Kinetic Isotope Effects in the Menschutkin-Type Reaction of Benzyl Benezenesulfonates with N,N-Dimethylanilines. Variation in the Transition-State Structure

Article abstract of DOI:10.1021/ja00319a030

Primary carbon-14 and secondary α-tritium isotope effects were measured for the Menschutkin-type reaction of m-bromobenzyl-methylene-14C X-substituted benzenesulfonates with Y-substituted N,N-dimethylanilines in acetone at 35 deg C (eq 1; Z=m-Br).The large carbon-14 (¹²k/¹⁴k = 1.117-1.151) and small α-tritium (^Hk/^Tk = 1.026-1.041) isotope effetcs were consistent with the S_N2 mechanism of the reaction.A monotonous trend was observed when the carbon isotope effects were plotted against the relative rates for varied Xs and a fixed Y (p-CH3), while a bell shape was observed for varied Xs and a fixed (p-Cl).Comparison of the results with those obtained for the unsubstituted benzyl esters indicated that the substitution on the benzyl moiety with the electron-withdrawing m-Br group made the transition state more product-like.Smaller α-tritium isotope effects for the m-Br series than those for the unsubstituted series verified tighter transition states for the former.The variation in the three-centered S_N2 transition states caused by the substituents on the leaving group, the nucleophile as well as the benzyl moiety, is discussed in terms of Thornton's rules and described on a potential energy map.