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 tetrabromophenolphthalein 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 viscosities 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 (e.g. 2,4-dinitrophenol and Bromophenol 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 considered and found unsatisfactory.