A phenolate anion reacts with persulfate ion in alkaline solution to yield a product in which a sulfate group enters the ring para or ortho to the phenolic group. Para substitution predominates. Subsequent acid-catalyzed hydrolysis yields the dihydric phenol.
The reaction was discovered by Karl Elbs in 1893 and named the Elbs persulfate oxidation. The reaction is generally applicable to ortho-, meta-, and para-substituted phenols with isomer distributions. The yields are not very high, particularly from para-substituted phenols, but the major contaminant is usually unchanged starting material that can be separated easily from the intermediate sulfate ester by solvent extraction. Other generally oxidizable groups such as an aldehyde or a double bond are usually not affected under the reaction conditions. The reaction was last thoroughly reviewed in 1951. T. R. Seshadri has made major contributions to the development of the Elbs oxidation. Nearly 30% of the references in this chapter are due to him and his colleagues.
By analogy with the Elbs persulfate oxidation of phenols, it might be expected that aromatic amines would react with persulfate to give p-aminoaryl sulfates. Although the Elbs reaction had been known since 1893, it was not until 60 years later that Boyland et al. reported the extension of this reaction to aromatic amines. In accordance with expectations, aminoaryl sulfates were indeed the major products of the reaction, but, unexpectedly, the substitution took place exclusively ortho to the amino group rather than predominantly in the para position as in the phenol oxidation. Para substitution takes place only if the ortho positions are occupied by substituents other than hydrogen. Boyland and Sims explored the preparative aspects of this reaction in a series of papers. It seems appropriate to name the reaction the Boyland–Sims oxidation. Primary, secondary, and tertiary aromatic amines are all converted to the corresponding o-aminoaryl sulfates under conditions similar to those used for the Elbs oxidation, that is, room temperature or below, aqueous alkali, and equimolar quantities of amine and persulfate.