Nitrous acid

Nitrous acid (molecular formula HNO2) is a weak 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
It can be prepared by adding any mineral acid to sodium nitrite.

Decomposition
It rapidly decomposes into nitrogen dioxide, nitric oxide and water when in solution.

2HNO2 &rarr; NO2 + NO + H2O

It also decomposes into nitric acid and nitrous oxide and water

4HNO2 &rarr; 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. 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

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).