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| Names | |||
|---|---|---|---|
| IUPAC name Nitrite | |||
| Systematic IUPAC name dioxidonitrate(1−) | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
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| UNII | |||
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| Properties | |||
| NO− 2 | |||
| Molar mass | 46.005 g·mol−1 | ||
| Conjugate acid | Nitrous acid | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Thenitriteion has thechemical formulaNO−
2. Nitrite (mostlysodium nitrite) is widely used throughout chemical and pharmaceutical industries.[1] The nitrite anion is a pervasive intermediate in thenitrogen cycle in nature. The name nitrite also refers to organic compounds having the –ONO group, which are esters ofnitrous acid.
Sodium nitrite is made industrially by passing a mixture of nitrogen oxides into aqueoussodium hydroxide orsodium carbonate solution:[2][1]
The product is purified by recrystallization. Alkali metal nitrites are thermally stable up to and beyond their melting point (441 °C for KNO2).Ammonium nitrite can be made fromdinitrogen trioxide, N2O3, which is formally theanhydride of nitrous acid:
The nitrite ion has a symmetrical structure (C2vsymmetry), with both N–O bonds having equal length and a bond angle of about 115°. Invalence bond theory, it is described as aresonance hybrid with equal contributions from two canonical forms that are mirror images of each other. Inmolecular orbital theory, there is asigma bond between each oxygen atom and the nitrogen atom, and a delocalizedpi bond made from thep orbitals on nitrogen and oxygen atoms which is perpendicular to the plane of the molecule. The negative charge of the ion is equally distributed on the two oxygen atoms. Both nitrogen and oxygen atoms carry alone pair of electrons. Therefore, the nitrite ion is aLewis base.
In the gas phase it exists predominantly as atrans-planar molecule.
Nitrite is the conjugate base of the weak acidnitrous acid:
Nitrous acid is also highly unstable, tending todisproportionate:
This reaction is slow at 0 °C.[2] Addition of acid to a solution of a nitrite in the presence of areducing agent, such as iron(II), is a way to makenitric oxide (NO) in the laboratory.
The formaloxidation state of the nitrogen atom in nitrite is +3. This means that it can be either oxidized to oxidation states +4 and +5, or reduced to oxidation states as low as −3. Standardreduction potentials for reactions directly involving nitrous acid are shown in the table below:[4]
| Half-reaction | E0 (V) |
|---|---|
| NO− 3 + 3 H+ + 2 e− ⇌ HNO2 + H2O | +0.94 |
| 2 HNO2 + 4 H+ + 4 e− ⇌ H2N2O2 + 2 H2O | +0.86 |
| N2O4 + 2 H+ + 2 e− ⇌ 2 HNO2 | +1.065 |
| 2 HNO2+ 4 H+ + 4 e− ⇌ N2O + 3 H2O | +1.29 |
The data can be extended to include products in lower oxidation states. For example:
Oxidation reactions usually result in the formation of thenitrate ion, with nitrogen in oxidation state +5. For example, oxidation withpermanganate ion can be used for quantitative analysis of nitrite (bytitration):
The product of reduction reactions with nitrite ion are varied, depending on thereducing agent used and its strength. Withsulfur dioxide, the products are NO and N2O; with tin(II) (Sn2+) the product ishyponitrous acid (H2N2O2); reduction all the way to ammonia (NH3) occurs withhydrogen sulfide. With thehydrazinium cation (N
2H+
5) the product of nitrite reduction ishydrazoic acid (HN3), an unstable and explosive compound:
which can also further react with nitrite:
This reaction is unusual in that it involves compounds with nitrogen in four different oxidation states.[2]
Nitrite is detected and analyzed by theGriess Reaction, involving the formation of a deep red-coloredazo dye upon treatment of aNO−
2-containing sample withsulfanilic acid and naphthyl-1-amine in the presence of acid.[5]
Nitrite is anambidentate ligand and can form a wide variety ofcoordination complexes by binding to metal ions in several ways.[2] Two examples are the red nitrito complex [Co(NH3)5(ONO)]2+ ismetastable,isomerizing to the yellownitro complex [Co(NH3)5(NO2)]2+. Nitrite is processed by several enzymes, all of which utilize coordination complexes.
Innitrification,ammonium is converted to nitrite. Important species includeNitrosomonas. Other bacterial species such asNitrobacter, are responsible for the oxidation of the nitrite into nitrate.
Nitrite can be reduced tonitric oxide orammonia by many species of bacteria. Under hypoxic conditions, nitrite may release nitric oxide, which causes potentvasodilation. Several mechanisms for nitrite conversion to NO have been described, including enzymatic reduction byxanthine oxidoreductase,nitrite reductase, andNO synthase (NOS), as well as nonenzymatic acidicdisproportionation reactions.
Azo dyes and other colorants are prepared by the process calleddiazotization, which requires nitrite.[1]
The addition ofnitrites andnitrates to processed meats such as ham, bacon, and sausages speeds up thecuring of meat and also impart an attractive colour.[8]
The academic and industrial consensus is that nitrites also reduces growth and toxin production ofClostridium botulinum.[9][10][11]On the other hand, a 2018 study (full text not available) by the British Meat Producers Association determined that legally permitted levels of nitrite do not affect the growth ofC. botulinum.[12] In the U.S., meat cannot be labeled as "cured" without the addition of nitrite.[13][14][15] In some countries, cured-meat products are manufactured withoutnitrate or nitrite, and without nitrite from vegetable sources.Parma ham, produced without nitrite since 1993, was reported in 2018 to have caused no cases of botulism. This is because the interior of the muscle is sterile and the surface is exposed to oxygen.[8] Other manufacture processes do not assure these conditions, and reduction of nitrite results in toxin production.[16]
In mice, food rich in nitrites together with unsaturated fats can preventhypertension by forming nitro fatty acids that inhibit solubleepoxide hydrolase, which is one explanation for the apparent health effect of theMediterranean diet.[17] Adding nitrites to meat has been shown to generate knowncarcinogens; theWorld Health Organization (WHO) advises that eating 50 g (1.8 oz) of nitrite processed meat a day would raise the risk of gettingbowel cancer by 18% over a lifetime.[8]
The recommended maximum limits by the World Health Organization indrinking water are 3 mg L−1 and 50 mg L−1 for nitrite andnitrate ions, respectively.[18] Ingesting too much nitrite and/or nitrate through well water is suspected to causemethemoglobinemia.[19]
95% of the nitrite ingested in modern diets comes from bacterial conversion of nitrates naturally found in vegetables.[20] However, potentially cancer-causing nitroso compounds are not made in the pH-neutral colon. They are mostly made in the acidic stomach.[21][22]
Nitrite reacts with the meat'smyoglobin by attaching to the heme iron atom, forming reddish-brown nitrosomyoglobin and the characteristic pink "fresh" color of nitrosohemochrome or nitrosyl-heme upon cooking.[23] In the US, nitrite has been formally used since 1925. According to scientists working for the industry groupAmerican Meat Institute, this use of nitrite started in theMiddle Ages.[24] Historians andepidemiologists argue that the widespread use of nitrite in meat-curing is closely linked to the development of industrial meat-processing.[25][26] French investigative journalistGuillaume Coudray [fr] asserts that the meat industry chooses to cure its meats with nitrite even though it is established that this chemical gives rise to cancer-causingnitroso-compounds.[27] Some traditional and artisanal producers avoid nitrites. As many researches nowadays try to point out the hazardous generation of nitrosamines as nitrites link with free peptides in the gastrointestinal system, the EU published a regulation which obliges the reduction of nitrites on meat curing from 150 to 80 ppm (mg/kg).[28]
Addition ofascorbic acid,erythorbic acid, or one of their salts enhance the binding of nitrite to the iron atom in myoglobin.[23] These chemicals also reduce the formation of nitrosamine in the stomach, but only when the fat content of a meal is less than 10%, beyond which they instead increase the formation of nitrosamine.[29][30]
Nitrites in the form ofsodium nitrite andamyl nitrite are components of manycyanideantidote kits.[31] Both of these compounds bind tohemoglobin and oxidize the Fe2+ ions to Fe3+ ions formingmethemoglobin. Methemoglobin, in turn, binds to cyanide (CN), creating cyanmethemoglobin, effectively removing cyanide from thecomplex IV of theelectron transport chain (ETC) inmitochondria, which is the primary site of disruption caused by cyanide. Another mechanism by which nitrites help treat cyanide toxicity is the generation ofnitric oxide (NO). NO displaces the CN from thecytochrome c oxidase (ETC complex IV), making it available for methemoglobin to bind.[32]
Inorganic chemistry,alkyl nitrites areesters of nitrous acid and contain the nitrosoxy functional group. Nitro compounds contain the C–NO2 group. Nitrites have the general formula RONO, where R is anaryl oralkyl group.Amyl nitrite and other alkyl nitrites have avasodilating action and must be handled in the laboratory with caution. They are sometimes used in medicine for the treatment of heart diseases. A classicnamed reaction for the synthesis of alkyl nitrites is theMeyer synthesis[33][34] in whichalkyl halides react with metallic nitrites to a mixture to nitroalkanes and nitrites.
Nitrite salts can react with secondaryamines to produceN-nitrosamines, which are suspected of causingstomach cancer. TheWorld Health Organization (WHO) advises that each 50 g (1.8 oz) of processed meat eaten a day would raise the risk of getting bowel cancer by 18% over a lifetime; processed meat refers to meat that has been transformed through fermentation, nitrite curing, salting, smoking, or other processes to enhance flavor or improve preservation. The World Health Organization's review of more than 400 studies concluded in 2015 that there was sufficient evidence that processed meats caused cancer, particularly colon cancer; the WHO'sInternational Agency for Research on Cancer (IARC) classified processed meats as carcinogenic to humans (Group 1).[8][35]
Nitrite (ingested) under conditions that result in endogenousnitrosation, specifically the production ofnitrosamine, has been classified asProbably carcinogenic to humans (Group 2A) by the IARC.[36]
Large doses of nitrite causes acute poisoning in the form ofmethemoglobinemia, which can lead to death.[37]
In trade journals of the 1960s, the firms who sold nitrite powders to ham-makers spoke quite openly about how the main advantage was to increase profit margins by speeding up production.
The results show that there is no change in levels of inoculatedC. botulinum over the curing process, which implies that the action of nitrite during curing is not toxic toC. botulinum spores at levels of 150ppm [parts per million] ingoing nitrite and below.
