A study of the bromination of alkenes in carbon tetrachloride

Abstract

The mechanism of the bromination of alkenes hasgenerally been believed to proceed through a bromonium ion intermediate. In recent years a bromine-alkene charge transfer complex has been suggested as an equilibrium step preceding the bromonium ion formation. This study was an attempt to test the hypothesis that a charge transfer complex is a necessary step in the bromination of alkenes. A nonpolar solvent, carbon tetrachloride, was used here to minimize effects due to solvent complexation. Individual bromination kinetic runs, competitive brominations, and iodine-alkene complexation experiments were conducted to test this hypothesis. The kinetic runs were followed by measuring the bromine absorbance with time using a Cary 14 spectrophotometer at approximately 23°C and concentration ranges of 10-3 to 10-2 molarity. The competitive reactions were conducted at 25°C at concentrations of 10-2 to 10-1 molarity. The final reaction mixtures were analyzed by gas-liquid chromatography. Iodine-alkene complexation constants were determined in carbon tetrachloride using the Benesi-Hildebrand method. Reproducibility of alkene bromination was shown to be possible in carefully purified carbon tetrachloride if impurities and light are excluded. The kinetic data for the bromination of E-3-hexene, Z-3-hexene, and E-4-octene were shown to be first order in alkene and second order in bromine. The apparent third order rate constants were shown to increase with increasing initial relative bromine concentration and to decrease with decreasing initial relative bromine concentration. These trends were shown to support the complexation hypothesis and are consistent with predictions made using an equation derived using the steady state-equilibrium approximation. The kinetic results clearly show that Z-3-hexene has a greater relative reactivity than E-4-octene. Under competitive conditions the relative reactivity of Z-3-hexene was much greater than predicted from the kinetic studies. This enhanced reactivity was shown to be consistent with a charge transfer complex formation, assuming that the Z-3-hexene complexes with bromine to a greater extent than does E-4-octene. This assumption is in accord with the iodine-alkene complexation data.