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

Authors:

• Chryssostomos Chatgilialoglu,
• K.U. Ingold,
• J.C. Scaiano

Abstract

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. Thus, for CCl, 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 involvement 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 °C which implies that a considerable activation barrier is involved in such processes. © 1982, American Chemical Society. All rights reserved.