Inorganic chemistry,nitroso refers to afunctional group in which thenitric oxide (−N=O) group is attached to an organicmoiety. As such, various nitroso groups can be categorized asC-nitroso compounds (e.g., nitrosoalkanes;R−N=O),S-nitroso compounds (nitrosothiols;RS−N=O),N-nitroso compounds (e.g.,nitrosamines,RN(−R’)−N=O), andO-nitroso compounds (alkyl nitrites;RO−N=O).

Nitroso compounds can be prepared by the reduction ofnitro compounds^[1] or by the oxidation ofhydroxylamines.^[2] Ortho-nitrosophenols may be produced by theBaudisch reaction. In theFischer–Hepp rearrangement, aromatic 4-nitrosoanilines are prepared from the correspondingnitrosamines.

Nitrosoarenes typically participate in amonomer–dimer equilibrium. The azobenzeneN,N'-dioxide (Ar(^–O)N⁺=⁺N(O^–)Ar) dimers, which are often pale yellow, are generally favored in the solid state, whereas the deep-green monomers are favored in dilute solution or at higher temperatures. They exist ascis andtrans isomers.^[4] The central "double bond" in the dimer in fact has a bond order of about 1.5.^[5]

When stored inprotic media,primary andsecondary nitrosoalkanesisomerize tooximes.^[6] Some tertiary nitrosoalkanes also isomerize to oximes through C-C bond fission, particularly if the bond iselectron-poor.^[7] Nitrosophenols and naphthols isomerize to the oximequinone in solution, but reversibly; nitrosophenol ethers typically dealkylate to facilitate the isomerization. Nitroso tertiary anilines generally do not dealkylate in that way.^[8]

Due to the stability of the nitric oxidefree radical, nitroso organyls tend to have very low C–Nbond dissociation energies: nitrosoalkanes have BDEs on the order of 30–40 kcal/mol (130–170 kJ/mol), while nitrosoarenes have BDEs on the order of 50–60 kcal/mol (210–250 kJ/mol). As a consequence, they are generally heat- and light-sensitive. Compounds containing O–(NO) or N–(NO) bonds generally have even lower bond dissociation energies. For instance,N-nitrosodiphenylamine, Ph₂N–N=O, has a N–N bond dissociation energy of only 23 kcal/mol (96 kJ/mol).^[9]

Organonitroso compounds serve as aligands givingtransition metal nitroso complexes.^[10]

Many reactions make use of an intermediate nitroso compound, such as theBarton reaction andDavis–Beirut reaction, as well as the synthesis ofindoles, for example:Baeyer–Emmerling indole synthesis,Bartoli indole synthesis. In theSaville reaction, mercury is used to replace a nitrosyl from a thiol group.

C-nitroso compounds are used in organic synthesis as synthons in some well-documented chemical reactions such as hetero Diels-Alder (HDA), nitroso-ene and nitroso-aldol reactions.^[11]

Nitrosyls are non-organic compounds containing the NO group, for example directly bound to the metal via the N atom, giving a metal–NO moiety. Alternatively, anonmetal example is the common reagentnitrosyl chloride (Cl−N=O). Nitric oxide is a stableradical, having an unpaired electron. Reduction of nitric oxide gives the nitrosylanion,NO⁻:

NO + e⁻ → NO⁻

Oxidation of NO yields thenitrosoniumcation,NO⁺:

NO → NO⁺ + e⁻

Nitric oxide can serve as aligand formingmetal nitrosyl complexes or just metal nitrosyls. These complexes can be viewed as adducts ofNO⁺,NO⁻, or some intermediate case.

Nitroso compounds react withprimary amines in acidic environments to formnitrosamines, which human metabolism converts to mutagenicdiazo compounds. Small amounts of nitro and nitroso compounds form during meatcuring; the toxicity of these compoundspreserves the meat againstbacterial infection. After curing completes, the concentration of these compounds appears to degrade over time. Their presence in finished products has been tightly regulated since several food-poisoning cases in the early 20th century,^[12] but consumption of large quantities of processed meats can still cause a slight elevation ingastric andoesophageal cancer risk today.^{[13][14][15][16]}

For example, during the 1970s, certainNorwegian farm animals began exhibiting elevated levels ofliver cancer. These animals had been fedherringmeal preserved withsodium nitrite. The sodium nitrite had reacted withdimethylamine in the fish and produceddimethylnitrosamine.^[17]

The effects of nitroso compounds vary dramatically across the gastrointestinal tract, and with diet. Nitroso compounds present in stool do not induce nitrosamine formation, because stool has neutralpH.^[18][19]Stomach acid catalyzes nitrosamine compound formation and is the main location of the reaction during digestion.^[20]

The formation process is inhibited when amine concentration is low (e.g. a low-protein diet or no fermented food). The process may also be inhibited in the case of highvitamin C (ascorbic acid) orerythorbic acid^[21] concentration (e.g. high-fruit diet).^[22][23][24] However, when 10% of the meal is fat, the effect reverses, and ascorbic acid markedly increases nitrosamine formation.^[20][25] Vitamin C and erythorbic acid are already commonly used in the meat industry because they enhance the binding of nitrite to myoglobin, encouraging the formation of the desired pink color.^[26]

• Nitrosamine, the functional group with the NO attached to an amine, such as R₂N–NO
• Nitrosobenzene
• Nitric oxide
• Nitroxyl

• Digestive system
• Nitrosamines
• Functional groups
• Nitroso compounds
• Nitrosyl compounds

• Articles with short description
• Short description is different from Wikidata