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 (e.g., nitrosoalkanes;), S-nitroso compounds (nitrosothiols;), N-nitroso compounds (e.g., nitrosamines,), and O-nitroso compounds (alkyl nitrites;).
See main article: Nitrosation. Nitroso compounds can be prepared by the reduction of nitro compounds[1] or by the oxidation of hydroxylamines. Ortho-nitrosophenols may be produced by the Baudisch reaction. In the Fischer–Hepp rearrangement, aromatic 4-nitrosoanilines are prepared from the corresponding nitrosamines.
Nitrosoarenes typically participate in a monomer–dimer equilibrium. The azobenzene N,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 as cis and trans isomers.[2] The central "double bond" in the dimer in fact has a bond order of about 1.5.[3]
When stored in protic media, primary and secondary nitrosoalkanes isomerize to oximes.[4] Some tertiary nitrosoalkanes also isomerize to oximes through C-C bond fission, particularly if the bond is electron-poor. Nitrosophenols and naphthols isomerize to the oxime quinone in solution, but reversibly; nitrosophenol ethers typically dealkylate to facilitate the isomerization. Nitroso tertiary anilines generally do not dealkylate in that way.
Due to the stability of the nitric oxide free radical, nitroso organyls tend to have very low C–N bond dissociation energies: nitrosoalkanes have BDEs on the order of, while nitrosoarenes have BDEs on the order of . 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 .[5]
Organonitroso compounds serve as a ligands giving transition metal nitroso complexes.[6]
Many reactions make use of an intermediate nitroso compound, such as the Barton reaction and Davis–Beirut reaction, as well as the synthesis of indoles, for example: Baeyer–Emmerling indole synthesis, Bartoli indole synthesis. In the Saville 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.[7]
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, a nonmetal example is the common reagent nitrosyl chloride . Nitric oxide is a stable radical, having an unpaired electron. Reduction of nitric oxide gives the nitrosyl anion, :
Oxidation of NO yields the nitrosonium cation, :
Nitric oxide can serve as a ligand forming metal nitrosyl complexes or just metal nitrosyls. These complexes can be viewed as adducts of,, or some intermediate case.