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.[3]
Nitrous acid is usually generated by acidification of aqueous solutionsofsodium nitrite with a mineral acid. The acidification is usually conducted at ice temperatures, and the HNO2 is consumed in situ.[4][5] Free nitrous acid or concentrated solutions are unstable and decompose rapidly.
Nitrous acid can also be produced by dissolving dinitrogen trioxide in water according to the equation
With N 2H+ 5, 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.
I2 + 2 e− ⇌ 2 I−Eo = +0.54 V
NO− 3 + 3 H+ + 2 e− ⇌ HNO2 + H2OEo = +0.93 V
HNO2 + H+ + e− ⇌ NO + H2OEo = +0.98 V
It can be seen that the values of Eo cell for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Base on the fact that dilute nitrous acid can oxidize iodide into iodine, it can be deduced that nitrous is a faster, rather than a more powerful, oxidizing agent than dilute nitric acid.[7]
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.[8] 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:[9]
It is mainly blue in colour
NaNO2 + HCl → HNO2 + NaCl
2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH
Reaction with two α-hydrogen atoms in ketones creates oximes, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of adipic acid.
^Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 156. ISBN0-08-029214-3. LCCN82-16524.
^Kameoka, Yohji; Pigford, Robert (February 1977). "Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide, and Alkaline Sodium Sulfite Aqueous". Ind. Eng. Chem. Fundamen. 16 (1): 163–169. doi:10.1021/i160061a031.
^ abHousecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 449. ISBN978-0-13-175553-6.
^Da Roza, R.; Friedberg, E. C.; Duncan, B. K.; Warner, H. R. (1977-11-01). "Repair of nitrous acid damage to DNA in Escherichia coli". Biochemistry. 16 (22): 4934–4939. doi:10.1021/bi00641a030. ISSN0006-2960. PMID334252.
^Hartman, Z.; Henrikson, E. N.; Hartman, P. E.; Cebula, T. A. (1994). "Molecular models that may account for nitrous acid mutagenesis in organisms containing double-stranded DNA". Environmental and Molecular Mutagenesis. 24 (3): 168–175. doi:10.1002/em.2850240305. ISSN0893-6692. PMID7957120.
