Kinetics of α-hydrogen exchange of sodium α-[4-(4-bromophenyl)-thiazol-2-yl]propionate in D2O solutions

Abstract

Data are reported for the rate of incorporation of deuterium into sodium α-[4-(4-bromophenyl)thiazol-2-yl]propionate in D₂O solutions studied as a function of pD by use of a quantitative n.m.r. technique. The first-order rate coefficients measured over the pD range 6–14 pass through a minimum at pD = 10. The data, measured at 30°, fit the kinetic expression k_obs=k_D3O+[D₃O⁺]+k_OD^–[OD^–] where k_D3O⁺ and k_OD^– have the values 4·1 × 10² and 7·3 × 10^–3 l. mole^–1 sec.^–1 respectively. No catalysis could be detected by other acidic or basic species. A mechanism involving a change in the position of the tautomeric equilibrium of the acid and its anion is advanced to explain the observed pD profile. Quantitative support for this interpretation is given by data for the tautomeric ratios of related model compounds.