nitrous acid
| Dictionary: nitrous acid |
n.
A weak inorganic acid, HNO2, existing only in solution or in the form of its salts.
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| Chemistry Dictionary: nitrous acid |
A weak acid, HNO2, known only in solution and in the gas phase. It is prepared by the action of acids upon nitrites, preferably using a combination that removes the salt as an insoluble precipitate (e.g. Ba(NO2)2 and H2SO4). The solutions are unstable and decompose on heating to give nitric acid and nitrogen monoxide. Nitrous acid can function both as an oxidizing agent (forms NO) with I − and Fe 2+, or as a reducing agent (forms NO3 −) with, for example, Cu 2+; the latter is most common. It is widely used (prepared in situ) for the preparation of diazonium compounds in organic chemistry. The full systematic name is dioxonitric(III) acid.
| Dental Dictionary: nitrous acid |
HNO2, a standard chemical reagent used in biologic and clinical laboratories.
| Wikipedia: Nitrous acid |
| Nitrous acid | |
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| IUPAC name |
Nitric (III) acid
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| Identifiers | |
| CAS number | 7782-77-6  |
| Properties | |
| Molecular formula | HNO2 |
| Molar mass | 47.013 g/mol |
| Appearance | Pale blue soution |
| 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 |
| (what is this?) (verify) 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 as a carbene.
Contents |
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]
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(from the microwave spectrum) |
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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.
- 2 HNO2 → NO2 + NO + H2O
It also decomposes into nitric acid and nitrous oxide and water.
- 4 HNO2 → 2 HNO3 + N2O + H2O
Chemistry
Nitrous acid is used to prepare diazonium salts:
- HNO2 + ArNH2 + H+ → ArN2+ + 2 H2O
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.[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
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Wikimedia Commons has media related to: nitrous acid |
References
- ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0-7506-3365-4 p. 462
- ^ Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0374.pdf
- ^ Prudent practices in the laboratory: handling and disposal of chemicals. National Academy Press. 1995. ISBN 0309052297. http://books.nap.edu/openbook.php?record_id=4911&page=165.
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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Mentioned in
- azotous
- nitrite
- azotite
- copper powder (metallurgy)
- diazotization (organic chemistry)
- Griess reagent (analytical chemistry)
- Riegler's test (analytical chemistry)
- elaidin reaction (analytical chemistry)
- mutagen
- nitranilic
- ricinelaidin
- Demjanov rearrangement (organic chemistry)
- Gmelin's test (pathology)
- elaidic
