Nitrous acid
Names IUPAC name Identifiers ChEBI ChEMBL ChemSpider ECHA InfoCard 100.029.057 EC Number 983 KEGG MeSH Nitrous+acid UNII Properties HNO2 Molar mass 47.013 g/mol Appearance Pale blue solution Density Approx. 1 g/ml Melting point Only known in solution or as gas Acidity (pKa) 3.15[2] Conjugate base Nitrite Hazards GHS labelling:[3] Danger H300, H314, H400 P260, P264, P264+P265, P270, P273, P280, P301+P316, P301+P330+P331, P302+P361+P354, P304+P340, P305+P354+P338, P316, P317, P321, P330, P363, P391, P405, P501 NFPA 704 (fire diamond) Flash point Non-flammable Related compounds Nitric acid Sodium nitritePotassium nitriteAmmonium nitrite Dinitrogen trioxide
Nitrous acid (molecular formula HNO2) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (NO−2) salts.[4] 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.
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.[4]
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:[5]: 1 [6]
3 HNO2 → 2 NO + HNO3 + H2O
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.[7][8]
Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:[5]: 2
N2O3 + H2O ⇌ 2 HNO2
Addition of dinitrogen trioxide to water is thus another preparatory technique.
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.[5]: 2
With I− and Fe2+ ions, NO is formed:[9]
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 + 4 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O
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 N2H+5, 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− Eo = +0.54 V NO−3 + 3 H+ + 2 e− ⇌ HNO2 + H2O Eo = +0.93 V HNO2 + H+ + e− ⇌ NO + H2O Eo = +0.98 V
It can be seen that the values of Eocell 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.[9]
Nitrous acid is used to prepare diazonium salts:
HNO2 + ArNH2 + H+ → ArN+2 + 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.[10] 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:[11] 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
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.[12][13]
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.[14] Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.[15]
- Demjanov rearrangement
- Nitric acid (HNO3)
- Nitrosyl-O-hydroxide
- Tiffeneau-Demjanov rearrangement
