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 as a carbene.

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.^[1]

dimensions of the trans form (from the microwave spectrum)	model of the trans form	cis form

Preparation

Nitrous acid can be prepared by adding any mineral acid to sodium nitrite.

Decomposition

Nitrous acid rapidly decomposes into nitrogen dioxide, nitric oxide, and water when in solution.

2HNO₂ ? NO₂ + NO + H₂O

It also decomposes into nitric acid and nitrous oxide and water.

4HNO₂ ? 2HNO₃ + N₂O + H₂O

Chemistry

Nitrous acid is used to prepare diazonium salts:

HNO₂ + ArNH₂ + H⁺ ? ArN₂⁺ + 2 H₂O

Such salts are widely is 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.^[2] 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:^[3]

NaNO₂ + HCl ? HNO₂ + NaCl

2 NaN₃ + 2 HNO₂ ? 3 N₂ + 2 NO + 2 NaOH

Atmosphere of the earth

Nitrous acid is involved in the ozone budget of the lower atmosphere: the troposphere. The heterogenous reaction of nitrous oxide (NO₂) 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

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