AN UNEXPECTED LINEAR BRONSTED CORRELATION

The reaction of barbiturate and 1,3-dimethylbarbiturate ions with o-, p- and 2,4-dinitrobenzaldehyde was studied, The reactions of barbitura te anion with o- and p-nitrobenzaldehyde exhibit a pH-rate profile dif ferent to that the corresponding to the reactions of barbiturate and 1 ,3-dimethylbarbiturate ions with 2,4-dinitrobenzaldehyde, The dependen ce 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 inte rmediate, T-, in the rate-determining step, Surprisingly, in the react ion of 2,4-dinitrobenzaldehyde with barbiturate and 1,3-dimethylbarbit urate anions, the Bronsted plot for general acid catalysis for carboxy lic acids of pK(a) between 2 to 5 gives a linear relationship with alp ha = 0.707 (r = 0.991), whereas a = 0 is expected, considering the pK( a) of the addition intermediate T-. On the other hand, the point for t he rate constant, considering water as a general acid catalyst, falls approximately 10(5) times above the corresponding Bronsted line, The t automerism existing in the intermediate T- permits these surprising fa cts to be explained.