Hyponitrous acid

Hyponitrous acid
Names
	Preferred IUPAC name Diazenediol
	Systematic IUPAC name N-(Hydroxyimino)hydroxylamine
	Other names Hyponitrous acid dimer
Identifiers
CAS Number	14448-38-5;
3D model (JSmol)	Interactive image;
3DMet	B00354;
ChEBI	CHEBI:14428;
ChemSpider	55636;
Gmelin Reference	141300
KEGG	C01818;
PubChem CID	61744;
CompTox Dashboard (EPA)	DTXSID40896989 ;
	InChI InChI=1S/H2N2O/c3-1-2-4/h(H,1,4)(H,2,3) Key: NFMHSPWHNQRFNR-UHFFFAOYSA-N;
	SMILES ON=NO;
Properties
Chemical formula	H2N2O2
Molar mass	62.0282 g/mol
Appearance	white crystals
Hazards
	Occupational safety and health (OHS/OSH):
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

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 (pK_a1 = 7.21, pK_a2 = 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

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(I) 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]

References

^ ^a ^b ^c Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5.
^ ^a ^b ^c 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.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[Wiberg&Holleman-1] Wiberg, Egon; Holleman, Arnold Frederick (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5.

[InorgChem-2] 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] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

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