Hyponitrous acid is the chemical compound H2N2O2. This can be formulated as HON=NOH and is an isomer of nitramide, (H2N−NO2). It forms white crystals that are explosive when dry.
Hyponitrous acid is a weak acid (pk1 = 7.21, pK2= 11.54) in aqueous solution and decomposes to N2O and water with a half-life of 16 days at 25°C at pH 1-3.
- H2N2O2→ H2O + N2O
As this reaction is not reversible, N2O should not be considered as the anhydride of H2N2O2.
Hyponitrous acid forms two series of salts, the "acid hyponitrites" containing [HON=N]− anion and hyponitrites containing the [ON=NO]2− anion.
The hyponitrite ion [ON=NO]2− can be prepared in aqueous solution by two methods. One method uses organic nitrites to synthesize the sodium salt:
- RONO + NH2OH + 2 EtONa → Na2N2O2 + ROH + 2 EtOH
Another method is by the reduction of sodium nitrite to give the silver salt:
- 2 NaNO2 + 4 NaHg + 2 H2O + 2 AgNO3 → Ag2N2O2 + 2 NaNO3 + 4 NaOH + 4 Hg
Addition of Ag+(aq) ions precipitates Ag2N2O2 from solution. The free acid is then prepared from silver hyponitrite and anhydrous HCl in ether:
- Ag2N2O2 + 2 HCl → H2N2O2 + 2 AgCl
There are two possible structures of hyponitrous acid, trans and cis, and the solid Na2N2O2•5H2O is confirmed to be the trans form. Spectroscopic data also indicate a trans configuration of the free acid. The cis-form can be prepared as the sodium salt Na2N2O2 by heating Na2O with gaseous N2O.