α-Deuterium Isotope Effects in Benzyl Halides. 2. Reaction of Nucleophiles with Substituted Benzyl Bromides. Evidence for a Change in Transition-State Structure with Electron-Donating Substituents

Article abstract of DOI:10.1021/ja00541a014

Rates and α-D isotope effects have been determined for the following substrates and nucleophiles: p-methoxybenzyl bromide (Et3N, SCN^-, N3^-, OH^-, S2O3^2-) benzyl bromide (Et3N, SCN^-, N3^-, OH^-, S2O3^2-), and p-nitrobenzyl bromide (Et3N, SCN^-, N3^-, S2O3^2-).In nearly all cases the second-order rate constant for each nucleophile goes though a minimum for the unsubstituted compound while the α-D isotope increases monotonically in the sequence p-NO2>p-H>p-OCH3.These results are consistent with an increasing "looseness" of the S_N2 transition state as the substituent on the aromatic ring becomes more electron donating.