Disulfides, liquid-phase oxidation

The chapter by C. J. Swan and D. L. Trimm, which also emphasizes the effect on catalytic activity of the precise form of a metal complex, shows too that, depending on the metal with which it is associated, the same ligand can act either as a catalyst or inhibitor. The model reaction studied was the liquid-phase oxidation of ethanethiol in alkaline solution, catalyzed by various metal complexes. The rate-determining step appears to be the transfer of electrons from the thiyl anion to the metal cation, and it is shown that some kind of coordination between the metal and the thiol must occur as a prerequisite to the electron transfer reaction (8, 9). In systems where thiyl entities replace the original ligands, quantitative yields of disulfide are obtained. Where no such displacement occurs, however, the oxidation rates vary widely for different metal complexes, and the reaction results in the production not only of disulfide but also of overoxidation and hydrolysis products of the disulfide. 

When used for gas treating, the Merox process simply involves the absorption of mercaptans from the gas by countercurrent contact with caustic soda solution, oxidation of the dissolved mercaptans to disulfides by contact with air, and separation of the disulfide liquid from the caustic solution by settling and decanting off the disulfide phase. Note that both CO2 and H2S are removed in a caustic prewash prior to contacting the gas with the Merox caustic solution. 

Ordinarily, the catalyst is activated by heating in a current of air or oxygen for several hours and next heated under continuous pumping for several hours. We shall discuss the active sites on catalysts later, but note now that activation is necessary for cation-radical formation. The substrate is usually added to the catalyst as a solution, say in benzene, carbon disulfide, and carbon tetrachloride, and the solvent is removed by pumping. Occasionally the substrate is added neat, either as a liquid or in the vapor phase. One-electron oxidation is usually immediate. 

Preformed disulfide-bonded dimer HF was placed in redox buffer from which aliquots were removed over time. Each aliquot was analyzed via reverse-phase high-performance liquid chromatography (HPLC) to quantify the relative amounts of HH, HF, and FF. Peptides HH and FF were detected within 30 min from the start of the reaction and only 2-3% of the heterodimer remained after 48 h, a 26-fold preference for the homodimer. The outcome was the same when reduced peptides H and F were placed in buffer and allowed to undergo air oxidation. Control experiments showed that there were no kinetic barriers to the formation of the HF heterodimer and that the disulfide exchange was reversible and under thermodynamic control. The free energy of specificity for the formation of homodimers. 

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