Utente:Grasso Luigi/sanbox1/Nitrosocomposti
Il termine nitroso si riferisce ad un gruppo funzionale in chimica organica che ha il gruppo NO collegato ad una porzione di molecola organica. I gruppi nitroso sono denominati come X-nitrosocomposti (e.g., C-nitrosocomposti (nitrosoalcani; R−N=O), S-nitrosocomposti (nitrosotioli; RS−N=O), N-nitrosocomposti (e.g., nitrosammine, R1N(−R2)−N=O), O-nitrosocomposti (alchil nitriti; RO−N=O).
Composti organonitroso
[modifica | modifica wikitesto]Il nitrosobenzene ((C6H5NO) viene preparato riducendo il nitrobenzene a fenilidrossilammina (C6H5NHOH), che viene poi ossidato.[1] I nitrosoareni tipicamente partecipano ad un equilibrio dimero-monomero. I dimeri, spesso di colore giallo pallido, sono favoriti allo stato solido, mentre i monomeri profondamente colorati sono favoriti in soluzione di diluizione o a temperature più elevate. Esistono come isomeri geometrici tipo cis- e trans-.[2]
I nitrosocomposti si possono preparare per riduzioni dei nitrocomposti o per ossidazione delle idrossilammine. Un buon esempio è (CH3)3CNO, detto 2-metil-2-nitrosopropano, o t-BuNO.[4] (CH3)3CNO è di colore blu ed esiste in soluzione in equilibrio col suo dimero, incolore, m.p. 80–81 °C.
Nel riarrangiamento di Fischer-Hepp, le 4-nitrosoaniline aromatiche sono preparate dalla corrispondenti nitrosammine. Un'altra reazione nota che coinvolge un nitrosocomposto è la reazione di Barton.
Organonitroso compounds serve as a ligands for transition metals.[5]
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 30 to 40 kcal/mol, while nitrosoarenes have BDEs on the order of 50 to 60 kcal/mol. 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, Ph2N–N=O, has a N–N bond dissociation energy of only 23 kcal/mol.[6]
Reazioni tipiche
[modifica | modifica wikitesto]Il nitrito può attivare due tipi di reazione, a seconda dell'ambiente fisico-chimico.
- Nitrosylation is adding a nitrosyl ion NO− 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+ to an amine –NH2 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−2 + H+ Template:Eqm HONO
- HONO + H+ Template:Eqm H2O + NO+
- C6H5NH2 + NO+ → C6H5N(H)NO + H+
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.
Composti inorganici: i nitrosili
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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−
Nitric oxide can serve as a ligand forming metal nitrosyl complexes or just metal nitrosyls. These complexes can be viewed as adducts of NO+, NO−, or some intermediate case.
In food
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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 contains nitrite and has a pH of 5 approximately, where almost all nitrite is present as NO−2 (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−2 to form NO.[7][8] Ascorbate and pH 5 thus favor nitrosylation of heme iron, forming nitrosylheme, 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: nitrosylheme is thus considered a benefit for the meat industry and for consumers.[9]
- In the stomach: secreted hydrogen chloride makes an acidic environment (pH=2) and ingested nitrite (with food or saliva) leads to nitrosation of amines, that yields nitrosamines (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: neutral pH does not favor nitrosation. No nitrosamine is formed in stools, even after addition of a secondary amine or nitrite.[10] Neutral pH favors NO− 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[11] 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).[12]
Voci correlate
[modifica | modifica wikitesto]- Nitrosammina, gruppo funzionale con NO collegato ad un'ammina, come R2N–NO
- Nitrosobenzene
- Ossido nitrico
- Nitrosile
Note
[modifica | modifica wikitesto]- ^ G. H. Coleman, C. M. McCloskey, F. A. Stuart, Nitrosobenzene, in Org. Synth., vol. 25, 1945, p. 80, DOI:10.15227/orgsyn.025.0080.
- ^ Dimerization of Aromatic C-Nitroso Compounds, in Chemical Reviews, vol. 116, 2016, pp. 258-286, DOI:10.1021/cr500520s.
- ^ E.Bosch, Structural Analysis of Methyl-Substituted Nitrosobenzenes and Nitrosoanisoles, in J. Chem. Cryst., vol. 98, 2014, DOI:10.1007/s10870-013-0489-8.
- ^ (EN) Organic Syntheses, vol. 52, http://www.orgsyn.org/demo.aspx?prep=cv6p0803.
- ^ tert-Butylnitroso complexes. Structural characterization of W(CO)5(N(O)Bu-tert) and [CpFe(CO)(PPh3)(N(O)Bu-tert)]+, in Organometallics, vol. 9, 1990, pp. 312–17, DOI:10.1021/om00116a004.
- ^ Yu-Ran Luo, Comprehensive Handbook of Chemical Bond Energies, Boca Raton, Fl., Taylor and Francis, 2007, ISBN 9781420007282.
- ^ Ascorbate–nitrite reaction: possible means of blocking the formation of carcinogenic N-nitroso compounds, in Science, vol. 177, n. 4043, July 1972, pp. 65–8, DOI:10.1126/science.177.4043.65.
- ^ <1842::aid-cncr2820581410>3.0.co;2-# Effects of vitamins C and E on N-nitroso compound formation, carcinogenesis, and cancer, in Cancer, vol. 58, 8 Suppl, October 1986, pp. 1842–50, DOI:10.1002/1097-0142(19861015)58:8+<1842::aid-cncr2820581410>3.0.co;2-#.
- ^ The use an control of nitrate and nitrite for the processing of meat products, in Meat Science, vol. 78, 2008, pp. 68–76, DOI:10.1016/j.meatsci.2007.05.030.
- ^ Absence of volatile nitrosamines in human feces, in Cancer Res., vol. 41, n. 10, October 1981, pp. 3992–4.
- ^ Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer?, in Carcinogenesis, vol. 17, n. 3, March 1996, pp. 515–23, DOI:10.1093/carcin/17.3.515.
- ^ Diet-induced endogenous formation of nitroso compounds in the GI tract, in Free Radic. Biol. Med., vol. 43, n. 7, October 2007, pp. 1040–7, DOI:10.1016/j.freeradbiomed.2007.03.011.