DIFFUSION-CONTROLLED AND CONCERTED BASE CATALYSIS IN DECOMPOSITION OF HEMITHIOACETALS

Rate and equilibrium constants are reported for the reactions of benzenethiol, p-nitrobenzenethiol, and thioacetic acid with acetaldehyde to form the corresponding hemithioacetals. The hydroxide ion catalyzed breakdown of these hemithioacetals occurs with rate constants near 1010 M−1sec−1 and is largely or entirely limited by the rate of diffusion-controlled encounter of the substrate and catalyst. The energy of activation is 2-3 kcal/ mole and the solvent deuterium isotope effect kOH-lkOD- is 1.25. In the synthesis reaction, formation of the carbon-sulfur bond and proton transfer (eq 11) occur rapidly and separation of hydroxide ion from the hemithioacetal (eq 10) is the rate-determining step. Weaker bases catalyze the breakdown reaction at a slower rate with a Brφnsted slope β of 0.8 ± 0.03. This indicates that carbon-sulfur bond formation or cleavage and proton transfer are in some sense concerted. Equilibrium constants for hemithioacetal formation are independent of the acidity of the thiol over a range of acidity of 107. © 1969, American Chemical Society. All rights reserved.