Absolute Rate Constants for the Reaction of Triethylsilyl Radicals with Organic Halides

Article abstract of DOI:10.1021/ja00383a022

The absolute rate constants for the reactions of Et3Si^. radicals with a number of organic halides have been measured in solution by using laser flash photolysis techniques.The reactivities cover a wide range; e.g., the rate constants at ca. 300 K are 4.3 * 10⁹, 1.1 * 10⁸, 2.5 * 10⁶, and <10⁵ M^-1 s^-1 for CH3CH2I, C6H5Br, (CH3)3CCl, and C6H5CH2F, respectively.Arrhenius parameters were determined for a few representative substrates.Thus, for CCl4, E_a = (0.78 +/- 0.24) kcal/mol and log (A/M^-1 s^-1) = 10.2 +/- 0.2, data which can be combined with relative rates from the literature to yield absolute rate constants for some additional substrates.Comparison of the Arrhenius parameters for halogen abstraction from several substrates suggests the involvment of charge-transfer interactions in some of these reactions.Such rate-enhancing interactions can have a greater influence on the preexponential factor than on the activation energy.An explanation for this phenomenon is advanced.Combination of our rate constants with data from literature also allows evaluation of the rates of inversion at the silicon center in triorganosilyl radicals as being in the range (3-12) * 10⁹ s^-1 at temperatures from 0 to 80 deg C which implies that a considerable activation barrier is involved in such process.