Solvent effects on the kinetics and thermodynamics of the fast proton-trasfer reactions of trichloroacetic acid and picric acid with phenyl diethyl nile blue base in aprotic solvents

Abstract

Rate and equilibrium constants have been determined by a microwave temperature-jump method for the reactions of trichloroacetic acid and picric acid with the basic indicator phenyl diethyl Nile Blue base (5-phenylimino-9-diethylamino-benzo[α]phenoxazine), over a range of temperature in five solvents—1-chlorobutane, 1-chloropentane, chlorobenzene, bromobenzene and iodobenzene. The rate constants are inversely correlated with the viscosities of the solvents; they are 8–24 times smaller than the values calculated for diffusion-controlled reactions by simple diffusion theory (Debye-Smoluchowski equation), and are more sensitive to viscosity. The results can be explained by preliminary formation of a weak complex, within which the reactant molecules can rotate into positions favourable to reaction.

The question whether the viscosity-dependence of the rate is attributable to the first step (encounter) or the second (rotation) is discussed.