Kinetics and thermodynamics of hydrogen-bond formation and proton-transfer in the reactions of tetrabromophenolphthalein ethyl ester with amine bases in chlorobenzene, by a microwave temperature-jump method

Abstract

The proton-transfer reactions of the substituted phenol tetrabromophenolphthalein ethyl ester with various aliphatic and aromatic amines in chlorobenzene have been studied over ranges of temperature. Equilibrium constants and thermodynamic parameters have been determined spectrophotometrically; rate constants and activation parameters for the forward and back reactions have been determined with the aid of a microwave temperature-jump apparatus. As in the case of the reactions with 2,4-dinitrophenol (preceding paper), the forward rate constants approach the value for diffusion control, and are not correlated with the equilibrium constants but appear to reflect steric factors. With some of the amines, spectrophotometric investigation revealed the presence also of a hydrogen-bonded species, whose formation may precede the proton-transfer step; equilibrium constants were determined for several of these complexes. The kinetics have been analysed on the basis of a two-stage mechanism, hydrogen-bond formation followed by proton-transfer. Various considerations, including the absence of a Brønsted-type relation between rate and equilibrium constants, indicate that the formation of a hydrogen-bonded complex or an encounter-complex is rate-limiting and the proton-transfer relatively fast.