Hyponitrous acid

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Hyponitrous acid
Tautomer wireframe models of hyponitrous acid
Preferred IUPAC name
Systematic IUPAC name
Other names
Hyponitrous acid dimer
3DMet B00354
ChemSpider 55636 N
Jmol-3D images Image
KEGG C01818
PubChem 61744
Molar mass 62.0282 g/mol
Appearance white crystals
Main hazards explosive when dry
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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.[1]

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.[1]

H2N2O2→ H2O + N2O

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

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

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:[2]

RONO + NH2OH + 2 EtONa → Na2N2O2 + ROH + 2 EtOH

Another method is by the reduction of sodium nitrite with sodium amalgam:[3]

2 NaNO2 + 4 Na(Hg) + 2 H2O → Na2N2O2 + 4 NaOH + 4 Hg

The insoluble silver salt can be precipitated from the solution by the addition of silver nitrate:

Na2N2O2 + 2 AgNO3 → Ag2N2O2 + 2 NaNO3

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.[2] 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.[2]


  1. ^ a b c d Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-12-352651-5
  2. ^ a b c Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 468. ISBN 978-0-13-175553-6. 
  3. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.