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Nitrites: properties, structure, nomenclature, formation

Thus, sodium nitrite, NaNO 2 , is an example of a nitrite compound or salt. These salts are found in nature, whether in soils, seas, mammals and plant tissues, as they are part of the biological nitrogen cycle. Therefore, NO  is a substance present in our organisms, and it is linked to vasodilator processes.

Full-space model of the nitrite anion. Source: Benjah-bmm27.

Nitrite is a form of nitrogen that is more reduced, or less oxidized, than nitrate, NO  . Nitric oxide, NO, is produced from it in regions of the body where there is oxygen deficiency. NO is a signaling molecule which exerts a vasodilator effect by relaxing the muscles and arteries.

Outside of biochemistry and medicine, nitrites are used as bactericides, their addition to meats being quite common. This with the purpose of curing them and prolonging their quality and freshness for a longer time.

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Nitrite properties

Basicity

Nitrites are basic salts, since the anion NO  is the conjugate base of nitrous acid, HNO 2 :

HNO 2 + H 2 O ⇌ NO  + H 3 O +

In water and in small quantities, it hydrolyzes to produce OH  ions :

NO  + H 2 O ⇌ HNO 2 + OH 

This basicity is intrinsic for all nitrite salts, since it depends on NO  and not on the cations that accompany it. However, these cations and their interactions with NO  do affect the solubilities of nitrite salts in water and other polar solvents.

Reactions

Nitrite reactions vary depending on which cations accompany NO  , or if it is an organic nitrite, RONO. In general terms, nitrites can decompose, oxidize or reduce to: metal oxides, NO  , NO 2 , NO, and even nitrogen gas, N 2 .

For example, ammonium nitrite, NH 4 NO 2 can decompose to N 2 and H 2 O.

Physical appearances

Almost all nitrites are solids with a crystalline appearance. Many are whitish or yellowish in color, although some are colored if they contain transition metal cations.

On the other hand, most organic nitrites are highly unstable and explosive volatile liquids.

Structure

Nitrite anion

Resonance structures of the nitrite anion. Source: Nothingserious / Public domain

In the first image the nitrite anion was shown with a full-space model. This model has the disadvantage that it is exactly the same as that of nitrogen dioxide, NO 2 . Instead, the upper image better describes the nitrite anion and how its negative charge behaves.

Solid

Inorganic nitrites are all solid crystalline compounds. Their interactions are purely electrostatic, with an attraction between NO  and M n + cations . Thus, for example, NaNO 2 has an orthorhombic crystal structure, and is made up of Na + and NO  ions .

The type of crystal structure will depend on the identity of M + n , so not all nitrites share an orthorhombic structure.

Organic nitrites

Organic nitrites, unlike inorganic ones, are not ionic but covalent compounds. They are therefore made up of molecules, which are characterized by having an R-ONO bond, where R can be an alkyl or aromatic group.

They are considered nitrous acid esters, since their hydrogen, H-ONO, is replaced by an R group:

General formula for an organic nitrite. Source: Pngbot via Wikipedia.

According to the image above, this ester could be written as RON = O, very similar to the formula for carbon esters, ROC = O. Note the great similarity this structure has to that of nitro compounds, RNO 2 , where now the main bond is R-NO 2 and not R-ONO. The only difference therefore lies in the atom with which NO 2 binds to the R group.

It is for this reason that nitrite esters and nitro compounds are considered bonding isomers, having the same atoms, but bonded differently.

Complexes

Nitrite complexes can have both inorganic and organic components. In them, a coordination bond is formed between a metal center and one of the NO  oxygens . That is, we do not speak of a purely electrostatic interaction, M n + NO  , but of a coordination M n + -ONO  .

Organic nitrites and their complexes will establish or not crystalline structures depending on whether their interactions manage to fix their particles in an orderly way in space.

Nomenclature

Inorganic and organic nitrites share a remarkably simple nomenclature. To name them, the words ‘nitrite of’ are placed first, followed by the name of the metal and its valence written in parentheses. Likewise, the endings –ico and –oso can be used if there is more than one valence.

For example, CuNO 2 can be named in two ways: copper (I) nitrite, or cuprous nitrite.

This naming rule also applies to organic nitrites. For example, CH 3 ONO is called methyl nitrite, since CH 3 corresponds to the R group bound to the oxygen of NO 2 .

The nomenclature can become complicated if there are other groups of equal or greater chemical relevance than NO 2 , or if they are metal complexes.

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Nitrification

Many inorganic nitrites are formed in nature in one of the steps of the nitrogen cycle: nitrification. This process consists of the oxidation of ammonia carried out by microorganisms, specifically, by nitrosomonas bacteria.

Nitrification also encompasses the subsequent oxidation of nitrite to nitrate; however, the formation of nitrite is the slow step of these reactions, since it requires more energy and must overcome a greater kinetic barrier.

The following equation exemplifies the above:

2NH +    + 3O 2    → 2NO    + 4H +      + 2H 2 O

Several enzymes participate in this reaction and hydroxylamine is produced, NH 2 OH, which is the product from which the nitrite anions will eventually originate.

It is thanks to nitrification that plants contain nitrites, and in turn, the animals that consume them. Nitrites are not only present in the soil, but also in the seas, and almost all of their natural production is due to the oxidative and anaerobic action of various microorganisms.

Nitrogen oxides in basic media

The nitrites of the alkali metals can be prepared industrially by bubbling nitrogenous oxides in solutions or basic media, either of their respective hydroxides or carbonates. For example, sodium nitrite is produced according to the following reaction:

NO + NO 2   + NaOH → 2NaNO 2   + H 2 O

In the case of ammonium nitrite, dinitrogen trioxide is bubbled into ammonia:

2NH 3    + H 2 O + N 2 O 3    → 2NH 4 NO 2

Reaction with nitric oxide

Alkyl nitrites, RONO, can be synthesized by reacting alkyl alcohols with nitric oxide, in contact with air and using organic solvents as the reaction medium. The general reaction would be the following:

R-OH → R-ONO

Nitrite application

Cured meats

The reddish color of meats is due to its curing with nitrite and nitrate salts. Source: Pxhere.

Nitrites have antibacterial effects, so they are added in moderate amounts to meat to slow down rotting. In addition to fulfilling this function, they react with the proteins of the meat to give them a more reddish and attractive color.

The downside to this practice is that some meats can have too many nitrite salts, and when cooked at high temperatures, they are transformed into nitrosoamines. Therefore, there is a risk of increasing the chances of contracting some type of cancer if the consumption of these cured meats is excessive.

Azo dyes

Nitrites are used in organic chemistry to carry out the diazotization reaction, with which azo dyes are synthesized.

Pigments

Some nitrite complexes, such as cobalt complexes, can be used as pigments for paints or porcelains due to their striking colorations.

Vasodilator

Nitrites are responsible for generating nitric oxide in the body in oxygen-deficient regions. NO is a signaling molecule, to which the blood vessels respond by relaxing and expanding. This expansion or dilation has the consequence of lowering blood pressure.

Examples of nitrites

Finally, some examples of nitrites will be listed with their respective formulas and names:

-NaNO 2 : sodium nitrite

-KNO 2 : potassium nitrite

-Mg (NO 2 ) 2 : magnesium nitrite

-Ca (NO 2 ) 2 : calcium nitrite

-CH 3 CH 2 ONO: ethyl nitrite

-NH 4 NO 2 : ammonium nitrite

-Zn (NO 2 ) 2 : zinc nitrite

-Pb (NO 2 ) 2 : lead (II) nitrite or plumbose nitrite

-Fe (NO 2 ) 3 : iron (III) nitrite or ferric nitrite

– (CH 3 ) 2 CHCH 2 CH 2 ONO: isoamyl nitrite

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