Proton chemical polarisation,autocatalysis and frontal kinetics of the oxidation of dialkylsulphides with nitric acid

The oxidation of dimethyl sulphide to dimethylsulphoxide with nitric acid displays CIDNP effects of protons and the phenomenon of frontal kinetics. The process is autocatalytic, with N₂O₄ as the catalyst and the primary oxidant of sulphide. In the presence of the inhibitor (methylmercaptan) the frontal reaction takes place: in purified samples—the usual volume reaction. The interaction of ethyl, n-propyl and n-butyl sulphides with NO₂ is accompanied by integral polarisation of the α-CH₂-protons. In all the cases sulphide is negatively polarised and sulphoxide positively polarised. The reaction mechanism proposed includes the formation of a radical pair during the interaction of sulphide with N₂O₄. Disproportionation of the radical pair leads to the formation of polarised sulphoxide and the decay results in re-generation of sulphide. The rate of oxidation of sulphide during the volume reaction is proportional to the product polarisation.