Kinetics and Mechanism of the Pyridinolysis of Benzyl Bromides in Dimethyl Sulfoxide

摘要

Nucleophilic substitution reaction of Y-benzyl bromide with X-pyridines are investigated in DMSO at 45.0∩. Biphasic rate dependence is observed on varying the substituents in the substrate (Y) as well as in the nucleophile (X). The two well-defined straight lines in the Hammett (ヱy) and Bronsted ( モx) plots are interpreted to indicate the changes in transition-state structure, a decrease in bond cleavage as the substituent on the substrate is changed from electron-donors ( ヱy < 0) to electron-acceptors ( ヱy > 0), and an increase in the extent of bond formation with the corresponding changes of the substituent on the pyridine. A Jencks` type analysis of separate polar (ヱ) and resonance (ヱr) effects can also be accounted for by the change of the transition-state structure, not by the variable combination of polar and resonance effects.