# Nitroso

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{{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](/source/Structural_formula) of nitroso group]]

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

==Synthesis==
{{Main|Nitrosation}}
Nitroso compounds can be prepared by the reduction of [nitro compound](/source/nitro_compound)s<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 [hydroxylamine](/source/hydroxylamine)s.<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](/source/Baudisch_reaction). In the [Fischer–Hepp rearrangement](/source/Fischer%E2%80%93Hepp_rearrangement), aromatic 4-nitrosoanilines are prepared from the corresponding [nitrosamine](/source/nitrosamine)s.

==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](/source/Dimerization_(chemistry)).  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](/source/Cis-trans_isomer).<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](/source/Cambridge_University_Press)|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](/source/protic_solvent), [primary](/source/primary_carbon) and [secondary](/source/secondary_carbon) nitrosoalkanes [isomerize](/source/rearrangement_reaction) to [oxime](/source/oxime)s.<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](/source/electron-poor).{{sfn|Williams|1988|p=36}}  Nitrosophenols and naphthols isomerize to the oxime [quinone](/source/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](/source/free_radical), nitroso organyls tend to have very low C–N [bond dissociation energies](/source/bond_dissociation_energy): 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](/source/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 [ligand](/source/ligand)s giving [transition metal nitroso complexes](/source/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](/source/Barton_reaction_(disambiguation)) and [Davis–Beirut reaction](/source/Davis%E2%80%93Beirut_reaction), as well as the synthesis of [indole](/source/indole)s, for example: [Baeyer–Emmerling indole synthesis](/source/Baeyer%E2%80%93Emmerling_indole_synthesis), [Bartoli indole synthesis](/source/Bartoli_indole_synthesis). In the [Saville reaction](/source/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](/source/Nonmetal_(chemistry)) example is the common reagent [nitrosyl chloride](/source/nitrosyl_chloride) ({{chem2|Cl\sN\dO}}). Nitric oxide is a stable [radical](/source/Radical_(chemistry)), having an unpaired electron. Reduction of nitric oxide gives the nitrosyl [anion](/source/anion), {{chem2|NO-}}:
:{{chem2|NO + e- -> NO- }}

Oxidation of NO yields the [nitrosonium](/source/nitrosonium) [cation](/source/cation), {{chem2|NO+}}:
:{{chem2|NO -> NO+ + e- }}

Nitric oxide can serve as a [ligand](/source/ligand) forming [metal nitrosyl complexes](/source/Metal_nitrosyl) 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](/source/Nitrosamine), the functional group with the NO attached to an amine, such as R<sub>2</sub>N–NO
* [Nitrosobenzene](/source/Nitrosobenzene)
* [Nitric oxide](/source/Nitric_oxide)
* [Nitroxyl](/source/Nitroxyl)

==References==
{{Reflist}}

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

Category:Functional groups
Category:Nitroso compounds
Nitrosyl compounds

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Adapted from the Wikipedia article [Nitroso](https://en.wikipedia.org/wiki/Nitroso) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Nitroso?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.