Disulfuric acid

Names
IUPAC name Disulfuric acid [1]
Other names Pyrosulfuric acid, Oleum
Identifiers
CAS Number	7783-05-3 Y
3D model (JSmol)	Interactive image Interactive image Interactive image
ChEBI	CHEBI:29211 Y
ChemSpider	56433 Y
ECHA InfoCard	100.029.069
EC Number	231-976-8
MeSH	Pyrosulfuric+acid
PubChem CID	62682
UNII	NTC1O8E83E N
CompTox Dashboard (EPA)	DTXSID7064817
InChI InChI=1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Y Key: VFNGKCDDZUSWLR-UHFFFAOYSA-N Y InChI=1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Key: VFNGKCDDZUSWLR-UHFFFAOYAZ
SMILES OS(=O)(=O)OS(O)(=O)=O OS(=O)(=O)OS(=O)(=O)O O=S(=O)(O)OS(=O)(=O)O
Properties
Chemical formula	H2O7S2
Molar mass	178.13 g·mol−1
Appearance	colorless
Melting point	36 °C (97 °F; 309 K)
Acidity (pKa)	2.5 (20 °C; in conc. H2SO4)[2]
Conjugate base	Disulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum, is a sulfur oxoacid.^[3] It is a major constituent of fuming sulfuric acid, oleum, and this is how most chemists encounter it. As confirmed by X-ray crystallography, the molecule consists of a pair of SO₂(OH) groups joined by an oxide.^[4]

Reactions

It is also a minor constituent of liquid anhydrous sulfuric acid due to the equilibria:

$${\displaystyle {\begin{aligned}{\ce {H2SO4))&{\ce {<=>H2O + SO3))\\{\ce {SO3 + H2SO4))&{\ce {<=> H2S2O7))\\{\ce {2H2SO4))&{\ce {<=> H2O + H2S2O7))\end{aligned))}$$

$${\displaystyle {\ce {H2SO4 + SO3 -> H2S2O7))}$$

Disulfuric acid can be seen as the sulfuric acid analog of an acid anhydride. The mutual electron-withdrawing effects of each sulfuric acid unit on its neighbour causes a marked increase in acidity. Disulfuric acid is strong enough to protonate "normal" sulfuric acid in the (anhydrous) sulfuric acid solvent system. There are salts of disulfuric acid, commonly called pyrosulfates, e.g. potassium pyrosulfate.

There are other related acids with the general formula H₂O·(SO₃)_x though none can be isolated.