation, proton abstraction is many orders of magnitude slower (k = 4.9×104 M−1 s−1 for deprotonation by OH) than what one would expect for a heteroatom proton-transfer process (processes that are so fast that they are sometimes diffusion-controlled).

The Ka of starting from or ending with the electronic excited states has also been determined. When process of deprotonating singlet state HNO to obtain singlet state NO has a pKa is about 23. On the other hand, when deprotonating triplet HNO to obtain triplet NO, the pKa is about −1.8. [9] [10]

Nitroxyl rapidly decomposes by a bimolecular pathway to nitrous oxide (k at 298 K = 8×106 M s): [9]

2 HNO → N2O + H2O

The reaction proceeds via dimerization to hyponitrous acid, H2N2O2, which subsequently undergoes dehydration. Therefore, HNO is generally prepared in situ as described above.

Nitroxyl is very reactive towards nucleophiles, including thiols. The initial adduct rearranges to a sulfinamide: [10]



In biological samples, nitroxyl can be detected using fluorescent sensors, many of which are based on the reduction of copper(II) to copper(I) with concomitant increase in fluorescence. [11]

Medicinal chemistry

Nitroxyl donors, known as nitroso compounds, show potential in the treatment of heart failure and ongoing research is focused on finding new molecules for this task.

See also

Related Research Articles

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In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (−OH) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, C
. Phenolic compounds are classified as simple phenols or polyphenols based on the number of phenol units in the molecule.

<span class="mw-page-title-main">Nitrous acid</span> Chemical compound

Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase and in the form of nitrite salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

<span class="mw-page-title-main">Dinitrogen pentoxide</span> Chemical compound

Dinitrogen pentoxide is the chemical compound with the formula N2O5. It is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It exists as colourless crystals that sublime slightly above room temperature, yielding a colorless gas.

<span class="mw-page-title-main">Cyclobutadiene</span> Chemical compound

Cyclobutadiene is an organic compound with the formula C4H4. It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is quite stable. Since the compound degrades by a bimolecular process, the species can be observed by matrix isolation techniques at temperatures below 35 K. It is thought to adopt a rectangular structure.

<span class="mw-page-title-main">Nitro compound</span> Organic compound containing an −NO₂ group

In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups. The nitro group is one of the most common explosophores used globally. The nitro group is also strongly electron-withdrawing. Because of this property, C−H bonds alpha (adjacent) to the nitro group can be acidic. For similar reasons, the presence of nitro groups in aromatic compounds retards electrophilic aromatic substitution but facilitates nucleophilic aromatic substitution. Nitro groups are rarely found in nature. They are almost invariably produced by nitration reactions starting with nitric acid.

<span class="mw-page-title-main">Lithium diisopropylamide</span> Chemical compound

Lithium diisopropylamide is a chemical compound with the molecular formula LiN(CH 2)2. It is used as a strong base and has been widely utilized due to its good solubility in non-polar organic solvents and non-nucleophilic nature. It is a colorless solid, but is usually generated and observed only in solution. It was first prepared by Hamell and Levine in 1950 along with several other hindered lithium diorganylamides to effect the deprotonation of esters at the α position without attack of the carbonyl group.

<span class="mw-page-title-main">Singlet oxygen</span> Oxygen with all of its electrons spin paired

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as 1
or 1
), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow.

<span class="mw-page-title-main">Bamford–Stevens reaction</span>

The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.

<span class="mw-page-title-main">Sulfamic acid</span> Chemical compound

Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205 °C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen.

<span class="mw-page-title-main">Nitroso</span> Class of functional groups with a –N=O group attached

In organic chemistry, nitroso refers to a functional group in which the nitric oxide group is attached to an organic moiety. As such, various nitroso groups can be categorized as C-nitroso compounds, S-nitroso compounds, N-nitroso compounds, and O-nitroso compounds.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.

<span class="mw-page-title-main">Dakin oxidation</span> Organic redox reaction that converts hydroxyphenyl aldehydes or ketones into benzenediols

The Dakin oxidation is an organic redox reaction in which an ortho- or para-hydroxylated phenyl aldehyde or ketone reacts with hydrogen peroxide in base to form a benzenediol and a carboxylate. Overall, the carbonyl group is oxidised, whereas the H2O2 is reduced.

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The McFadyen–Stevens reaction is a chemical reaction best described as a base-catalyzed thermal decomposition of acylsulfonylhydrazides to aldehydes.

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Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a strong electrophile and oxidizing agent. It is sometimes called Tilden's reagent, after William A. Tilden, who was the first to produce it as a pure compound.

In organic chemistry, the Baudisch reaction is a process for the synthesis of nitrosophenols using metal ions. Although the products are of limited value, the reaction is of historical interest as an example of metal-promoted functionalization of aromatic substrates.

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In organic chemistry, a nitrate ester is an organic functional group with the formula R−ONO2, where R stands for any organyl group. They are the esters of nitric acid and alcohols. A well-known example is nitroglycerin, which is not a nitro compound, despite its name.

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Nitrolic acids are organic compounds with the functional group RC(NO2)=NOH. They are prepared by the reaction of nitroalkanes with base and nitrite sources:


  1. "Nitroxyl". PubChem . Retrieved August 24, 2022.{{cite web}}: CS1 maint: url-status (link)
  2. Doctorovich, F.; Bikiel, D.; Pellegrino, J.; Suárez, S. A.; Larsen, A.; Martí, M. A. (2011). "Nitroxyl (azanone) trapping by metalloporphyrins". Coordination Chemistry Reviews. 255 (23–24): 2764–2784. doi:10.1016/j.ccr.2011.04.012.
  3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  4. Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN   978-0-12-352651-9.
  5. Nagasawa, H. T.; Kawle, S. P.; Elberling, J. A.; DeMaster, E. G.; Fukuto, J. M. (1995). "Prodrugs of Nitroxyl as Potential Aldehyde Dehydrogenase Inhibitors vis-a-vis Vascular Smooth Muscle Relaxants". J. Med. Chem. 38 (11): 1865–1871. doi:10.1021/jm00011a005. PMID   7783118.
  6. Cohen, A. D.; Zeng, B.-B.; King, S. B.; Toscano, J. P. (2003). "Direct observation of an acyl nitroso species in solution by time-resolved IR spectrocopy". J. Am. Chem. Soc. 125 (6): 1444–1445. doi:10.1021/ja028978e. PMID   12568581.
  7. Sha, Xin; Isbell, T. Scott; Patel, Rakesh P.; Day, Cynthia S.; King, S. Bruce (2006). "Hydrolysis of Acyloxy Nitroso Compounds Yields Nitroxyl (HNO)". J. Am. Chem. Soc. 128 (30): 9687–9692. doi:10.1021/ja062365a. PMID   16866522.
  8. White, George Clifford (1986). The handbook of chlorination (2nd ed.). New York: Van Nostrand Reinhold. p. 169. ISBN   978-0-442-29285-0.
  9. 1 2 Shafirovich, V.; Lymar, S. V. (2002). "Nitroxyl and its anion in aqueous solutions: Spin states, protic equilibria, and reactivities toward oxygen and nitric oxide". Proceedings of the National Academy of Sciences of the United States of America. 99, 7340 (11): 7340–7345. doi: 10.1073/pnas.112202099 . PMC   124232 . PMID   12032284.
  10. 1 2 Bianco, C. L.; Toscano, J. P.; Bartberger, M. D.; Fukuto, J. M. (2017). "The chemical biology of HNO signaling". Archives of Biochemistry and Biophysics. 617: 129–136. doi:10.1016/ PMC   5318259 . PMID   27555493.
  11. Rivera-Fuentes, Pablo; Lippard, Stephen J. (2015). "Metal-Based Optical Probes for Live Cell Imaging of Nitroxyl (HNO)". Acc. Chem. Res. 38 (11): 2427–2434. doi:10.1021/acs.accounts.5b00388. hdl: 1721.1/107934 . PMID   26550842.