Oxidation of Thiols Mercaptans

Gentle oxidation of mercaptans and thiols leads to coupled products— disulfides. The oxidants used for this purpose are air [9], hydrogen peroxide [186], dichromates [663], chlorochromates [619], manganese dioxide [816], copper permanganate [896], ferric chloride [907], and diethyl azodicar-boxylate [977] (equations 544-546). 

In compounds containing oxidizable functional groups, these groups may be left intact 163, 186] or oxidized concurrently [907] (equations 547 and 548). 

More energetic oxidation of thiols leads to sulfonic acids, Thiophenol dissolved in a solution of potassium hydroxide in dimethylformamide is oxidized by oxygen at 23.5 °C over a 22-h period to benzenesulfonic acid in 91% yield [53], Lauryl mercaptan gives dodecanesulfonic acid in 100% isolated yield upon treatment with potassium peroxymonosulfate, 2KHS05 KHS04 K2S04, at room temperature for 30 min [205]. 

Whereas 2-(2-pyridyl)ethanethiol is oxidized with hydrogen peroxide to 2-(2-pyridyl)ethyl disulfide, its oxidation by nitric acid yields 2-(2-pyr-idyl)ethanesulfonic acid (equation 549) [474]. 

In 2-mercaptobenzothiazole, energetic oxidation with hydrogen peroxide leads to the replacement of the mercapto group by hydrogen, probably via oxidation to sulfinic acid (equation 550) [1197]. 

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FIGURE 16.76 Oxidation of thiols (mercaptans) with nitric acid gives sulfonic acids. 

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