Abstract
Temperature-jump studies of the reactions of tartaric acid and bitartrate and tartrate anions with boric acid at three different hydrogen ion concentrations allowed the determination of the rate constants for the reactions of tartaric acid and tartrate anion. The complexation rate constant for tartaric acid is 475 M-1 sec-1, which is considerably larger than the rate constant for tartrate anion (215 M-1 sec-1). Only a composite rate constant could be determined for the ambident bitartrate anion (430 M-1 sec-1), which cannot be experimentally separated into rate constants for the individual reactions. No catalytic effect of acid on the rates of the individual reactions was noted within the limits of the experiments. A concerted mechanism is proposed for the complex formation. The sequence includes attack of a nucleophilic alcoholic oxygen on the electron-deficient boron with concurrent release of water (in the tartaric acid reaction) or hydroxyl (in the bitartrate reaction). The leaving of water is assisted in the former case by the acidic carboxyl proton.