Nitroso

Structural formula 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, R1N(−R2)−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 nonmetal example is the common reagent nitrosyl chloride (Cl−N=O).

Nitric oxide is a stable radical, having an unpaired electron.

Reduction of nitric oxide gives the hyponitrite anion, NO:

NO + e → NO

Oxidation of NO yields the nitrosonium cation, NO+:

NO → NO+ + e

Nitrosyl as a ligand

Linear and bent metal nitrosyls
Main article: Metal nitrosyl

Nitric oxide can serve as a ligand in complexes. The resulting complexes are called metal nitrosyls, and can bond to a metal atom in two extreme modes: as NO+ and as NO. It is generally assumed that NO+ coordinates linearly, the M−N−O angle being 180°, whereas NO 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 compounds or by the oxidation of hydroxylamines. A good example is (CH3)3CNO, known formally as 2-methyl-2-nitrosopropane, or t-BuNO, which is prepared by the following sequence:[1]

(CH3)3CNH2 → (CH3)3CNO2
(CH3)3CNO2 → (CH3)3CNHOH
(CH3)3CNHOH → (CH3)3CNO

(CH3)3CNO is blue and exists in solution in equilibrium with its dimer, which is colorless, m.p. 80–81 °C.

In the Fischer–Hepp rearrangement aromatic 4-nitrosoanilines are prepared from the corresponding nitrosamines. Another named reaction involving a nitroso compound is the Barton reaction.

Organonitroso compounds serve as a ligands for transition metals.[2]

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.

In food

Nitrosyl-heme

In foodstuffs and in the gastro-intestinal tract, nitrosation and nitrosylation do not have the same consequences on consumer health.

See also

References

  1. Calder, A.; Forrester, A. R.; Hepburn, S. P. "2-Methyl-2-nitrosopropane and Its Dimer". Org. Synth. 52: 77.; Coll. Vol. 6,, p. 803
  2. Pilato, R. S.; McGettigan, C.; Geoffroy, G. L.; Rheingold, A. L.; Geib, S. J. (1990). "tert-Butylnitroso complexes. Structural characterization of W(CO)5(N(O)Bu-tert) and [CpFe(CO)(PPh3)(N(O)Bu-tert)]+". Organometallics 9: 312–17. doi:10.1021/om00116a004.
  3. "Ascorbate–nitrite reaction: possible means of blocking the formation of carcinogenic N-nitroso compounds". Science 177 (4043): 65–8. July 1972. Bibcode:1972Sci...177...65M. doi:10.1126/science.177.4043.65. PMID 5041776.
  4. "Effects of vitamins C and E on N-nitroso compound formation, carcinogenesis, and cancer". Cancer 58 (8 Suppl): 1842–50. October 1986. doi:10.1002/1097-0142(19861015)58:8+<1842::aid-cncr2820581410>3.0.co;2-#. PMID 3756808.
  5. 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.
  6. "Absence of volatile nitrosamines in human feces". Cancer Res. 41 (10): 3992–4. October 1981. PMID 7285009.
  7. "Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer?". Carcinogenesis 17 (3): 515–23. March 1996. doi:10.1093/carcin/17.3.515. PMID 8631138.
  8. "Diet-induced endogenous formation of nitroso compounds in the GI tract". Free Radic. Biol. Med. 43 (7): 1040–7. October 2007. doi:10.1016/j.freeradbiomed.2007.03.011. PMID 17761300.
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