Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite () salts.

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Image:Trans-nitrous-acid-2D-dimensions.png | Dimensions of the anti form<br />(from the microwave spectrum)

Image:Trans-nitrous-acid-3D-balls.png | Model of the anti form

Image:Cis-nitrous-acid-3D-balls.png | syn form

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Decomposition and preparation

Free, gaseous nitrous acid is unstable, rapidly disproportionating to nitric oxides:

:2 HNO<sub>2</sub> → NO<sub>2</sub> + NO + H<sub>2</sub>O

In aqueous solution, the nitrous acid also disproportionates, for a net reaction producing nitric oxide and nitric acid:

:3&nbsp;HNO<sub>2</sub>&nbsp;→ 2&nbsp;NO&nbsp;+ HNO<sub>3</sub>&nbsp;+

Consequently applications of nitrous acid usually begin with mineral acid acidification of sodium nitrite. The acidification is usually conducted at ice temperatures, and the HNO<sub>2</sub> consumed in situ.

Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:

: 2 HNO<sub>2</sub> + 2 KI + 2 H<sub>2</sub>SO<sub>4</sub> → I<sub>2</sub> + 2 NO + 2 H<sub>2</sub>O + 2 K<sub>2</sub>SO<sub>4</sub>

: 2 HNO<sub>2</sub> + 2 FeSO<sub>4</sub> + 2 H<sub>2</sub>SO<sub>4</sub> → Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 2 NO + 2 H<sub>2</sub>O + K<sub>2</sub>SO<sub>4</sub>

With Sn<sup>2+</sup> ions, N<sub>2</sub>O is formed:

: 2 HNO<sub>2</sub> + 4 HCl + 2 SnCl<sub>2</sub> → 2 SnCl<sub>4</sub> + N<sub>2</sub>O + 3 H<sub>2</sub>O

With SO<sub>2</sub> gas, NH<sub>2</sub>OH is formed:

: 2 KNO<sub>2</sub> + 6 H<sub>2</sub>O + 4 SO<sub>2</sub> → 3 H<sub>2</sub>SO<sub>4</sub> + K<sub>2</sub>SO<sub>4</sub> + 2 NH<sub>2</sub>OH

With Zn in alkali solution, NH<sub>3</sub> is formed:

: 5 H<sub>2</sub>O + KNO<sub>2</sub> + 3 Zn → NH<sub>3</sub> + KOH + 3 Zn(OH)<sub>2</sub>

With , both HN<sub>3</sub> and (subsequently) N<sub>2</sub> gas are formed:

: HNO<sub>2</sub> + [N<sub>2</sub>H<sub>5</sub>]<sup>+</sup> → HN<sub>3</sub> + H<sub>2</sub>O + H<sub>3</sub>O<sup>+</sup>

: HNO<sub>2</sub> + HN<sub>3</sub> → N<sub>2</sub>O + N<sub>2</sub> + H<sub>2</sub>O

Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I<sup>−</sup> to I<sub>2</sub>, but dilute nitric acid cannot.

: I<sub>2</sub> + 2 e<sup>−</sup> ⇌ 2 I<sup>−</sup> E<sup>o</sup> = +0.54&nbsp;V

: + 3 H<sup>+</sup> + 2 e<sup>−</sup> ⇌ HNO<sub>2</sub> + H<sub>2</sub>O E<sup>o</sup> = +0.93&nbsp;V

: HNO<sub>2</sub> + H<sup>+</sup> + e<sup>−</sup> ⇌ NO + H<sub>2</sub>O E<sup>o</sup> = +0.98&nbsp;V

It can be seen that the values of E for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Based 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. 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:

It is mainly blue in colour

: 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

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.

Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl nitrites, which are potent vasodilators:

:(CH<sub>3</sub>)<sub>2</sub>CHCH<sub>2</sub>CH<sub>2</sub>OH + HNO<sub>2</sub> → (CH<sub>3</sub>)<sub>2</sub>CHCH<sub>2</sub>CH<sub>2</sub>ONO + H<sub>2</sub>O

The carcinogens called nitrosamines are produced, usually not intentionally, by the reaction of nitrous acid with secondary amines:

:HNO<sub>2</sub> + R<sub>2</sub>NH → R<sub>2</sub>N-NO + H<sub>2</sub>O

Atmosphere of the Earth

Nitrous acid is involved in the ozone budget of the lower atmosphere, the troposphere. The heterogeneous reaction of nitric oxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric aerosols, the product readily photolyses to hydroxyl radicals.

DNA damage and mutation

Treatment of Escherichia coli cells with nitrous acid causes damage to the cell's DNA including deamination of cytosine to uracil, and these damages are subject to repair by specific enzymes. Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.

See also

  • Demjanov rearrangement
  • Nitric acid (HNO<sub>3</sub>)
  • Nitrosyl-O-hydroxide
  • Tiffeneau-Demjanov rearrangement

References