View source for Nitroso

{{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 (chemistry)|moiety]]. As such, various nitroso groups can be categorized as ''C''-nitroso compounds (e.g., nitroso[[alkane]]s; {{chem2|R\sN\dO}}), ''S''-nitroso compounds ([[S-Nitrosothiol|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 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]]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]]. In the [[Fischer–Hepp rearrangement]], aromatic 4-nitrosoanilines are prepared from the corresponding [[nitrosamine]]s.

==Properties==
[[File:VIYMEX.png|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 [[Dimerization (chemistry)|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-trans isomer|''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 Press|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 solvent|protic media]], [[primary carbon|primary]] and [[secondary carbon|secondary]] nitrosoalkanes [[rearrangement reaction|isomerize]] to [[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]].{{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 energy|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|''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]]s 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 [[indole]]s, 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==
[[Image:Metal-nitrosyl-coordination-modes-2D.png|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 (chemistry)|nonmetal]] example is the common reagent [[nitrosyl chloride]] ({{chem2|Cl\sN\dO}}). Nitric oxide is a stable [[Radical (chemistry)|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|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| ]]
[[Category:Nitrosyl compounds|Nitrosyl compounds]]