# Nitrous acid

Nitrous acid (molecular formula HNO $_2$) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (NO$^−$ $_2$) salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

## Structure

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.
* Dimensions of the anti form (from the microwave spectrum)
* Model of the anti form
* syn form

## Preparation and decomposition

Free, gaseous nitrous acid is unstable, rapidly disproportionating to nitric oxides:
    2 HNO$_2$ → NO$_2$ + NO + H$_2$O
In aqueous solution, the nitrogen dioxide also disproportionates, for a net reaction producing nitric oxide and nitric acid:$^{:1}$
    3 HNO$_2$ → 2 NO + HNO$_3$ + H$_2$O
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$_2$ consumed in situ.
Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:$^{:2}$
    N$_2$O$_3$ + H$_2$O ⇌ 2 HNO$_2$
Addition of dinitrogen trioxide to water is thus another preparatory technique.

## Chemical applications

Nitrous acid is the main chemophore in the Liebermann reagent, used to spot-test for alkaloids.
At high acidities (pH ≪ 2), nitrous acid is protonated to give water and nitrosonium cations.$^{:2}$

### Reduction

With I$^−$ and Fe$^{2+}$ ions, NO is formed:
    2 HNO$_2$ + 2 KI + 2 H$_2$SO$_4$ → I$_2$ + 2 NO + 2 H$_2$O + 2 K$_2$SO$_4$
    2 HNO$_2$ + 2 FeSO$_4$ + 2 H$_2$SO$_4$ → Fe$_2$(SO$_4$)$_3$ + 2 NO + 2 H$_2$O + K$_2$SO$_4$
With Sn$^{2+}$ ions, N$_2$O is formed:
    2 HNO$_2$ + 6 HCl + 2 SnCl$_2$ → 2 SnCl$_4$ + N$_2$O + 3 H$_2$O + 2 KCl
With SO$_2$ gas, NH$_2$OH is formed:
    2 HNO$_2$ + 6 H$_2$O + 4 SO$_2$ → 3 H$_2$SO$_4$ + K$_2$SO$_4$ + 2 NH$_2$OH
With Zn in alkali solution, NH$_3$ is formed:
    5 H$_2$O + KNO$_2$ + 3 Zn → NH$_3$ + KOH + 3 Zn(OH)$_2$
With N $_2$H$^+$ $_5$, both HN$_3$ and (subsequently) N$_2$ gas are formed:
    HNO$_2$ + [N$_2$H$_5$]$^+$ → HN$_3$ + H$_2$O + H$_3$O$^+$
    HNO$_2$ + HN$_3$ → N$_2$O + N$_2$ + H$_2$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$^−$ to I$_2$, but dilute nitric acid cannot.
    I$_2$ + 2 e$^−$ ⇌ 2 I$^−$ E$^o$ = +0.54 V
    NO$^−$ $_3$ + 3 H$^+$ + 2 e$^−$ ⇌ HNO$_2$ + H$_2$O E$^o$ = +0.93 V
    HNO$_2$ + H$^+$ + e$^−$ ⇌ NO + H$_2$O E$^o$ = +0.98 V
It can be seen that the values of E$^o$ $_{cell}$ 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.

### Organic chemistry

Nitrous acid is used to prepare diazonium salts:
    HNO$_2$ + ArNH$_2$ + H$^+$ → ArN$^+$ $_2$ + 2 H$_2$O
where Ar is an aryl group.
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. 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$_2$ + HCl → HNO$_2$ + NaCl
    2 NaN$_3$ + 2 HNO$_2$ → 3 N$_2$ + 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$_3$)$_2$CHCH$_2$CH$_2$OH + HNO$_2$ → (CH$_3$)$_2$CHCH$_2$CH$_2$ONO + H$_2$O
The carcinogens called nitrosamines are produced, usually not intentionally, by the reaction of nitrous acid with secondary amines:
    HNO$_2$ + R$_2$NH → R$_2$N-NO + H$_2$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.