Chemistry:Hyponitrous acid

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Hyponitrous acid
Tautomer wireframe models of hyponitrous acid
Hyponitrous acid Ball and Stick (Tautomer 1).png
Hyponitrous acid Ball and Stick (Tautomer 2).png
Preferred IUPAC name
Systematic IUPAC name
Other names
Hyponitrous acid dimer
3D model (JSmol)
Molar mass 62.0282 g/mol
Appearance white crystals
Conjugate base Hyponitrite
Main hazards explosive when dry
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Tracking categories (test):

Hyponitrous acid is a chemical compound with formula H2N2O2 or HON=NOH. It is an isomer of nitramide, H2N−NO2.

Hyponitrous acid forms two series of salts, the hyponitrites containing the [ON=NO]2− anion, and the "acid hyponitrites" containing the [HON=NO] anion.[1]

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:

H2N2O2H2O + N2O

Since this reaction is not reversible, N2O should not be considered as the anhydride of H2N2O2.[1]

The cis acid is not known,[1] but its sodium salt can be obtained.[2]


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

Ag2N2O2 + 2 HCl → H2N2O2 + 2 AgCl

Spectroscopic data indicate a trans configuration for the resulting acid.[2]
It can also be synthesized from hydroxylamine and nitrous acid:

NH2OH + HNO2H2N2O2 + H2O

Biological aspects

In enzymology, a hyponitrite reductase is an enzyme that catalyzes the chemical reaction[3]

H2N2O2 + 2 NADH + 2 H+ ↔ 2 NH2OH + 2 NAD+


  1. 1.0 1.1 1.2 Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5. 
  2. 2.0 2.1 Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry (3rd ed.). Pearson. p. 468. ISBN 978-0-13-175553-6. 
  3. "ENZYME - Hyponitrite reductase".