Nitrous acid
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2008-07-02T13:32:05Z
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robot Adding: [[hu:Salétromossav]]
{{Chembox new
| Name = Nitrous acid
| ImageFile = Nitrous acid acsv.svg
| ImageSize = 200px
| ImageName = Nitrous acid
| OtherNames = Nitrous acid
| OtherNames =
| Section1 = {{Chembox Identifiers
| CASNo = 7782-77-6
}}
| Section2 = {{Chembox Properties
| Formula = [[Hydrogen|H]][[Nitrogen|N]][[Oxygen|O]]<sub>2</sub>
| MolarMass = 47.0134 g/mol
| Density = ? g/cm<sup>3</sup>
| Solvent = other solvents
| SolubleOther =
| MeltingPt =
| BoilingPt = }}
}}
'''Nitrous acid''' (molecular formula [[Hydrogen|H]][[Nitrogen|N]][[Oxygen|O]]<sub>2</sub>) is a weak and monobasic [[acid]] known only in [[solution]] and in the form of [[nitrite]] salts.
Nitrous acid is used to make [[diazo|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]].
==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 [[infrared spectroscopy|IR measurements]] indicate it is [[Gibbs free energy|more stable]] by around 2.3 kJ mol<sup>−1</sup>.<ref>{{Greenwood&Earnshaw}} p. 462</ref>.
{|align="center" class="wikitable"
|<center>[[Image:Trans-nitrous-acid-2D-dimensions.png|200px]]</center>||<center>[[Image:Trans-nitrous-acid-3D-balls.png|150px]]</center>||<center>[[Image:Cis-nitrous-acid-3D-balls.png|130px]]</center>
|-
|<center>dimensions of the ''trans'' form<br>(from the [[rotational spectroscopy|microwave spectrum]])</center>||<center>[[ball-and-stick model|model]] of the ''trans'' form</center>||<center>''cis'' form</center>
|}
==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.
2HNO<sub>2</sub> → NO<sub>2</sub> + NO + H<sub>2</sub>O
It also decomposes into [[nitric acid]] and [[nitrous oxide]] and water.
4HNO<sub>2</sub> → 2HNO<sub>3</sub> + N<sub>2</sub>O + H<sub>2</sub>O
==Chemistry==
Nitrous acid is used to prepare [[diazonium salt]]s:
:HNO<sub>2</sub> + [[Aryl|Ar]]NH<sub>2</sub> + H<sup>+</sup> → ArN<sub>2</sub><sup>+</sup> + 2 H<sub>2</sub>O
Such salts are widely is used in [[organic synthesis]], e.g., for the [[Sandmeyer reaction]] and in the preparation [[azo dye]]s, brightly-colored compounds that are the basis of a qualitative test for [[aniline]]s.<ref>Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941). http://www.orgsyn.org/orgsyn/pdfs/CV1P0374.pdf</ref> 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]]:<ref>{{cite book | title = Prudent practices in the laboratory: handling and disposal of chemicals | year = 1995 | publisher = [[National Academy Press]] | city = Washington, D.C. | isbn = 0309052297 | url = http://books.nap.edu/openbook.php?record_id=4911&page=165}}</ref>
: NaNO<sub>2</sub> + HCl → HNO<sub>2</sub> + NaCl
: 2 NaN<sub>3</sub> + 2 HNO<sub>2</sub> → 3 N<sub>2</sub> + 2 NO + 2 NaOH
==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]] (NO<sub>2</sub>) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric [[aerosol]]s, product readily [[photolysis|photolyses]] to [[hydroxyl]] [[radical]]s.
==See also==
{{commonscat|nitrous acid}}
* [[Demjanov rearrangement]]
* [[Nitric acid]] ([[Hydrogen|H]][[Nitrogen|N]][[Oxygen|O]]<sub>3</sub>)
* [[Tiffeneau-Demjanov rearrangement]]
==References==
{{reflist}}
[[Category:Nitrites]]
[[Category:Acids]]
[[Category:Nitrogen metabolism]]
{{inorganic-compound-stub}}
[[bg:Азотиста киселина]]
[[cs:Kyselina dusitá]]
[[de:Salpetrige Säure]]
[[es:Ácido nitroso]]
[[eo:Nitrita acido]]
[[fr:Acide nitreux]]
[[it:Acido nitroso]]
[[he:חומצה חנקיתית]]
[[hu:Salétromossav]]
[[nl:Waterstofnitriet]]
[[ja:亜硝酸]]
[[pl:Kwas azotowy(III)]]
[[pt:Ácido nitroso]]
[[ru:Азотистая кислота]]
[[sk:Kyselina dusitá]]
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[[uk:Азотиста кислота]]
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