Résumé
Nitrous acid (molecular formula HNO2) is a weak and monoprotic acid known only in solution, in the gas phase and in the form of nitrite (NO-2) salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes. In the gas phase, the planar nitrous acid molecule can adopt both a syn and an anti form. The anti form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3 kJ/mol. Image:Trans-nitrous-acid-2D-dimensions.png | Dimensions of the ''anti'' form(from the [[rotational spectroscopy|microwave spectrum]]) Image:Trans-nitrous-acid-3D-balls.png | [[ball-and-stick model|Model]] of the ''anti'' form Image:Cis-nitrous-acid-3D-balls.png | ''syn'' form Nitrous acid is usually generated by acidification of aqueous solutions of sodium nitrite with a mineral acid. The acidification is usually conducted at ice temperatures, and the HNO2 is consumed in situ. Free nitrous acid is unstable and decomposes rapidly. Nitrous acid can also be produced by dissolving dinitrogen trioxide in water according to the equation N2O3 + H2O → 2 HNO2 Nitrous acid is the main chemphore in the Liebermann reagent, used to spot-test for alkaloids. Dinitrogen trioxide Gaseous nitrous acid, which is rarely encountered, decomposes into nitrogen dioxide, nitric oxide, and water: 2 HNO2 → NO2 + NO + H2O Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution: 2 NO2 + H2O → HNO3 + HNO2 In warm or concentrated solutions, the overall reaction amounts to production of nitric acid, water, and nitric oxide: 3 HNO2 → HNO3 + 2 NO + H2O The nitric oxide can subsequently be re-oxidized by air to nitric acid, making the overall reaction: 2 HNO2 + O2 → 2 HNO3 With I− and Fe2+ ions, NO is formed: 2 HNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4 2 HNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4 With Sn2+ ions, N2O is formed: 2 HNO2 + 6 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O + 2 KCl With SO2 gas, NH2OH is formed: 2 HNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH With Zn in alkali solution, NH3 is formed: 5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2 With N2H5+, both HN3 and (subsequently) N2 gas are formed: HNO2 + [N2H5]+ → HN3 + H2O + H3O+ HNO2 + HN3 → N2O + N2 + H2O 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.
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