The decomposition of triphenylimidazole-para-acetate follows specific base catalysis and can be conveniently followed by fluorescence.

Abstract

The kinetics of the decomposition reaction of 4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl acetate (1) in basic alcoholic media was investigated, using a simple fluorescence (FL) spectrophotometric procedure. The process was conveniently studied using FL, since the triphenylimidazole-derived ester 1 and its reaction products (the corresponding phenol 2 and phenolate 2- ) are all highly fluorescent (ΦFL >37%). By carefully selecting excitation and emission wavelengths, observed rate constants k1 in the order of 10-3 to 10-2 s-1 were obtained from either reactant consumption (λex =300nm, λem =400nm) or product formation (λex =350nm, λem =475nm); these were shown to be kinetically equivalent. Intensity-decay time profiles also gave a residual FL intensity parameter, shown to be associated to the distribution of produced species 2 and 2- , according to the basicity of the medium. Studying the reaction in both methanol (MeOH) and isopropanol (iPrOH), upon addition of HO- , provided evidence that the solvent's conjugate base is the active nucleophilic species. When different bases were used (tBuO- , HO- , DBU and TEA), bimolecular rate constants kbim ranging from 4.5 to 6.5Lmol-1 s-1 were obtained, which proved to be non-dependent on the base pKaH , suggesting specific base catalysis for the decomposition of 1 in alcoholic media.