Nitroso


Nitroso
Wire-frame model of nitroso moiety

Nitroso refers to a functional group in organic chemistry which has the general formula RNO. Nitroso compounds are a class of organic compounds containing the nitroso functional group, R−N=O.

Nitrosyls are molecules with the general formula RNO, where R represents an unspecified substituent.

Nitroso and bent Nitrosyl are synonyms; nitroso is used in organic chemistry, while bent nitrosyl is used in inorganic chemistry. Neither Nitroso nor bent Nitrosyl are identical to linear Nitrosil, which possesses sp hybridization. A common example of a nitroso compound is nitrosyl chloride, NOCl (although its structure is better represented ONCl).

Nitrosyl also refers to the discrete molecule nitric oxide, NO. 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

Contents

Nitrosyl as a ligand

Linear and bent metal nitrosyls

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 distinct modes: as NO+ and as NO. 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°.

Nitroso 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-nitroso-anilines are prepared from the corresponding nitrosamines. Another named reaction involving a nitroso compound is the Barton reaction.

Nitrosation vs. nitrosylation

Nitrite can enter two kinds of reaction, depending on the physico-chemical environment.

  • 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 occurs at acidic pH, particularly in the stomach, as shown in the figure below. Primary amines R-NH2 lead to unstable nitrosamines (that turn to alcool), but secondary amines R1-NH-R2 lead to stable nitrosamines, most of which are carcinogens in rodents.

Formation of n-nitrosamine.gif

In food

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 contains nitrite and has a pH of 5 approximately, where almost all nitrite is present as NO2 (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 NO2 to form NO.[2][3] 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.[4]
  • 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 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 favour nitrosation. No nitrosamine is formed in stools, even after addition of a secondary amine or nitrite.[5] 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[6] 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).[7]

See also

References

  1. ^ 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. Link
  2. ^ PMID 5041776
  3. ^ PMID 3756808
  4. ^ 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
  5. ^ PMID 7285009
  6. ^ PMID 8631138
  7. ^ PMID 17761300

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Look at other dictionaries:

  • nitroso — nitroso, sa (Del lat. nitrōsus). 1. adj. Que tiene nitro o se le parece en alguna de sus propiedades. 2. Quím. Se dice del ácido nitroso o de sus derivados. ☛ V. ácido nitroso …   Diccionario de la lengua española

  • Nitroso- — Ni*tro so (? or ?). (Chem.) A prefix (also used adjectively) designating the group or radical { NO}, called the nitroso group, or its compounds. [1913 Webster] …   The Collaborative International Dictionary of English

  • Nitroso... — Nitroso...,   Bezeichnung der chemischen Nomenklatur für die Gruppe NO als Substituent in organischen Verbindungen …   Universal-Lexikon

  • nitroso — |ô| adj. Nitrado; salitroso. • Plural: nitrosos |ó| …   Dicionário da Língua Portuguesa

  • nitroso — nitroso, sa adjetivo 1. Que tiene nitro o alguna de sus propiedades. 2. Área: química [Compuesto oxidado del nitrógeno] que tiene menos nitrógeno que el ácido nítrico …   Diccionario Salamanca de la Lengua Española

  • nitroso- — [nī trō′sō, nī trō′sə] [< L nitrosus, full of natron < nitrum: see NITER] combining form of or containing nitrosyl [nitrosamine]: also, before a vowel, nitros …   English World dictionary

  • nitroso — ► adjetivo 1 QUÍMICA Que contiene nitrógeno o tiene alguna de sus propiedades. 2 QUÍMICA Se aplica a los compuestos oxidados del nitrógeno en grado inferior al ácido nítrico. * * * nitroso, a (del lat. «nitrōsus») 1 adj. De [o del] nitro. ⊚… …   Enciclopedia Universal

  • Nitroso — Estructura general de un compuesto nitroso. En química orgánica, nitroso se refiere a un grupo funcional que tiene la fórmula general RNO. Los compuestos nitroso pueden ser preparados por reducción de nitroderivados, o por la oxidación de las… …   Wikipedia Español

  • nitroso — nī.ˈtrō(ˌ)sō adjective Etymology: nitros : containing or being the univalent group −NO used especially of organic compounds; compare nitro, nitros , nitrosyl 1 * * * /nuy troh soh/, adj. Chem. (esp. of organic compounds) containing the nitroso… …   Useful english dictionary

  • nitroso — /nuy troh soh/, adj. Chem. (esp. of organic compounds) containing the nitroso group; nitrosyl. [1880 85; independent use of NITROSO ] * * * …   Universalium


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