DETAILED KINETIC-ANALYSIS OF THE ELECTROPHILIC BROMINATION OF 2-CYCLOHEXEN-1-OL BENZOATE USING A NEWLY DEVELOPED MULTICHANNEL STOP-FLOW APPARATUS

A new stop-flow instrument is presented. It is equipped with a 32-channel parallel diode-array detector and a data-acquisition system interfaced to a standard MS-DOS PC, yielding a total throughput of 10 000 spectra/s. The apparatus includes a program capable of fitting simultaneously data taken over a range of wavelengths, concentrations, and temperature. The program can be adapted quickly to any kinetic model. This apparatus allowed a comprehensive kinetic analysis of the electrophilic addition reaction of molecular bromine to 2-cyclohexen-1-ol benzoate in 1,2-dichloroethane. Under the employed reagent concentrations, the formation of the expected olefin-bromine charge-transfer complex could not be detected. Data were taken over the 300-480-nm range. We found k(3)(1) = 368 (25) and k(3)(2) = 215 (30) M(-2) s(-1) for the third-order (second order in bromine) reactions leading respectively to the trans-dibromo adducts (1 and 2) and to the intermediate trans-3-bromocyclohexano-cis-1,2-(2'-phenyl-1',3'-dioxolan-2'-ylium) tribromide (5). The species 5 then collapses to the cis-bromo adducts 3 and 4 with k(1) = 0.282 (0.006) s(-1). The ratio k(3)(1)/k(3)(2) = 1.7 is in agreement with that previously reported for the yields of the trans- and cis-dibromo adducts.