Abstract

103
M. J. White and F. J. Leeper
"Kinetics of the Thiazolium Ion-Catalysed Benzoin Condensation."
J. Org. Chem., 2001, 66, 5124-5131. Full Text

The formation of benzoin (Ph-CHOH-CO-Ph) from two molecules of benzaldehyde, catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide in methanol buffered with Et₃N/Et₃NH⁺.Cl^- has been studied. Initial-rate studies at various concentrations of PhCHO (0.1-1.7 M) showed that the reaction is close to being first order in PhCHO. Following the reaction in deuteriomethanol, ¹H NMR spectroscopy allowed rate constants for all three kinetically significant steps to be determined. These show that all three steps are partially rate-determining. A normal deuterium kinetic isotope effect for the overall reaction (k_H/k_D = ca. 3.4) is observed using PhCDO, and a large inverse solvent isotope effect (k_D/k_H = ca. 5.9) is observed using deuteriomethanol, consistent with the kinetic scheme presented here.

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