Kinetics and mechanism of the autoxidation of 2-aminoethanethiol and ethanethiol catalyzed by cobalt(II) 4,4',4",4"'-tetrasulfophthalocyanine in aqueous solution

Abstract

The kinetics of autoxidation of 2-aminoethanethiol to 2-aminoethyl disulfide and ethanethiol to ethyl disulfide as catalyzed by cobalt(II) 4,4',4",4"'-tetrasulfophthalocyanine (Co^(II)TSP) were investigated. The following general rate law was found to hold for both substrates over the pH range 8.8-13.5: v = -d[RS-]/dt = k_(obsd)[CoTSP]_T[RS^-]. Evidence for a mechanism that proceeds via a dimeric catalytic center that is bridged by the RS^- anion has been obtained. In the proposed mechanism, electron transfer from the Co(II) metal center to bound dioxygen was considered to be the rate-determining step. Hydrogen peroxide and mercaptan radical were identified as reaction intermediates in the autoxidation of both substrates. These intermediates react further to produce the corresponding disulfide, RSSR, as the final product of the autoxidation. k_(obsd) was found to be of the following general form: k_(obsd) = (k_(31)K_(21) + (k_(32)K_(22)K'_1/ɑH+) + (k_(33)K_(23)K'_1K'_2/ɑ_H^(+2)))/[(1 + (K'_1/ɑ_H^+) + (K'_1K'_2/ɑH^(+2)))(1 + (ɑ_H^+/K'_(a1)^2))], where k_(31) and K_(21) are the rate constants for the electron transfer and the equilibrium constants for substrate complexation of the ith catalytic center, respectively, K'_2 and K'_2 are the apparent acid dissociation constants of the pyrrole groups of Co^(II)TSP-RS^--Co^(II)TSP and Co^(II)TSP^-RS--co^II)TSP-, respectively, and K'_(a1) is the apparent acid dissociation constant of RCH_2CH_2SH

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