Pressure sensitivity of reactions of substitution at C-H, C-Cl, and C-Br bonds in the radical telomerization of ethylene, propylene, and vinyl chloride by polyhalomethanes

Abstract

1. The competitive kinetics was studied with a calculation of the constants of chain transfer in the radical telomerization of vinyl chloride (60°), ethylene and propylene (95°) with CCl4, CHCl3, and CBrCl3 at pressures up to 2400 kg/cm2. The peculiarities of these ten systems permitted an unambiguous isolation of the elementary step of substitution with the participation of aliphatic radicals and a consideration of the dependence of its volume of activation on the nature of the attacking radical and the type of bond to be broken. 2. Using several methods of calculation it was shown that the volumes of activation of substitution reactions vary in the series C-H C-C1>C-Br for CCl3[CH2CH(CH3)]n; and remain constant for CCl3(CH2CHCl)n at n=1–3. A high sensitivity of chain transfer to pressure for all the indicated bonds forα-chloro-orα-methyl-substituted aliphatic radicals was detected. The volumes of activation of homolytic substitution with cleavage of C-Hal bonds depend less on the nature of the attacking radical than chain transfer at the C-H bond. 3. A change in the polar characteristics of the activated complexes of reactions of homolytic substitution at the C-H and C-Hal bonds does not make any significant contribution to the volumes of activation. The values of the parameters of activation of such reactions are evidently determined by the volume of the atoms and groups around the reaction center and by differences in the length of the bond to be broken (formed) in the transition state.