The H/(CH₃)₃SiH/C₂H₄Reaction System and the Addition of (CH₃)₃Si to Ethene. An End-Product Study

Me 3 Si and C 2 H 5 radicals have been generated by the reaction of H atoms with mixtures of Me 3 SiH and C 2 H 4 and the subsequent reactions studied by end-product analysis. The primary radicals undergo self- and cross-combination and disproportionation reactions. The Me 3 Si radical also adds to the C 2 H 4, generating another radical, Me 3 SiCH 2 CH 2 . This radical in turn undergoes a variety of self- and cross-combination as well as disproportionation reactions with all the radicals present. A number of relative rate constants were extracted from the data. In particular the ratio k (H + Me 3 SiH)/ k (H+C 2 H 4 ) was found to be in good agreement with recent absolute determinations of these two rate constants. For the addition reaction we obtained the relative rate constant k (Me 3 Si + C 2 H 4 )/ k 1/2 (2Me 3 Si) = (8.7 ± 0.2) × 10 - 10 cm 3/2 s - 1/2 at room temperature. From studies in the temperature range 300−470 K the following values of the Arrhenius parameters for the addition reaction were obtained: 11.6 ≤ E a /kJ mol - 1 ≤ 14.1 and 7.1 × 10 - 13 ≤ A /cm 3 s - 1 ≤ 1.9 × 10 - 12 . In addition, it was found that alkyl and silyl radicals closely obey the geometric mean rule and that disproportionation reactions between alkyl and silyl radicals are of only minor importance. The reactivity of the silyl radical is much higher than that of the alkyl radical in adding to C 2 H 4, and this can be explained in terms of polar effects.