Laser flash photolysis in aqueous basic solutions of the ortho acid derivatives 1-(phenyldimethoxymethyl)benzimidazole 2 and 4-bromo-1-(phenyldimethoxymethyl)imidazole 3 results in production of the phenyldimethoxymethyl cation, which has λmax at 260 nm. The cation decays in reactions with water (k = 9.9 × 104 s−1) and hydroxide ion (2.5 × 108 M−1 s−1) to finally yield methyl benzoate, whose formation was monitored at 234 nm. In solutions with pH 10–12, rate constants measured at this wavelength are the same as those obtained at 260 nm, but with pH > 13 and pH < 9, rate constants at 234 nm are smaller. With pH 9–10 and pH 12–13, single exponential kinetics are not observed at 234 nm. This behavior is interpreted in terms of a scheme where at each pH there are two consecutive first-order reactions, cation → phenyldimethoxyhydroxymethane (5) → ester, and the pH dependencies of the rate constants are such that they cross twice over the pH range of this study. The intermediate 5 is the tetrahedral intermediate formed in the methanolysis of methyl benzoate, and the 234-nm buildup at pH > 13 and pH < 9 directly measures its breakdown. At pH > 13 the rate constant is independent of pH with k = 9 × 106 s−1. This represents the rapid expulsion of methoxide from the conjugate base of 5. At pH < 9 the rate constants are proportional to hydroxide ion concentration, with [Formula: see text]. In these solutions the neutral intermediate predominates and the dependence on [OH−] of its rate of conversion to ester is interpreted in terms of breakdown of the anion and protonation of this species by water occurring at comparable rates. Thus, [Formula: see text] represents a situation where there is partial rate-limiting deprotonation of the neutral intermediate by hydroxide. The intermediate of this study bears a close resemblance to the tetrahedral intermediate of the hydrolysis of methyl benzoate. The observation that the anionic forms of such intermediates undergo breakdown at rates similar to those associated with the establishment of proton transfer equilibrium explains why the ester undergoes carbonyl oxygen exchange in base at a rate slower than hydrolysis. Keywords: tetrahedral intermediate, flash photolysis, ester hydrolysis.
Measurements of Rate Constants for HO2 + NO and NH2 + NO Reactions by
Time-Resolved Photoionization Mass Spectrometry
