Nitroso



In organic chemistry, nitroso refers to a functional group in which the nitric oxide (\sN\dO) group is attached to an organic moiety. As such, various nitroso groups can be categorized as C-nitroso compounds (e.g., nitrosoalkanes; R\sN\dO), S-nitroso compounds (nitrosothiols; RS\sN\dO), N-nitroso compounds (e.g., nitrosamines, RN(\sR’)\sN\dO), and O-nitroso compounds (alkyl nitrites; RO\sN\dO).

Synthesis
Nitroso compounds can be prepared by the reduction of nitro compounds 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.

Properties


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.

When stored in protic media, primary and secondary nitrosoalkanes isomerize to oximes.

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 30 - 40 kcal/mol, while nitrosoarenes have BDEs on the order of 50 - 60 kcal/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. Organonitroso compounds serve as a ligands giving transition metal nitroso complexes.

Reactions
Many reaction exists which make use of an intermediate nitroso compound, such as the Barton reaction and Davis–Beirut reaction, as well as in 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.

Nitrosation vs. nitrosylation
Nitrite can enter two kinds of reaction, depending on the physico-chemical environment. Many primary alkyl N-nitroso compounds, such as CH3N(H)NO, tend to be unstable with respect to hydrolysis to the alcohol. Those derived from secondary amines (e.g., (CH3)2NNO derived from dimethylamine) are more robust. It is these N-nitrosamines that are carcinogens in rodents.
 * Nitrosylation is adding a nitrosyl ion NO- to a metal (e.g. iron) or a thiol, leading to nitrosyl iron Fe\sNO (e.g., in nitrosylated heme = nitrosylheme) or S-nitrosothiols (RSNOs).
 * Nitrosation is adding a nitrosonium ion NO+ to an amine –NH2 leading to a nitrosamine. This conversion occurs at acidic pH, particularly in the stomach, as shown in the equation for the formation of N-phenylnitrosamine:
 * NO2- + H+ <-> HONO
 * HONO + H+ <-> H2O + NO+
 * C6H5NH2 + NO+ -> C6H5N(H)NO + H+

Nitrosyl in inorganic chemistry
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 (Cl\sN\dO). Nitric oxide is a stable radical, having an unpaired electron. Reduction of nitric oxide gives the nitrosyl anion, NO-:
 * NO + e- -> NO-

Oxidation of NO yields the nitrosonium cation, NO+:
 * NO -> NO+ + e-

Nitric oxide can serve as a ligand forming metal nitrosyl complexes or just metal nitrosyls. These complexes can be viewed as adducts of NO+, NO-, or some intermediate case.