The aerobic and peroxide-induced coupling of aqueous thiols-II. Reaction mechanisms, model analysis, and a comparison of the model and experimental results

Kinetic results of the preceeding paper are used to formulate the most probable reaction mechanisms for the peroxide-induced and aerobic coupling of aqueous thiols. Parameters in the proposed mechanisms are evaluated for n-propylthiol using the results of independent measurements. A remarkably good agreement is achieved between the model and experimental results. The peroxide reaction, which is not affected by either copper ion or radical scavengers, is shown to be a nucleophilic substitution (S_N 2) reaction which proceeds by a two-step mechanism as follows: RS^- + H₂O₂ {A figure is presented} RSOH + OH^- rate-determining RS^- + RSOH → RSSR + OH^- In the reaction the thiolate anion acts as the nucleophile while hydrogen peroxide and the transient sulfenic acid, RSOH, function as electrophiles. The most plausible mechanism for the copper-catalyzed, aerobic coupling reaction is as follows: Cu⁺ (RSSR)₂⁺ + RS^- {A figure is presented} (RSSR)Cu⁺ (RS^-) + RSSR (RSSR)Cu⁺ (RS^-) + RS^- {A figure is presented} (RSSR)Cu⁺ (RS^-)₂^- (RSSR)Cu⁺ (RS^-)₂^- + O₂ {A figure is presented} Cu⁺(RSSR)₂⁺ + HO₂^- rate-determining RS^- + H₂O₂ {A figure is presented} RSOH + OH^- rate-determining RS^- + RSOH → RSSR + OH^- For the n-propylthiol substrate, the pertinent parameters in the mechanism are 1n (K₁) = -11,000 K/T + 33.6, where K₁ is dimensionless, 1n (K₂) = 4270 K/T -10.5, where K₂ is in 1/mol, 1n (k₂) = 30.0 - 5260 K/T, and 1n (k₂) = 26.8 - 6190 K/T, where k₁ and k₂ are in 1/(mol min).