Solvent Effects in the Kinetics of Fast Proton-transfer Reactions: Diffusion, Preliminary Complexes, and Steric Factors

Abstract

The kinetics of the reaction of tri-n-butylamine with the substituted phenol tetrabromophenol- phthalcin ethyl ester (Bromophthalein Magenta E) in various solvents have been investigated, by a microwave temperature-jump method. The rate constants are inversely correlated with the viscos ities of the solvents, and are 15 to 30 times less than the values calculated for diffusion control. Bromophthalein Magenta E shows close similarities with other substituted phenols (c.g. 2,4-dinitro- phenol and Bromophcnol Blue) in its reactions with tertiary aliphatic amines. All the data on these reactions are satisfactorily correlated by a theory which envisages diffusion-controlled formation of weakly-bound complexes, during whose life-time the reactant molecules can rotate into positions where the active centres are adjacent so that reaction can proceed. Alternative models are con sidered and found unsatisfactory.