A kinetic, product and kinetic isotope effect investigation of the bromination of 1,1-diphenylethylenes and of their 2,2-dideuterio derivatives

Abstract

The kinetics of bromination of 1,1-diphenylethylene (1a), 4-trifluoromethyl-1,1-diphenylethylene (1b), and 1-(3-trifluoromethylphenyl)-1-(4-trifluoromethylphenyl)ethylene (1c) and of their 2,2-dideuterio derivatives have been investigated in 1,2-dichloroethane. The rate law was always second-order in Br₂ and first-order in olefin, with the following k₃: 1a (L = H), 1.7 (0.1) × 10⁷; 1b (L = H), 8.0 (0.1) × 10⁴; 1c (L = H), 52 (2) dm⁶ mol^-2 s^-1. Olefin 1a (L = H) gave dibromide 4a and vinyl bromide 3a in a ratio changing from 99∶1 at 10^-2 mol dm^-3 to 5∶95 at 10^-4 mol dm^-3 Br₂ and olefin. The k₃ was independent of the reagent concentrations and of the extent of proton loss from the intermediate, showing that the last step was not rate limiting and the formation of the intermediate was completely rate determining. In the whole investigated concentration range, only dibromides 4b and 4c were instead obtained from 1b and 1c, respectively. A negligible KIE, 0.97 (0.01) was found for 1a, whether the dibromide or the vinyl bromide was the main product, while significant inverse KIEs were obtained for 1b, 0.75 (0.05), for 1c, 0.70 (0.05), on deuteriation of the methylene group, and for cis-1,2-diphenylethylene, 2. The kinetic and product distribution data are discussed in terms of nature of the intermediates, depending on substituent effect and reagent concentration.