An unexpected linear Brønsted correlation

Abstract

The reaction of barbiturate and 1,3-dimethylbarbiturate ions with o-, p- and 2,4-dinitrobenzaldehyde was studied. The reactions of barbiturate anion with o- and p-nitrobenzaldehyde exhibit a pH-rate profile different to that the corresponding to the reactions of barbiturate and 1,3-dimethylbarbiturate ions with 2,4-dinitrobenzaldehyde. The dependence of the rate constant on the viscosity of the medium in the pH range 2–4, for all the reactions, indicates the contribution of a diffusion-controlled proton transfer from the hydronium ion to an addition intermediate, T−, in the rate-determining step. Surprisingly, in the reaction of 2,4-dinitrobenzaldehyde with barbiturate and 1,3-dimethylbarbiturate anions, the Brønsted plot for general acid catalysis for carboxylic acids of pKa between 2 to 5 gives a linear relationship with α = 0·707 (r = 0·991), whereas α = 0 is expected, considering the pKa of the addition intermediate T−. On the other hand, the point for the rate constant, considering water as a general acid catalyst, falls approximately 105 times above the corresponding Brønsted line. The tautomerism existing in the intermediate T− permits these surprising facts to be explained.