Mechanism of the reaction between silyl radicalsand chloroethylenes: a case study of the Et3Si-C2Cl4 reaction

The photolysis and radiolysis of C2Cl4 solutions in Et3SiH (Et-triethyl) were studied at 298 K. The main products, Et3SiCl and C2Cl3H, are formed in equal yields and via a free radical chain mechanism, as indicated by the high quantum yields (approx. 500) and G values (approx. 1600). The reactions C2Cl3 + Et3SiH → C2Cl3H + Et3Si (3) and Et3Si + C2Cl4 → Et3SiCl + C2Cl3 (4) constitute the chain propagation step. Competitive studies yield k4/k11 of 018 +/- 0.01 (2sigma) where Et3Si + t-BuCl → Et3SiCl + t-Bu (11). The mechanistic implications and consequences of the observation that the reaction of Et3Si radicals with C2Cl4 results almost exclusively in Cl transfer rather than addition are discussed, and the conclusions are generalized for similar reactions of other chloroethylenes. 21 references, 3 figures, 4 tables