Nitrous acid

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 IR measurements indicate it is more stable by around 2.3 kJ mol⁻¹.

Preparation

When cold, dilute solutions of nitrite ion, NO₂⁻ are carefully acidified, a light blue solution of nitrous acid is produced. Free nitrous acid is unstable and decomposes rapidly. It can be produced by dissolving dinitrogen trioxide in water according to the equation

N₂O₃ + H₂O → 2 HNO₂

Decomposition

In anything other than very dilute, cold solutions, nitrous acid rapidly 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

Inorganic chemistry

Reduction of the acid gives different products, depending on the reducing agent:

With I⁻ and Fe²⁺ ions, NO is formed:

2 KNO₂ + 2 KI + 2 H₂SO₄ → I₂ + 2 NO + 2 H₂O + 2 K₂SO₄2 KNO₂ + 2 FeSO₄ + 2 H₂SO₄ → Fe₂(SO₄)₃ + 2 NO + 2 H₂O + K₂SO₄

With Sn²⁺ ions, N₂O is formed:

2 KNO₂ + 6 HCl + 2 SnCl₂ → 2 SnCl₄ + N₂O + 3 H₂O + 2 KCl

With SO₂ gas, NH₂OH is formed:

2 KNO₂ + 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⁺, HN₃, and subsequently, N₂ gas is 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⁻ {E^o = +0.54 V}NO₃⁻ + 3 H⁺ + 2 e⁻ ⇌ HNO₂ + H₂O {E^o = +0.93 V}HNO₂ + H⁺ + e⁻ ⇌ NO + H₂O {E^o = +0.98 V}

It can be seen that the values of E_cell^o 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.

Organic chemistry

Nitrous acid is used to prepare diazonium salts:

HNO₂ + ArNH₂ + H⁺ → ArN₂⁺ + 2 H₂O

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

NaNO₂ + HCl → HNO₂ + NaCl2 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.

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

(CH₃)₂CH-CH₂-CH₂-OH + HNO₂ → (CH₃)₂CH-CH₂-CH₂-ONO + H₂O

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.