Kinetic studies of the reaction of phenacyl bromide derivatives with sulfur nucleophiles

Abstract

The reaction of the substituted phenacyl bromides 1a-e and 2a-e with thioglycolic acid 3 and thiophenol 6 in methanol underwent nucleophilic substitution S N2 mechanism to give the corresponding 2-sulfanylacetic acid derivatives 4a-e, 5a-e and benzenethiol derivatives 9a-e, 10a-e. The reactants and products were identified by mass spectra, infrared and nuclear magnetic resonance. We measured the kinetics of these reactions conductometrically in methanol at a range of temperatures. The rates of the reactions were found to fit the Hammett equation and correlated with σ-Hammett values. The ρ values for thioglycolic acid were 1.22-1.21 in the case of 4-substituted phenacyl bromide 1a-e, while in the case of the nitro derivatives 2a-e they were 0.39-0.35. The ρ values for thiophenol were 0.97-0.83 in the case of 4-substituted phenacyl bromide 1a-e, while in the case of the nitro derivatives 2a-e they were 0.79-0.74. The Brønsted-type plot was linear with a α=-0.41±0.03. The kinetic data and structure-reactivity relationships indicate that the reaction of 1a-e and 2a-e with thiol nucleophiles proceeds by a concerted mechanism. The plot of log k 45 versus log k 30, the plot log(k x,3-NO2/ k H) versus log(k x/k H), and the Brønsted-type correlation indicate that the reactions of the thiol nucleophiles with the substituted phenacyl bromides 1a-e and 2a-e are attributed to the electronic nature of the substituents. Copyright © 2011 John Wiley & Sons, Ltd.