{{Short description|1=Class of functional groups with a –N=O group attached}} {{Redirect|Nitrosyl|the analogous concept in metalorganic chemistry|metal nitrosyl complex}} [[Image:Nitroso-compound-2D.svg| thumb | right | Structural formula of nitroso group]]

In organic chemistry, '''nitroso''' refers to a functional group in which the nitric oxide ({{chem2|\sN\dO}}) group is attached to an organic moiety. As such, various nitroso groups can be categorized as ''C''-nitroso compounds (e.g., nitrosoalkanes; {{chem2|R\sN\dO}}), ''S''-nitroso compounds (nitrosothiols; {{chem2|RS\sN\dO}}), ''N''-nitroso compounds (e.g., nitrosamines, {{chem2|RN(\sR’)\sN\dO}}), and ''O''-nitroso compounds (alkyl nitrites; {{chem2|RO\sN\dO}}).

==Synthesis== {{Main|Nitrosation}} Nitroso compounds can be prepared by the reduction of nitro compounds<ref>{{cite journal| author = G. H. Coleman| author2 = C. M. McCloskey| author3 = F. A. Stuart | title = Nitrosobenzene | journal=Org. Synth.|volume = 25 | pages = 80| year = 1945 | doi=10.15227/orgsyn.025.0080}}</ref> or by the oxidation of hydroxylamines.<ref>{{OrgSynth|first1=A. |last1=Calder |first2=A. R. |last2=Forrester |first3=S. P. |last3=Hepburn |title=2-Methyl-2-nitrosopropane and Its Dimer |collvol=6 |collvolpages=803 |volume=52 |page=77 |prep=cv6p0803}}</ref> Ortho-nitrosophenols may be produced by the Baudisch reaction. In the Fischer–Hepp rearrangement, aromatic 4-nitrosoanilines are prepared from the corresponding nitrosamines.

==Properties== thumb|right|132px|Structure of 2-nitrosotoluene dimer<ref>{{cite journal|author=E.Bosch|title=Structural Analysis of Methyl-Substituted Nitrosobenzenes and Nitrosoanisoles|journal=J. Chem. Cryst.|year=2014|volume=98|issue=2|page=44| doi=10.1007/s10870-013-0489-8|s2cid=95291018}}</ref>

Nitrosoarenes typically participate in a monomer–dimer equilibrium. The azobenzene ''N'',''N'-''dioxide (Ar(<sup>–</sup>O)N<sup>+</sup>=<sup>+</sup>N(O<sup>–</sup>)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.<ref>{{cite journal|author1=Beaudoin, D. |author2=Wuest, J. D. |title=Dimerization of Aromatic ''C''-Nitroso Compounds|journal=Chemical Reviews|year=2016|volume=116|issue=1 |pages=258–286|doi=10.1021/cr500520s|pmid=26730505}}</ref> The central "double bond" in the dimer in fact has a bond order of about 1.5.<ref>{{cite book|title=Nitrosation|first=D.&nbsp;L.&nbsp;H.|last=Williams|publisher=Cambridge University|location=Cambridge, UK|year=1988|isbn=0-521-26796-X|url=https://archive.org/details/nitrosation0000will|url-access=registration|page=36}}</ref>

When stored in protic media, primary and secondary nitrosoalkanes isomerize to oximes.<ref name=TildenLect>{{cite journal |doi=10.1039/CS9770600001|title=Electrophilic C-nitroso-compounds|year=1977|last1=Kirby|first1=G.&nbsp;W.|journal=Chemical Society Reviews|volume=6|page=2|postscript=none}} (Tilden lecture).</ref> Some tertiary nitrosoalkanes also isomerize to oximes through C-C bond fission, particularly if the bond is electron-poor.{{sfn|Williams|1988|p=36}} 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.{{sfn|Williams|1988|pp=59-61}}

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 {{cvt|30|-|40|kcal/mol|kJ/mol|-1}}, while nitrosoarenes have BDEs on the order of {{cvt|50|-|60|kcal/mol|kJ/mol|-1}}. 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, Ph<sub>2</sub>N–N=O, has a N–N bond dissociation energy of only {{cvt|23|kcal/mol|kJ/mol|0}}.<ref>{{Cite book|title=Comprehensive Handbook of Chemical Bond Energies|last=Luo|first=Yu-Ran|publisher=Taylor and Francis|year=2007|isbn=9781420007282|location=Boca Raton, FL}}</ref>

Organonitroso compounds serve as a ligands giving transition metal nitroso complexes.<ref name=GBRA>{{cite journal |doi=10.1021/cr0000731 |title=Interactions of Organic Nitroso Compounds with Metals |date=2002 |last1=Lee |first1=Jonghyuk |last2=Chen |first2=Li |last3=West |first3=Ann H. |last4=Richter-Addo |first4=George B. |journal=Chemical Reviews |volume=102 |issue=4 |pages=1019–1066 |pmid=11942786 }}</ref>

==Reactions== 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.<ref>{{cite journal|author1=Bianchi, P. |author2=Monbaliu, J. C. M. |title=Three decades of unveiling the complex chemistry of ''C''-nitroso species with computational chemistry|journal=Organic Chemistry Frontiers|year=2022|volume=9|pages=223–264|doi=10.1039/d1qo01415c}}</ref>

==Nitrosyl in inorganic chemistry== thumb|right|100px|Linear and bent metal nitrosyls 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 ({{chem2|Cl\sN\dO}}). Nitric oxide is a stable radical, having an unpaired electron. Reduction of nitric oxide gives the nitrosyl anion, {{chem2|NO-}}: :{{chem2|NO + e- -> NO- }}

Oxidation of NO yields the nitrosonium cation, {{chem2|NO+}}: :{{chem2|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 {{chem2|NO+}}, {{chem2|NO-}}, or some intermediate case.

==In human health== {{Transcluded section|source=Nitrosamine formation during digestion}}{{:Nitrosamine formation during digestion}}

== See also == * Nitrosamine, the functional group with the NO attached to an amine, such as R<sub>2</sub>N–NO * Nitrosobenzene * Nitric oxide * Nitroxyl

==References== {{Reflist}}

{{Functional Groups}} {{Nitric oxide signaling}} {{Nitrogen compounds}} {{Authority control}}

Category:Functional groups Category:Nitroso compounds Nitrosyl compounds