Nitrous acid

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Nitrous acid (molecular formula Template:Chem) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (Template:Chem) salts.<ref name=G&E/> 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.

StructureEdit

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.<ref name="G&E">Template:Greenwood&Earnshaw p. 462.</ref>

Preparation and decompositionEdit

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

2 HNO2 → NO2 + NO + H2O

In aqueous solution, the nitrogen dioxide also disproportionates, for a net reaction producing nitric oxide and nitric acid:<ref name="Nitros">Template:Cite book</ref>Template:Rp<ref>Template:Cite journal</ref>

3 HNO2 → 2 NO + HNO3 + Template:H2O-nl

Consequently applications of nitrous acid usually begin with mineral acid acidification of sodium nitrite. The acidification is usually conducted at ice temperatures, and the HNO2 consumed in situ.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:<ref name="Nitros" />Template:Rp

N2O3 + H2O Template:Eqm 2 HNO2

Addition of dinitrogen trioxide to water is thus another preparatory technique.

Chemical applicationsEdit

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

At high acidities (Template:Math), nitrous acid is protonated to give water and nitrosonium cations.<ref name="Nitros" />Template:Rp

ReductionEdit

With I and Fe2+ ions, NO is formed:<ref name="InorgChem">Template:Cite book</ref>

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 Template:Chem, 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 Template:Pad Eo = +0.54 V
Template:Chem + 3 H+ + 2 e ⇌ HNO2 + H2O Template:Pad Eo = +0.93 V
HNO2 + H+ + e ⇌ NO + H2O Template:Pad Eo = +0.98 V

It can be seen that the values of ETemplate:Su 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.<ref name="InorgChem" />

Organic chemistryEdit

Nitrous acid is used to prepare diazonium salts:

HNO2 + ArNH2 + H+Template:Chem + 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.<ref>Template:Cite journal</ref> 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:<ref>Template:Cite book</ref> 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.

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

(CH3)2CHCH2CH2OH + HNO2 → (CH3)2CHCH2CH2ONO + H2O

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

HNO2 + R2NH → R2N-NO + H2O

Atmosphere of the EarthEdit

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

DNA damage and mutationEdit

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.<ref>Template:Cite journal</ref> Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.<ref>Template:Cite journal</ref>

See alsoEdit

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ReferencesEdit

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Template:Nitrogen compounds Template:Nitrites Template:Authority control