Kinetic medium effects of cationic cosolutes in aqueous solution: the effects of alkylammonium bromides on the neutral hydrolysis of 1-benzoyl-1,2,4-triazole

Abstract

We have investigated the kinetic effects of alkylammonium, alkyldimethylammonium, alkyltrimethylammonium and tetraalkylammonium bromides on the water-catalysed hydrolysis of 1-benzoyl-1,2,4-triazole at pH 4 and 25 °C. All cosolutes retard the reaction. The retardation is attributed to a dominant stabilisation of the initial state through hydrophobic interactions with the cosolute. The results are analysed in terms of pairwise Gibbs energy interaction parameters. These G(C) values show that alkyl groups shorter than propyl have no significant influence on the medium effect. The extensively hydrated ammonium groups prevent the formation of a well developed hydrophobic hydration shell by methylene moieties in its vicinity. We observe additivity of kinetic effects of methylene groups outside the ionic hydration sphere on the hydrolysis reaction. Pairwise group interaction parameters were obtained for the CH₂-group of the different cosolutes; the contribution of the CH₂-group to the overall cosolute effect is -94 J kg mol^-2 for both alkylammonium bromides and alkyldimethylammonium bromides, and -110 J kg mol^-2 for alkyltrimethylammonium bromides. Ammonium bromide, which is primarily hydrophilic, also retards the reaction. We suggest that competition between ammonium bromide and the transition state for water molecules results in the strict orientational requirements for the water molecules in the activated complex not being met. Hence the hydrolysis is retarded.