Thermodynamics and kinetics of some acid base reactions in benzonitrile and acetonitrile

Abstract

The proton transfer between an acid and a base involves a change of the total amount of charges in the reacting system: ▪ or a transfer of charge: ▪ Therefore the equilibria and the reaction mechanisms of acid basereactions depend strongly on the ability of the solvent to stabilize ions and to participate in the proton transfer process. We have studied the reactions between five-substituted phenols ( AH) and acridine orange ( B) in benzonitrile and between 4-nitrobenzoic acid and 4-nitrophenol and B in acetonitrile. In these systems, free cations are formed according to eqn. (1) and the anions are stabilized by hydrogen bonding to additional phenol in a second step (homoconjugation): ▪ Thermodynamic data were evaluated from spectrometric titrations of acridine orange base with the acids and of Bu 4N-salts of the phenols with phenol solutions at 10-50°C. Proton transfer rates were obtained by temperature jump relaxation measurements in solutions for the kinetic runs were prepared under nitrogen in an all glass apparatus and were filled into the measuring cell without opening the system to the atmosphere. The equilibrium (1) does not depend much on the solvent, if AH = 4-nitrophenol, whereas the formation constant of AHA − increases from k 2 = (4.6 ± 0.5· 10 3 in acetonitrile to K 2 = (2.5 ± 0.2)·10 4 in benzonitrile at 25°C. This has the remarkable consequence that in benzonitrile in spite of its lower polarity more ions are formed than in acetonitrile with the same concentrations of reactants. In acetonitrile acid and B are k 1 = 7.2·10 8 and k −1 = 1.8·10 8 ( M −1 s −1) at 25°C. The rates are significantly slower than those calculated for the diffusion controlled process. Similar observations had previously been made with other proton transfer reactions in acetonitrile [1]. In benzonitrile the proton transfer rates in reaction (1) are correlated with the equilibrium constants. The fastest observed rates are diffusion controlled. Consequences of the kinetic results for the interpretation of properties of the two solvents will be discussed.