| Nitrous acid | |
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Nitrous acid |
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Hydroxidooxidonitrogen |
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| Identifiers | |
| CAS number | 7782-77-6 |
| PubChem | 24529 |
| ChemSpider | 22936 |
| EC number | 231-963-7 |
| KEGG | C00088 |
| MeSH | Nitrous+acid |
| ChEBI | CHEBI:25567 |
| ChEMBL | CHEMBL1161681 |
| Gmelin Reference | 983 |
| 3DMet | B00022 |
| Jmol-3D images | Image 1 |
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O=NO |
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InChI=1S/HNO2/c2-1-3/h(H,2,3) |
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| Properties | |
| Molecular formula | HNO2 |
| Molar mass | 47.013 g/mol |
| Appearance | Pale blue solution |
| Density | Approx. 1 g/ml |
| Melting point |
Only known in solution |
| Acidity (pKa) | 3.398 |
| Hazards | |
| EU Index | Not listed |
| Flash point | Non-flammable |
| Related compounds | |
| Other anions | Nitric acid |
| Other cations | Sodium nitrite Potassium nitrite Ammonium nitrite |
| Related compounds | Dinitrogen trioxide |
 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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.
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.
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(from the microwave spectrum) |
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When cold, dilute solutions of nitrite ion, NO2 are carefully acidified, a light blue solution of nitrous acid is produced. Free nitrous acid is unstable and decomposes rapidly.
In anything other than very dilute, cold solutions, nitrous acid rapidly decomposes into nitrogen dioxide, nitric oxide, and water:
Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution:
In warm or concentrated solutions, the overall reaction amounts to production of nitric acid, water, and nitric oxide:
Nitrous acid can be oxidized to nitrate by powerful oxidizing agents, such as acidified potassium permanganate:
Reduction of the acid gives different products, depending on the reducing agent:
With I and Fe ions, NO is formed:
With Sn ions, N2O is formed:
With SO2 gas, NH2OH is formed:
With Zn in alkali solution, NH3 is formed:
With N2H5, HN3, and subsequently, N2 gas is formed:
Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I to I2, but dilute nitric acid cannot.
It can be seen that the values of Ecell for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Base on the fact that dilute nitrous acid can oxidize iodide into iodine, it can be deduced that nitrous is a more faster, rather than a more powerful oxidizing agent than dilute nitric aid.
Nitrous acid is used to prepare diazonium salts:
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: It is aminly blue in colour
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:
Nitrous acid is involved in the ozone budget of the lower atmosphere: the troposphere. The heterogeneous reaction of nitric oxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric aerosols, the product readily photolyses to hydroxyl radicals.
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