The aerobic and peroxide-induced coupling of aqueous thiols—I. Kinetic results and engineering significance

Abstract

Alkaline thiols undergo a rapid oxidative coupling reaction in the presence of oxygen or hydrogen peroxide; the oxygen reaction is catalyzed by trace copper whereas the peroxide reaction occurs spontaneously without the necessity of metal catalysis. Depending on the nature of the substrate—its molecular size and whether it contains any polar group—the disulfide, which is the coupling product, may be soluble or sparingly soluble in water. Water-soluble disulfides undergo further, although much slower, oxidation reactions to form higher oxidation products. In the oxygen reaction molecular oxygen is reduced sequentially to peroxide and then water. For n-propylthiol, the oxygen reaction has a first-order dependence on oxygen pressure and copper concentration; the reaction order with respect to thiolate changes from pseudo zeroth order to first and, then, second order as the reaction approaches completion. The peroxide reaction, on the other hand, has a strictly first-order dependence on the concentrations of thiolate and the non-dissociated hydrogen peroxide. The activation energies of the oxygen and peroxide reactions are, respectively, 9.5 ± 1.2 and 12.3 kcal/mol. A similar dependency on the kinetic parameters appears to hold for other thiols, although some variations are observed which arise from special features of individual systems. For aminothiols, the dependence on copper switches to second order below a certain catalyst concentration; for alkylthiols which form insoluble liquid disulfides, the oxygen reaction shows a pronounced sensitivity to stirring that arises from the tendency of the copper-thiolate and disulfide complexes to cluster and segregate at the water-disulfide interface. When coupled to the fact that many mercapto and sulfhydryl compounds of interest form disulfides which have low aqueous solubilities, the ease of the aerobic coupling reaction suggests that it may be used to separate soluble mercapto and sulfhydryl substances from aqueous media.