Carbon isotope effect studies of the mechanism of the hofmann elimination reaction of para-substituted (2-phenylethyl-1-14C)- and (2-phenylethyl-2-14C)trimethylammonium bromides

Article abstract of DOI:10.1021/ja00023a034

Carbon-14 kinetic isotope effects (KIE) were measured for the NaOEt-promoted elimination reaction of para-substituted (2-phenylethyl)trimethylammonium bromides successively labeled at the α- and β-carbons in ethanol at 40°C. The substantial KIE observed for labeling at both carbons (k/^αk = 1.050, 1.044, 1.040, 1.032, 1.019 and k/^βk = 1.040, 1.040, 1.044, 1.044, 1.042 for p-CH₃O, H, p-Cl, p-CF₃, and p-NO₂, respectively) indicates an E2 rather than an El or E1cB-irrev mechanism, which has substantial E1cB character in all systems studied. Since both primary β-deuterium and nitrogen-15 leaving group isotope effects have been determined by others employing similar aromatic substituents under these same reaction conditions, this system affords the best test to date of the present theories used to predict structural changes in transition states for elimination reactions and of the successive labeling approach.