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.

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.^[1] 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:^[2]

NaNO₂ + HCl → HNO₂ + NaCl

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

Atmospheric relevance

Nitrous acid is an important atmospheric intermediate. It is produced by the heterogeneous reaction of NO₂ and water on various surfaces such as atmospheric aerosols. It is readily photolysed to produce hydroxyl radicals, which are intricately involved in the ozone budget of the troposphere (lower atmosphere).

See also

• Demjanov rearrangement
• Nitric acid (HNO₃)
• Tiffeneau-Demjanov rearrangement

References

1. ^ Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0374.pdf
2. ^ (1995) Prudent practices in the laboratory: handling and disposal of chemicals. National Academy Press. ISBN 0309052297.