Hyponitrous acid explained

Hyponitrous acid is a chemical compound with formula or HON=NOH. It is an isomer of nitramide, H₂N−NO₂; and a formal dimer of azanone, HNO.

Hyponitrous acid forms two series of salts, the hyponitrites containing the [ON=NO]²⁻ anion, and the "acid hyponitrites" containing the [HON=NO]⁻ anion.

Structure and properties

There are two possible structures of hyponitrous acid, trans and cis. trans-Hyponitrous acid forms white crystals that are explosive when dry. In aqueous solution, it is a weak acid (pK_a1 = 7.21, pK_a2 = 11.54),^[1] and decomposes to nitrous oxide and water with a half life of 16 days at 25 °C at pH 1–3:

H2N2O2 -> H2O + N2OSince this reaction is not reversible, should not be considered as the anhydride of .^[2]

The cis acid is not known,^[2] but its sodium salt can be obtained.

Preparation

Hyponitrous acid (trans) can be prepared from silver(I) hyponitrite and anhydrous HCl in ether:

Ag2N2O2 + 2 HCl -> H2N2O2 + 2 AgClSpectroscopic data indicate a trans configuration for the resulting acid.^[3]
It can also be synthesized from hydroxylamine and nitrous acid:

NH2OH + HNO2 -> H2N2O2 + H2O

Biological aspects

In enzymology, a hyponitrite reductase is an enzyme that catalyzes the chemical reaction^[4]

H2N2O2 + 2 NADH + 2 H+ <-> 2 NH2OH + 2 NAD+

Notes and References

1. Book: Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. D. D.. Perrin. 2nd. IUPAC Chemical Data. 29. Pergamon. Oxford. 1982. 1984. 1969. 82-16524. 0-08-029214-3. Entry 120.
2. Book: Egon . Wiberg . Arnold Frederick . Holleman . 2001 . Inorganic Chemistry . Elsevier . 0-12-352651-5.
3. Book: Inorganic Chemistry . limited . 3rd . Chapter 15: The group 15 elements . Catherine E. Housecroft . Alan G. Sharpe. Pearson . 2008 . 978-0-13-175553-6 . 468.
4. Web site: ENZYME - 1.7.1.5 Hyponitrite reductase.