Isothionic acid

Cysteic acid is obtained in nearly quantitative yield from cysteine with aqueous hydrogen peroxide in the presence of iron(II) ions.397 Molybdates and tungstates have also been used as effective catalysts for similar transformations.398 An excellent route for the oxidation of 2-thioethanol to isothionic acid has been developed.399 Heteropolyoxometallates supported on alumina400 can also be used to oxidize a range of organo-sulfur compounds. For example, alkyl monosulfides to sulfoxides and sulfones, and thiols to sulfonic acids are a few possibilities (Figure 3.98). 

The viscous fatty acid chloride is mixed with soHd sodium isothionate and is heated under vacuum and agitation without water or solvent HCl is evolved, leaving the product as a finely divided powder. The direct esterification of fatty acid and sodium isethionate has been patented (69). 

If the central sulfur atom of the hydrophilic group is directly bonded to a carbon atom, the resulting compound is called a sulfonate. Because aromatic ring systems will easily react with sulfuric acid or sulfur trioxide, alkylbenzene sulfonates were one of the earliest types of surface-active compounds to be produced. The sulfonate group is the hydrophilic part in many surfactants in the trade. Besides alkylbenzene sulfonates, alkane sulfonates, olefine sulfonates, of-sulfo fatty acid methyl esters, taurates and isothionates should also be mentioned. 

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