Nitrous acid 379302 223076666 2008-07-02T13:32:05Z DorganBot 2844817 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]] [[sv:Salpetersyrlighet]] [[uk:Азотиста кислота]] [[zh:亚硝酸]]