Nitrous acid

Not to be confused with nitric acid.

"Hono" redirects here. For the place in Sweden, see Hönö.

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.^[3] 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.

Nitrous acid

Names
IUPAC name Nitrous acid[1]
Identifiers
CAS Number	7782-77-6 Y
3D model (JSmol)	Interactive image
3DMet	B00022
ChEBI	CHEBI:25567 Y
ChEMBL	ChEMBL1161681 Y
ChemSpider	22936 Y
ECHA InfoCard	100.029.057
EC Number	231-963-7
Gmelin Reference	983
KEGG	C00088 N
MeSH	Nitrous+acid
PubChem CID	24529
UNII	T2I5UM75DN Y
CompTox Dashboard (EPA)	DTXSID7064813
InChI InChI=1S/HNO2/c2-1-3/h(H,2,3) Y Key: IOVCWXUNBOPUCH-UHFFFAOYSA-N Y
SMILES O=NO
Properties
Chemical formula	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
NFPA 704 (fire diamond)	4 0 2 OX
Flash point	Non-flammable
Related compounds
Other anions	Nitric acid
Other cations	Sodium nitrite Potassium nitrite Ammonium nitrite
Related compounds	Dinitrogen trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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.^[3]

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

Preparation and decomposition

See also: Dinitrogen trioxide

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:^{[4]: 1 [5]}

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.^[6][7]

Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:^[4]: 2

N₂O₃ + H₂O ⇌ 2 HNO₂

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.^[4]: 2

Reduction

With I⁻ and Fe²⁺ ions, NO is formed:^[8]

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 N
₂H⁺
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⁻   E^o = +0.54 V

NO⁻
₃ + 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^o
_cell 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.^[8]

Organic chemistry

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.^[9] 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:^[10] 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 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.^[11][12]

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.^[13] Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.^[14]

See also

• Demjanov rearrangement
• Nitric acid (HNO₃)
• Nitrosyl-O-hydroxide
• Tiffeneau-Demjanov rearrangement

References

1. "Nitrous Acid".
2. Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 156. ISBN 0-08-029214-3. LCCN 82-16524.
3. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. p. 462.
4. Williams, D. L. H. (1988). Nitrosation. Cambridge, UK: Cambridge University. ISBN 0-521-26796-X.
5. Kameoka, Yohji; Pigford, Robert (February 1977). "Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide, and Alkaline Sodium Sulfite Aqueous". Ind. Eng. Chem. Fundamen. 16 (1): 163–169. doi:10.1021/i160061a031.
6. Petit, Y.; Larchevêque, M. (1998). "Ethyl Glycidate from (S)-Serine: Ethyl (R)-(+)-2,3-Epoxypropanoate". Org. Synth. 75: 37. doi:10.15227/orgsyn.075.0037.
7. Smith, Adam P.; Savage, Scott A.; Love, J. Christopher; Fraser, Cassandra L. (2002). "Synthesis of 4-, 5-, and 6-methyl-2,2'-bipyridine by a Negishi Cross-coupling Strategy: 5-methyl-2,2'-bipyridine". Org. Synth. 78: 51. doi:10.15227/orgsyn.078.0051.
8. Housecroft, Catherine E.; Sharpe, Alan G. (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 449. ISBN 978-0-13-175553-6.
9. Clarke, H. T.; Kirner, W. R. (1922). "Methyl Red". Organic Syntheses. 2: 47. doi:10.15227/orgsyn.002.0047.
10. Prudent practices in the laboratory: handling and disposal of chemicals. Washington, D.C.: National Academy Press. 1995. doi:10.17226/4911. ISBN 978-0-309-05229-0.
11. Spataro, F; Ianniello, A (November 2014). "Sources of atmospheric nitrous acid: state of the science, current research needs, and future prospects". Journal of the Air & Waste Management Association. 64 (11): 1232–1250. Bibcode:2014JAWMA..64.1232S. doi:10.1080/10962247.2014.952846. PMID 25509545.
12. Anglada, Josef M.; Solé, Albert (November 2017). "The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals". The Journal of Physical Chemistry A. 121 (51): 9698–9707. Bibcode:2017JPCA..121.9698A. doi:10.1021/acs.jpca.7b10715. PMID 29182863.
13. Da Roza, R.; Friedberg, E. C.; Duncan, B. K.; Warner, H. R. (1977-11-01). "Repair of nitrous acid damage to DNA in Escherichia coli". Biochemistry. 16 (22): 4934–4939. doi:10.1021/bi00641a030. ISSN 0006-2960. PMID 334252.
14. Hartman, Z.; Henrikson, E. N.; Hartman, P. E.; Cebula, T. A. (1994). "Molecular models that may account for nitrous acid mutagenesis in organisms containing double-stranded DNA". Environmental and Molecular Mutagenesis. 24 (3): 168–175. Bibcode:1994EnvMM..24..168H. doi:10.1002/em.2850240305. ISSN 0893-6692. PMID 7957120.