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Nitrous acid (molecular formula
HNO2) is a weak and monobasic
acid known only in
solution and in the form of
nitrite salts.
Nitrous acid is used to make diazides from amines; this occurs by nucleophilic attack of the amine onto the nitrite, reprotonation by the surrounding solvent, and double-elimination of water. The diazide can then be liberated to give a carbene or carbenoid.
Structure
In the gas phase, the planar nitrous acid molecule can adopt both a
cis and a
trans form. The
trans form predominates at room temperature, and
IR measurements indicate it is
more stable by around 2.3 kJ mol
−1.
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dimensions of the trans form(from the microwave spectrum)||
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cis form
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Preparation
When cold, dilute solutions of
nitrite ion, NO
2− are carefully acidified, a light blue solution of nitrous acid is produced. Free nitrous acid is unstable and decomposes rapidly.
Decomposition
In anything other than very dilute, cold solutions, nitrous acid rapidly decomposes into nitrogen dioxide, nitric oxide, and water:
:2 HNO2 → NO2 + NO + H2O
Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution:
:2 NO2 + H2O → HNO3 + HNO2
In warm or concentrated solutions, the overall reaction amounts to production of nitric acid, water, and nitric oxide:
:3 HNO2 → HNO3 + 2 NO + H2O
Chemistry
Nitrous acid is used to prepare
diazonium salts:
:HNO
2 + ArNH
2 + H
+ → ArN
2+ + 2 H
2O
where Ar is an aryl group.
Such salts are widely used in organic synthesis, e.g., for the Sandmeyer reaction and in the preparation azo dyes, brightly-colored compounds that are the basis of a qualitative test for anilines. Nitrous acid is used to destroy toxic and potentially-explosive sodium azide. For most purposes, nitrous acid is usually formed in situ by the action of mineral acid on sodium nitrite:
: NaNO2 + HCl → HNO2 + NaCl
: 2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH
Reaction with two α-hydrogen atoms in ketones creates oximes, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of adipic acid.
Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl nitrites, which are potent vasodilators:
:(CH3)2CH-CH2-CH2-OH + HNO2 → (CH3)2CH-CH2-CH2-ONO + H2O
Atmosphere of the earth
Nitrous acid is involved in the
ozone budget of the lower
atmosphere: the
troposphere. The
heterogeneous reaction of
nitrogen monoxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric
aerosols, product readily
photolyses to
hydroxyl radicals.
See also
Demjanov rearrangement
Nitric acid (HNO3)
Tiffeneau-Demjanov rearrangement
References
Category:Nitrogen oxoacids
Category:Nitrogen metabolism
Category:Oxidizing agents
