.alpha.-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

Rates and ..cap alpha..-D isotope effects have been determined for the following substrates and nucleophiles: p-methoxybenzyl bromide (Et/sub 3/N, SCN/sup -/, N/sub 3//sup -/, OH/sup -/, S/sub 2/O/sub 3//sup 2 -/), benzyl bromide (Et/sub 3/N, SCN/sup -/, N/sub 3//sup -/, OH/sup -/, S/sub 2/O/sub 3//sup 2 -/), and p-nitrobenzyl bromide (Et/sub 3/N, SCN/sup -/, N/sub 3//sup -/, S/sub 2/O/sub 3//sup 2 -/). In nearly all cases the second-order rate constant for each nucleophile goes through a minimum for the unsubstituted compound while the ..cap alpha..-D isotope increases monotonically in the squence p-NO/sub 2/ > p-H > p-OCH/sub 3/. These results are consistent with an increasing looseness of the S/sub N/2 transition state as the substituent on the aromatic ring becomes more electron donating. 4 figures, 3 tables.