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EspañolNITROUS ACID
| Nitrous acid | |
|---|---|
 Nitrous acid | |
| General | |
| Systematic name | Dioxonitric(III) acid |
| Other names | Nitrous acid |
| Molecular formula | HNO2 |
| Molar mass | 47.0134 g/mol |
| CAS number | 7782-77-6 |
| Properties | |
| Density | ? g/cm3 |
| Solubility (water) | |
| Melting point | ? °C |
| Boiling point | ? °C |
| Acid dissociation constant p''K''a | 3.34 |
'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.
| Contents |
| Preparation |
| Decomposition |
| Chemistry |
| Atmospheric relevance |
| See also |
| References |
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 → 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. Prudent practices in the laboratory: handling and disposal of chemicals, , , , National Academy Press, 1995,
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