Nitrous acid

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Nitrous acid
Nitrous acid
Identifiers
CAS number [7782-77-6]
Properties
Molecular formula HNO2
Molar mass 47.0134 g/mol
Density  ? g/cm3
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and 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

[edit] 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].
dimensions of the trans form
(from the microwave spectrum)
model of the trans form
cis form

[edit] Preparation

Nitrous acid can be prepared by adding any mineral acid to sodium nitrite.

[edit] 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

[edit] 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.[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

[edit] Atmosphere of the earth

Nitrous acid is involved in the ozone budget of the 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.

[edit] See also

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[edit] References

  1. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.  p. 462
  2. ^ Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0374.pdf
  3. ^ (1995) Prudent practices in the laboratory: handling and disposal of chemicals. National Academy Press. ISBN 0309052297. 
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