View source for Woodward–Hoffmann rules

[[Image:Nitroso-compound-2D.png | thumb | right | [[Wire-frame model]] of nitroso moiety]]

'''Nitroso''' refers to a [[functional group]] in [[organic chemistry]] which has the NO group attached to an organic moiety.  As such, various nitroso groups can be categorized as C-nitroso compounds (e.g., nitrosoalkanes; R-N=O), S-nitroso compounds (nitrosothiols; RS-N=O), N-nitroso compounds (e.g., nitrosamines, R<sup>1</sup>N(-R<sup>2</sup>)-N=O), and O-nitroso compounds (alkyl nitrites; RO-N=O).  

'''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 non-metal example is the common reagent nitrosyl chloride (Cl-N=O).

Nitric oxide is a stable [[Radical (chemistry)|radical]], having an unpaired electron.  

Reduction of nitric oxide gives the [[hyponitrite]] [[anion]], NO<sup>&minus;</sup>:

:NO + e<sup>&minus;</sup> → NO<sup>&minus;</sup>

Oxidation of NO yields the [[nitrosonium]] [[cation]], NO<sup>+</sup>:

:NO → NO<sup>+</sup> + e<sup>&minus;</sup>

==Nitrosyl as a ligand==
[[Image:Metal-nitrosyl-coordination-modes-2D.png|thumb|right|100px|Linear and bent metal nitrosyls]]
{{Main|Metal nitrosyl}}
Nitric oxide can serve as a [[ligand]] in [[complex (chemistry)|complexes]].  The resulting complexes are called metal nitrosyls, and can bond to a metal atom in two extreme modes: as NO<sup>+</sup> and as NO<sup>&minus;</sup>.  It is generally assumed that NO<sup>+</sup> coordinates linearly, the M−N−O angle being 180°, whereas NO<sup>&minus;</sup> forms a bent geometry, with an M−N−O angle of approximately 120°.  However, the results of many studies have shown that the ionic descriptions of the NO ligand do not correlate with metal-NO geometry.  A more realistic description of electron-counting in metal-nitrosyl chemistry is given by the Enemark-Feltham notation.

==Organonitroso compounds==
Nitroso compounds can be prepared by the reduction of [[nitro compound]]s or by the oxidation of [[hydroxylamine]]s. A good example is (CH<sub>3</sub>)<sub>3</sub>CNO, known formally as [[2-methyl-2-nitrosopropane]], or ''t''-BuNO, which is prepared by the following sequence:<ref>A. Calder, A. R. Forrester, and S. P. Hepburn "2-Methyl-2-nitrosopropane and Its Dimer" ''[[Organic Syntheses]]'', Coll. Vol. 6, p.803; Vol. 52, p.77. [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0803 Link]</ref>
:(CH<sub>3</sub>)<sub>3</sub>CNH<sub>2</sub> → (CH<sub>3</sub>)<sub>3</sub>CNO<sub>2</sub>
:(CH<sub>3</sub>)<sub>3</sub>CNO<sub>2</sub> → (CH<sub>3</sub>)<sub>3</sub>CNHOH
:(CH<sub>3</sub>)<sub>3</sub>CNHOH → (CH<sub>3</sub>)<sub>3</sub>CNO
(CH<sub>3</sub>)<sub>3</sub>CNO is blue and exists in solution in [[Chemical equilibrium|equilibrium]] with its [[dimer (chemistry)|dimer]], which is colorless, m.p.&nbsp;80–81 °C.

In the [[Fischer-Hepp rearrangement]] aromatic 4-nitroso-anilines are prepared from the corresponding [[nitrosamine]]s. Another named reaction involving a nitroso compound is the [[Barton reaction]].

Organonitroso compounds serve as a ligands for transition metals.<ref>Pilato, R. S.; McGettigan, C.; Geoffroy, G. L.; Rheingold, A. L.; Geib, S. J. "tert-Butylnitroso complexes. Structural characterization of W(CO)<sub>5</sub>(N(O)Bu-tert) and [CpFe(CO)(PPh<sub>3</sub>)(N(O)Bu-tert)]<sup>+</sup>" ''Organometallics'' 1990, vol. 9, pp. 312–17. {{DOI|10.1021/om00116a004}}</ref>

== Nitrosation vs. nitrosylation ==
[[Nitrite]] can enter two kinds of reaction, depending on the physico-chemical environment. 
* [[Nitrosylation]] is adding a nitrosyl ion NO<sup>−</sup> to a metal (e.g. iron) or a thiol, leading to nitrosyl iron Fe-NO (e.g., in nitrosylated heme = nitrosylheme) or S-nitrosothiols (RSNOs).
* [[Nitrosation]] is adding a nitrosonium ion NO<sup>+</sup> to an amine -NH<sub>2</sub> 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:
: NO<sub>2</sub><sup>-</sup>  +  H<sup>+</sup>  <math>\overrightarrow{\leftarrow}</math>  HONO
: HONO  +  H<sup>+</sup>  <math>\overrightarrow{\leftarrow}</math>  H<sub>2</sub>O  +  NO<sup>+</sup>
: C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>  +  NO<sup>+</sup>   →   C<sub>6</sub>H<sub>5</sub>N(H)NO  +  H<sup>+</sup>
Many primary alkyl N-nitroso compounds, such as CH<sub>3</sub>N(H)NO, tend to be unstable with respect to hydrolysis to the alcohol.  Those derived from secondary amines (e.g., (CH<sub>3</sub>)<sub>2</sub>NNO derived from [[dimethylamine]]) are more robust.  It is these N-nitrosamines that are carcinogens in rodents.  

===In food===
[[Image:Nitrosyl-Heme.png|thumb|right|350px|Nitrosyl-heme]]
In foodstuffs and in the gastro-intestinal tract, nitrosation and nitrosylation do not have the same consequences on consumer health. 
* In [[cured meat]]: Meat processed by [[Curing (food preservation)|curing]] contains nitrite and has a pH of 5 approximately, where almost all nitrite is present as NO<sub>2</sub><sup>−</sup> (99%). Cured meat is also added with [[sodium ascorbate]] (or erythorbate or Vitamin C). As demonstrated by S. Mirvish, ascorbate inhibits nitrosation of amines to nitrosamine, because ascorbate reacts with NO<sub>2</sub><sup>−</sup> to form NO.<ref>{{cite journal | pmid = 5041776 | volume=177 | issue=4043 | title=Ascorbate-nitrite reaction: possible means of blocking the formation of carcinogenic N-nitroso compounds |date=July 1972 | journal=Science | pages=65–8 | doi=10.1126/science.177.4043.65|bibcode = 1972Sci...177...65M }}</ref><ref>{{cite journal | pmid = 3756808 | volume=58 | issue=8 Suppl | title=Effects of vitamins C and E on N-nitroso compound formation, carcinogenesis, and cancer |date=October 1986 | journal=Cancer | pages=1842–50}}</ref> Ascorbate and pH 5 thus favor nitrosylation of heme iron, forming nitrosyl-heme, a red pigment when included inside myoglobin, and a pink pigment when it has been released by cooking. It participates to the "bacon flavor" of cured meat: nitrosyl-heme is thus considered a benefit for the meat industry and for consumers.<ref>Honikel, K. O., 2008, "The use an control of nitrate and nitrite for the processing of meat products", ''Meat Science'', 78, 68-76. doi 10.1016/j.meatsci.2007.05.030</ref>  

* In the [[stomach]]: secreted HCl makes an acidic environment (pH=2) and ingested nitrite (with food or saliva) leads to nitrosation of amines, that yields [[nitrosamine]]s (potential carcinogens). Nitrosation is low if amine concentration is low (e.g., low-protein diet, no fermented food) or if Vitamin C concentration is high (e.g., high fruit diet). Then S-nitrosothiols are formed, that are stable at pH 2. 

* In the [[Colon (anatomy)|colon]]: neutral pH does not favor nitrosation. No nitrosamine is formed in stools, even after addition of a secondary amine or nitrite.<ref>{{cite journal | pmid = 7285009 | volume=41 | issue=10 | title=Absence of volatile nitrosamines in human feces |date=October 1981 | journal=Cancer Res. | pages=3992–4}}</ref> Neutral pH favors NO<sup>−</sup> release from S-nitrosothiols, and nitrosylation of iron. The previously called NOC (N-nitroso compounds) measured by Bingham's team in stools from red meat-fed volunteers<ref>{{cite journal | pmid = 8631138 | volume=17 | issue=3 | title=Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer? |date=March 1996 | journal=Carcinogenesis | pages=515–23}}</ref> were, according to Bingham and Kuhnle, largely non-N-nitroso ATNC (Apparent Total Nitroso Compounds), e.g.,  S-nitrosothiols and nitrosyl iron (as nitrosyl heme).<ref>{{cite journal | doi = 10.1016/j.freeradbiomed.2007.03.011 | pmid = 17761300 | volume=43 | issue=7 | title=Diet-induced endogenous formation of nitroso compounds in the GI tract |date=October 2007 | journal=Free Radic. Biol. Med. | pages=1040–7}}</ref>

== 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==
<references/>

{{Functional Groups}}

[[Category:Functional groups]]
[[Category:Nitroso compounds| ]]
[[Category:Nitrosyl compounds|Nitrosyl compounds]]