Kinetic and equilibrium solvent isotope effects on the deprotonation of a salicylate ion by hydroxide ion and general bases

Abstract

Kinetic and equilibrium solvent isotope effects have been measured for proton transfer from 4-(3-nitrophenylazo)salicylate ion (HA) to general bases (B) to examine whether a primary kinetic isotope effect is observed when proton transfer is occurring between bases of roughly equal strength for which ΔpK= pKHA– pKBH+= 0. The kinetic solvent isotope effects for catalysis by three tertiary amines (ΔpK–0.85, –0.27, and + 0.58) are kBH2O/kBD2O 2.0 ± 0.3, 2.2 ± 0.3, and 1.7 ± 0.2. Since for proton transfer to hydroxide ion (ΔpK–5.0) the observed isotope effect has a value kOH–H2O/kOD–D2O of 1.5 ± 0.1, the primary isotope effect for these proton transfers must be small and independent of base strength over the range studied. An explanation in terms of the reaction mechanism is given.