Hyponitrous acid

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 (pKa1 = 7.21, pKa2 = 11.54), and decomposes to nitrous oxide and water with a half life of 16 days at 25 °C at pH 1–3:

Since this reaction is not reversible,  should not be considered as the anhydride of.

The cis acid is not known, but its sodium salt can be obtained.

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

Spectroscopic data indicate a trans configuration for the resulting acid. It can also be synthesized from hydroxylamine and nitrous acid:

Biological aspects
In enzymology, a hyponitrite reductase is an enzyme that catalyzes the chemical reaction