Heavy Atom Isotope Effect Studies of Elimination Reaction Mechanisms. 1. A Kinetic and Carbon-14 Kinetic Isotope Effect Study of the Base-Promoted Dehydrochlorination of Substituted 1-Phenylethyl-2-14C Chlorides

Article abstract of DOI:10.1021/ja00350a038

Upon treatment with the solvent conjugate base, the primary reaction of substituted 1-phenyl chloride is elimination to substituted styrenes in t-BuOH-10percent v/v Me2SO at 60 deg C and in bis(2-hydroxyethyl) ether-10percent v/v Me2SO at 45 deg C.Kinetics studies using eight substituted chlorides show that these reactions are strongly accelerated by both electron-donating and electron-withdrawing substituents, probably indicating a fairly reactant-like E2 transition state with the mechanism changing from E1-like E2 to E1cB-like E2 as the substituents change from EDG to EWG.In this first carbon isotope effect study of an elimination reaction of an alkyl chloride, carbon-14 kinetic isotope effects have been measured in the alkoxide/bis(2-hydroxyethyl) ether-Me2SO system for 1-(4-methylphenyl)ethyl-2-14C chloride, k/^βk = 1.038, 1-phenylethyl-2-14C chloride, k/^βk = 1.058, and 1-(4-chlorophenyl)ethyl-2-14C chloride, k/^βk = 1.068.Clearly none of these compounds reacts by an irreversible E1 mechanism, for which no β-carbon isotope effect should be observed.The trend in the results is what would be expected from increased C_β-H bond rupture as the E2 transition state shifts from E1-like to E1cB-like.Theoretical calculations involving E1-like E2 transition-state models show that in this region only for a reaction coordinate motion that strongly couples C_β-H bond rupture, C_α-C_β double bond formation, and C_α-Cl bond rupture does the calculated value for k/^βk come into the experimentally observed range, and then only for reactant-like models.