Nitrite
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Nitrite is a polyatomic ion, which has the chemical formula NO2-. Nitrites are salts and esters of nitrous acid.
Contents
Structure
The nitrite ion has a symmetrical structure (C2v symmetry), with both N–O bonds having equal length and a bond angle of about 115°. In valence bond theory, it is described as a resonance hybrid with equal contributions from two canonical forms that are mirror images of each other. In molecular orbital theory, there is a sigma bond between each oxygen atom and the nitrogen atom, and a delocalized pi bond made from the p 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 a lone pair of electrons. Therefore, the nitrite ion is a Lewis base.
Properties
Nitrite is the conjugate base of the weak acid nitrous acid:
- HNO2 ⇌ H+ + NO−
2; pKa ≈ 3.3 at 18 °C
The formal oxidation 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. Standard reduction potentials for reactions directly involving nitrous acid are shown in the table below:
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
In organic chemistry, nitrites are esters 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 an aryl or alkyl group. Isopropyl nitrite and other alkyl nitrites are used in medicine for the treatment of heart diseases.
Preparation
Nitrite salts can be prepared by bubbling a mixture of nitrogen dioxide and nitric oxide through an alkali solution, in the absence of air.
Reduction of nitrates with a reducing agent of choice also yields nitrites.
Organic nitrites can be prepared by reacting an acidified nitrite salt with an alcohol.
Safety
Nitrite (ingested) under conditions that result in endogenous nitrosation has been classified as "Probably carcinogenic to humans" by IARC.
