The cationic dimerization of 1,1-diphenylethylene. III. Kinetics of initiation by aluminum chloride

Kinetics of the reaction of AlCl₃ with 1,1-diphenylethylene (DPE) in CH₂Cl₂ solution have been investigated at low temperature by ultraviolet (UV) spectrophotometry. Initiation proceeds in three successive stages. The first, very fast and leading to a low and constant concentration of carbocations, is assigned to residual traces of cocatalyst. A second stage, slower but more extensive, yields about one carbocation for two aluminum in a reaction that is first order in AlCl₃ and first order in monomer (k_i = 2.5 10^?2 liter mole^?1 sec^?1 at ?30°; E_i = 10 kcal mole^?1). This stage, which takes place after consumption of residual cocatalyst, should be attributed to a direct initiation. In a proposed mechanism the cationic intermediate produced by reaction of DPE with AlCl₃ transfers a proton to DPE, thus leading to a stable diphenylethyl cation and to alkenyldichloroaluminum (Al-Cl₂CH?CPh₂). A third stage develops slowly and the yield in carbocations finally reaches 65–70% with respect to AlCl₃. This stage is tentatively assigned to a second direct initiation by the less active organoaluminum compound formed in the preceding stage.