Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum, is a
sulfur oxoacid. It is a major constituent of fuming sulfuric acid,
oleum
Oleum ( Latin ''oleum'', meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Ol ...
, and this is how most chemists encounter it. As confirmed by
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
, the molecule consists of a pair of SO
2(OH) groups joined by an oxide.
Reactions
It is also a minor constituent of liquid anhydrous
sulfuric acid
Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
due to the equilibria:
:
H2SO4<=>H2O + SO3
:
SO3 + H2SO4<=> H2S2O7
:
2H2SO4 <=> H2O + H2S2O7 ''Global
The acid is prepared by reacting excess
sulfur trioxide
Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
(SO
3) with sulfuric acid:
H2SO4 + SO3 -> H2S2O7
Disulfuric acid can be seen as the sulfuric acid
analog of an
acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid.
In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form when one equivalen ...
. 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
In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid, ...
"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 H
2O·(SO
3)
''x'' though none can be isolated.
See also
*
Pyrophosphoric acid
Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, HO)2P(O)sub>2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid cr ...
*
Disulfurous acid
*
Oleum
Oleum ( Latin ''oleum'', meaning oil), or fuming sulfuric acid, is a term referring to solutions of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid). Ol ...
*
Sulfuric acid
Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
*
Sulfur oxoacid
*
Trithionic acid
References
{{Hydrogen compounds
Hydrogen compounds
Pyrosulfates
Sulfur oxoacids
Acid anhydrides