Oxides and oxoacids

The most important oxides of all three elements (E) are EO₂ and EO₃. Sulfur additionally forms several oxides of low thermodynamic stability, for instance S₈O with a structure consisting of an S8 ring. Sulfur dioxide SO₂ is the main result of burning organic sulfur and sulfur compounds in air and is a severe air pollutant giving rise (after oxidation to H₂SO₄;) to acid rain. SO₂ is a bent molecule and comprise both Lewis acid and basic properties with one lone-pair. For Reactions with strong oxidizing agents the liquid is a good solvent. SO₂ dissolves in water giving acid solutions consisting of the pyramidal hydrogensulfite (HSO₃^-) and sulfite (SO₃^2-) ions. The supposed sulfurous acid H₂SO₃, though, is exist only in very low concentrations. SeO₂ and TeO₂ comprise polymeric structures and give oxoacid salts identical to those from sulfur.

Sulfur trioxide SO₃ is prepared industrially as a route to sulfuric acid, through oxidizing SO₂ with oxygen by using a vanadium oxide catalyst. It can be present like a monomeric planar molecule but readily provides cyclic S₃O₉ trimers and linear polymers with corner-sharing SO₄ units. The extremely exothermic reaction with water gives sulfuric acid H₂SO₄, that is the world's main industrial chemical, being used in several large-scale processes for creating soaps, fertilizers, dyestuffs, and detergents, and synthetic fibers. Anhydrous sulfuric acid goes through a series of acidbase equilibria like

It is a extremely strong acid medium, where HNO₃ (a strong acid in water) acts as a base:

The resultant 'nitrating mixture' is employed for preparing aromatic nitro compounds through electrophilic reactions of NO₂⁺ .

Reaction of HF with SO₃ provides fluorosulfonic acid HSO₃F that is than sulfuric acid more strongly acidic. In mixtures with SO₃ and powerful fluoride acceptors like SbF₅ it provides superacid media, that are able of protonating even several organic compounds.

Diagram 2. Reactions in 'superacid' solutions.

SeO₃ and selenic acid H₂SeO₄ are identical to the sulfur analogs apart from they are more strongly oxidizing. Tellurium behaves in a different way, as telluric acid has the octahedral Te(OH)₆ structure, that, as supposed from Pauling's rules, is a extremely weak acid.

There are several other oxoacids of sulfur, of which the most significant are peroxodisulfate S₂O₈^2-that comprise a peroxo (O-O) bond, and compounds with S-S bonds include thiosulfate S₂O₃^2-dithionite S₂O₄^2-and tetrathionate S₂O₆^2- . The reaction

is employed for the quantitative estimation of I₂ in aqueous solution.