The carbanion mechanism of olefin-forming elimination. Part VI. Mechanism of the dehydrochlorination of 1,1-diaryl-2,2-dichloroethanes

Abstract

Rate constants for dehydrochlorination of the title compounds induced by NaOMe in MeOH and Bu^tOK in Bu^tOH are reported. Primary deuterium isotope effects for the reaction of 1,1-dichloro-2,2-bis-(p-chlorophenyl)ethane (DDD) with NaOMe in MeOH have been measured and these, together with the activation parameters, point to the importance of proton tunnelling. The rate of dehydrochlorination is faster than the rate calculated to pertain to carbanion formation from the latter compound. The Hammett ρ value (1·93) for the Ar₂CH·CHCl₂ series with NaOMe in MeOH is significantly less than that for the Ar₂CH·CCl₃ series, which has been shown to eliminate via carbanion intermediates. The sensitivity of DDD dehydrochlorination rates to [NaOMe] is less than that for Ph₂CH·CCl₃. These three items of evidence suggest that compounds in the Ar₂CH·CHCl₂ series undergo predominantly concerted E2 elimination, but the reaction may involve a ca. 20% contribution from an E1cB pathway.