Kinetics of the Addition Reactions of Thiobenzoic Acids to Styrenes or Ethynylbenzenes as the Model of Polyaddition. Study on the Rate-Determining Step and Substituent Effect

Abstract

The reaction mechanism and substituent effect on the addition reaction of thiobenzoic acid to styrene or ethynylbenzene were investigated in detail. Both addition reactions proceeded by a radical chain mechanism, and the rate-determining step was the chain transfer step between each intermediate carbon radical [φĊHCH2SC(=O)φ or φĊ=CHSC(=O)φ] and thiobenzoic acid. The substituent effect on the chain transfer step between the para-X substituted intermediate carbon radicals and para-Y thiobenzoic acids in the styrene and ethynylbenzene systems was better correlated by the modified Hammett’s equation considering resonance effect than by the simple Hammett’s equation. In both styrene and ethynylbenzene systems, the reaction constant ρ for p-X intermediate carbon radicals was negative, and that for p-Y thiobenzoic acids was positive, suggesting that a perturbation between SOMO of each intermediate carbon radical and LUMO of thiobenzoic acid in the transition state is the important controlling factor for reactivity in the adduct formation of thiobenzoic acid with styrene or ethynylbenzene. That is, the intermediate carbon radicals are nucleophilic. Therefore, the overall reaction rate of the adduct formation increases by introducing a strong electron-donating (+I) group into the para position of styrene or ethynylbenzene and a large −I group into the para position of thiobenzoic acid. The results are compared with those of the addition reaction of thiophenol to styrene or ethynylbenzene. The polyaddition mechanism of 1,4-benzenebiscarbothioic acid to 1,4-divinylbenzene or 1,4-diethynylbenzene is discussed.