Nitrous acid (molecular formula ) is a weak and monoprotic acid known only in

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...

, in the gas phase and in the form of

nitrite The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also ...

() salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give

azo dye Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C-N ...

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. p. 462. Image:Trans-nitrous-acid-2D-dimensions.png , Dimensions of the ''anti'' form
(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

Preparation

Nitrous acid is usually generated by acidification of

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be r ...

s of

sodium nitrite Sodium nitrite is an inorganic compound with the chemical formula NaNO2. It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. From an industrial perspective, it is the most important nitrite ...

with a

mineral acid A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water. Cha ...

. 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

Reactions

Nitrous acid is the main chemphore in the Liebermann reagent, used to spot-test for alkaloids.

Decomposition

Gaseous nitrous acid, which is rarely encountered, decomposes into

nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...

nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...

, and water: :2 HNO₂ → NO₂ + NO + H₂O Nitrogen dioxide disproportionates into

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

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

Reduction

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 , both HN₃ and (subsequently) N₂ gas are formed: : HNO₂ + ₂H₅sup>+ → 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 : + 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'' 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 Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...

, 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⁺ → + 2 H₂O where Ar is an

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used ...

group. Such salts are widely used in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

, e.g., for the Sandmeyer reaction and in the preparation

s, brightly colored compounds that are the basis of a qualitative test for

aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starti ...

s. 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

: 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

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

s 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 Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: about 2.5 billion kilograms of this white crystalline powder are produced annuall ...

. Nitrous acid reacts rapidly with aliphatic alcohols to produce

alkyl nitrites In organic chemistry, alkyl nitrites are a group of organic compounds based upon the molecular structure , where R represents an alkyl group. Formally they are alkyl esters of nitrous acid. They are distinct from nitro compounds (). The first ...

, which are potent

vasodilators Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction ...

: :(CH₃)₂CHCH₂CH₂OH + HNO₂ → (CH₃)₂CHCH₂CH₂ONO + H₂O The carcinogens called

nitrosamine In organic chemistry, nitrosamines (or more formally ''N''-Nitrosamines) are organic compounds with the chemical structure , where R is usually an alkyl group. They feature a nitroso group () bonded to a deprotonated amine. Most nitrosamines a ...

s are produced, usually not intentionally, by the reaction of nitrous acid with secondary amines: :HNO₂ + R₂NH → R₂N-NO + H₂O

Atmosphere of the Earth

Nitrous acid is involved in the

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...

budget of the lower

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

, the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. Fro ...

. The

heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

reaction of

(NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric

aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthropogen ...

s, the product readily photolyses to

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...

radical Radical may refer to: Politics and ideology Politics * Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe an ...

References

{{Authority control Nitrogen oxoacids Nitrogen cycle Oxidizing agents Mineral acids Nitrogen(III) compounds