Kinetics of the Thiosulfate-Bromoacetate Reaction in the Presence of Electrolytes

Abstract

The reaction between thiosulfate and bromoacetate ions in solvent mixtures of different dielectric constant and in the presence of added salts fails to give results conforming to the predictions of the BrSnsted primary salt effect. The deviation from theory is particularly high in solvents of low dielectric constant and in the presence of higher valence type electrolytes. A mechanism for the over-all reaction is proposed to account for the apparent abnormalities. New experimental data are added to those previously obtained and are correlated with the BrSnsted-Christiansen-Scatchard theoretical expressions which relate the log of the molar reaction rate constants to the dielectric constant of the solvent mixture in which reaction takes place, the ionic strength of the solutions making up the reaction mixture, and the size and charge of the intermediate complex assumed to be formed in ionic reactions. The essential condition in the postulated mechanism is that the reaction proceeds through the formation of a complex or ion pair between the added cation and thiosulfate ion, followed by reaction of this complex with bromoacetate ion.The reaction between bromoacetate and thiosulfate ions in several solvent systems has been investigated extensively (3-10). The conclusions reached by Ciapetta and Tomilinson (10) are pertinent to the remarks to be made in the present discussion. They may be summarized as follows.(A) Specific solvent effects are minor. Observed rate constant data obtained with different isodielectric constant solvents, but with identical concentrations of reactants and added salt, are of the same order of magnitude.(B) Strontium and calcium nitrate mixed with reactants produce high rate constants. Rate constants level off to almost constant values when these salts are present in excess of the reactants.(C) When lanthanum nitrate is the added salt, reaction rate constants are as high as one hundredfold times those anticipated from the BrSnstedChristiansen-Scatchard theoretical expressions (1,2). When the reaction is allowed to take place in solvents of low dielectric constant, the rate constants increase in value with increase in the added salt concentration, reaching a maximum when the added salt concentration equals that of reactants, then fall off with further increases in added salt.(D) LaMer and Davis (3) have shown that when solvents range in dielectric constant from 20 to 35 so that constant DT values are obtained, the observed energies of activation for the reactions with added salt are the same as those in which salt addition is absent. They concluded that abnormal rate constants were the result of certain electrostatic attractions leading to possible ion pair formations. Ciapetta and Tomilinson postulated an ion pair (LaS~O~BrAc) ~ since they observed that the maximum in the rate constant value was usually obtained when the salt concentration added with reactants was equal to that of the reactants. They concluded further that rate constants were lower in the presence of greater concentrations of ...

The Influence of Ion-Pairs on Reaction Mechanism The Bromoacetate-Thiosulfate Reaction

“…Corsaro (7) has reported data for reaction rates with systems containing equimolar amounts of bromoacetate-thiosulfate ions and thorium nitrate additions less than reactant ion concentrations. The reactions follow an apparent third order mechanism since they were second order with respect to thiosulfate ion concentration, and consistent third rate constants were calculated from the appropriate rate expression.…”

“…Previous studies of the bromoacetatethiosulfate ionic reaction (4,7,8,(17)(18)(19)(20)(21) usually have concerned themselves with the effect of added salts to sodium reactant mixtures. In this investigation reactant mixtures consisted of sodium, cesium, potassium, and calcium thiosulfate with the corresponding bromoacetate salt.…”

“…The bromoacetate-thiosulfate reaction shows serious deviation from theory which cannot be reconciled with the assumption that "free" ions are reactants. Corsaro (7) has summarized the nature and magnitude of these deviations and proposed a mechanism which accounts for them based on the postulate that ion-pairs are participants in reaction. Davies and Williams (8) developed a two term rate expressidn for reactions involving bromoacetate and thiosulfate ions which took into account the ion-pair concentration as a reactant and were able to correlate the rate data with a modified BrSnsted primary salt effect expression which they derive.…”

“…The bromoacetate-thiosulfate reaction shows serious deviation from theory which cannot be reconciled with the assumption that "free" ions are reactants. Corsaro (7) has summarized the nature and magnitude of these deviations and proposed a mechanism which accounts for them based on the postulate that ion-pairs are participants in reaction.…”

“…It is the objective of this paper to demonstrate the presence of ion-pairs in reactions involving thiosulfate and bromoacetate ions, and the contributions these ion-pairs make to the mechanism of the reaction. Previous studies of the bromoacetatethiosulfate ionic reaction (4,7,8,(17)(18)(19)(20)(21) usually have concerned themselves with the effect of added salts to sodium reactant mixtures. In this investigation reactant mixtures consisted of sodium, cesium, potassium, and calcium thiosulfate with the corresponding bromoacetate salt.…”