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

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 mechanis m, and the rate-determining step was the chain transfer step between e ach intermediate carbon radical [phiCHCH2SC(=O)phi or phiC=CHSC(=O)phi ] and thiobenzoic acid. The substituent effect on the chain transfer s tep between the para-X substituted intermediate carbon radicals and pa ra-Y thiobenzoic acids in the styrene and ethynylbenzene systems was b etter correlated by the modified Hammett's equation considering resona nce effect than by the simple Hammett's equation. In both styrene and ethynylbenzene systems, the reaction constant rho 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 intermed iate carbon radical and LUMO of thiobenzoic acid in the transition sta te is the important controlling factor for reactivity in the adduct fo rmation of thiobenzoic acid with styrene or ethynylbenzene. That is, t he intermediate carbon radicals are nucleophilic. Therefore, the overa ll reaction rate of the adduct formation increases by introducing a st rong electron-donating (+I) group into the para position of styrene or ethynylbenzene and a large -I group into the para position of thioben zoic acid. The results are compared with those of the addition reactio n of thiophenol to styrene or ethynyl-benzene. The polyaddition mechan ism of 1,4-benzenebiscarbothioic acid to 1,4-divinylbenzene or 1,4-die thynylbenzene is discussed.