Search

A kinetic study has been made of the decomposition of tetraphenyl-phosphonium bromide with hydroxide or methoxide anions in mixtures of dimethyl sulphoxide with methanol. Both reactions display drastic rate increase as the DMSO content of the medium is increased. The rate of the OH− reaction is faster than that of the OMe−, due to a change in mechanism, and the kinetic factor k'OH-/k'OMe-depends largely on solvent composition. The transition state solvation is believed to exihibit a large contribution in rate enhancement. Solvent effects are shown to be far more important in determining the electrostatic phenomena than other solvent properties like dielectric constant. The thermodynamic parameters of the activated complex are considered.

The rate of the acid-catalysed hydrolysis of benzyl acetate in DMSO-water mixtures increases slightly first then decreases with increase of DMSO content of the medium in a normal manner. However, the rate of the base-catalysed reaction in the same solvent system increases with addition of DMSO. This behaviour was attributed to the ability of the DMSO molecules to desolvate hydroxide anions through formation of 1:2 association complexes with water molecules. A kinetic equation involving the simultaneous direct as well as inverse proportionality of the rate to the water concentration was explained. The effects of the dielectric constant and solvation changes on the rate constant and thermodynamic parameters were discussed.

The kinetics of decomposition of benzyltriphenylphosphonium chloride by sodium hydroxide was studied in binary aqueous solvent mixtures ranging from pure water to about 70% (v/v) of dipolar aprotic solvents such as DMSO or acetone in presence of a series of aliphatic and aromatic amines. The rate, which is directly proportional to the organic solvent content, was found to increase to a maximum at 0.1 M amine concentration and then to decrease. The small increase in rate due to the addition of amine is explained on the basis of an amine-water interaction, where the concentration of OH− ions in the solution increases slightly due to the ability of the amines to abstract protons from water molecules in the medium. However, the decrease in rate beyond 0.1 M amine concentration is explained as due to the decrease of hydroxide ion concentration by formation of a weakly ionised quaternary ammonium hydroxide. The observed activation energies, thermodynamic parameters and radii of the activated complex for all solvent compositions are computed. The effect of the bulk dielectric constant on the reaction rate was investigated in the light of the available electrostatic theories.

The rates for the consecutive acid hydrolysis of ditert-butylsuccinate were measured in a series of an aqueous dipolar aprotic solvent, namely, acetone. The observed rates of the two steps of the reaction were analysed into four separate terms due to the unimolecular (AAL1) and bimolecular (AAC2) mechanisms. These rates were correlated with the measured acidity function H0 of the medium, where linear relationships exist at lower acetone contents for the unimolecular rate constants (k′I1 & k′II1). The bimolecular rate constants (k′I2 & k′II2), however, follow a non-linear trend indicating the operation of the AAC2 mechanism. The activation parameters and the rate-acidity function relationships were computed and discussed as evidences of mechanistic pathway and solvation effects involved in the reaction.

The alkaline decomposition of benzyltriphenylphosphonium chloride was studied kinetically in a series of DMSO-H 2 O and DMSO-MeOH mixtures at several temperatures in presence of HO - or MeO - anions, respectively. The reaction rate showed great variations with relative changes of the constituents of the mixed medium, and rate enhancements were generally accompanied by increase in methoxide or hydroxide ion activities. The rate of the alkoxide-promoted decomposition was found to be always smaller than that of the hydroxide-promoted one. Comparative results of the kinetics, rate constants and activation parameters point to different mechanisms with different rates for the two reactions. The role of changing solvation states of reactants and transition complexes as the solvent composition is varied, though large, was found to be of a minor contribution to the rate changes as compared to the effect of change in mechanism. The thermodynamic parameters of activation showed strong dependence on solvent composition and were related to structural changes and solvation power of the reaction medium.

Comparative study of the role of solvent effects of aqueous methyl cellosolve and tetrahydrofuran solvent systems on the alkaline decomposition of tetraphenylphosphonium bromide was carried out. The reaction rate increases remarkably as the medium changes from purely aqueous to a water-organic cosolvent reaction mixture. The relative changes in the solvation states of both reactants and transition state as the structural properties of the two solvent binaries change were shown to be the direct reasons for the difference in the effects on rate and activation energies.

A kinetic investigation of the consecutive acid-catalysed hydrolysis of ditert.-butyl malonate in dioxane-water mixtures is given. The rate constants of the first and second hydrolysis steps decrease by addition of the organic solvent, and both steps proceed concurrently by AA11 and AAc2 mechanisms. The kinetic ratio kI/kII depends on the solvent composition. The results are compared with those obtained previously for ditert.-butyl succinate in many aspects such as rate, percentage AA11 fission and the role of the inductive as well as steric effects. The maximum concentration of the intermediate, halfester, decreases with decrease in the ionising power of the solvent and has nearly the same value for malonate and succinate in isocomposition media. The effect of the dielectric constant on the rate constant and the activation thermodynamic parameters are discussed.

The rates of the acid hydrolysis of potassium ethyl malonate in presence of KCl, NaCl, Li2SO4 and MgSO4 in water and in 50% by weight of dioxane—water mixture were measured. The concentration of the added salts covered the range 0–1M. A negative salt effect on the rate of the half-ester hydrolysis was observed. The applicability ofBronsted's equation was tested and deviations found were explained as due among other reasons, to the operation of an iondipole type of interaction. The correlation between the hydration numbers as well as the radii of the cations of the added electrolytes and the reaction rate were discussed. The activation energy was found to increase with an increase of the concentration of electrolyte due to the relative changes in solvation of the transition state as a result of hydration of the electrolytes added. The thermodynamic properties of the activated complex were calculated and discussed as functions of the salt concentration. A remarkable linear compensation between ΔH ≠ and ΔS ≠ was found.

The acid-catalysed hydrolysis of potassium ethyl malonate in presence of 0.05N-HCl in water and in dioxane-water as well as acetone-water mixtures containing up to about 90% by weight of the organic solvent was studied over the temperature range 30–55°C. The reaction rate decreases continuously with decreasing water content of the solvent mixtures until it reaches a minimum at about 80% dioxane and increases again. The isocomposition activation energy shows only slight changes with solvent composition. The kinetics of the reaction was investigated from the viewpoints of solvent composition, water concentration and dielectric constant. The radii and thermodynamic parameters of the activated complex were calculated and discussed. The conclusions drawn show that the reaction is better treated as an ion-dipole rather than an ion-ion interaction.

The rate of the acid-catalysed hydrolysis of tert-butyl acetate was found to decrease pronouncly in the presence of increasing amounts of DMSO. The observed activation energy decreased progressively with increasing DMSO content of the medium and the change was attributed to a gradual interconversion of the two concurrent and competing reaction mechanisms AALl and AAc2. The overall reaction rate was analysed theoretically as two individual rate constants corresponding to the respective mechanisms. The percentage contribution of each mechanism was then estimated and found to be concordant with the values determined experimentally by others using 180 tracer technique. The proportion of the AALl mechanism was found to increase both with increasing temperature and decreasing DMSO content. The relative abundance of the transition state belonging to each of the involved mechanisms was discussed in the light of the solvating power of the binary DMSO-H20 solvent system. The effect of the molar concentration of water as well as the dielectric constant of the medium on the reaction kinetics was studied. The thermodynamic parameters of activation showed strong dependence on solvent composition and their values were determined by the relative contributions of the two mechanistic routes of the reaction.

The rates of the acid hydrolysis of potassium ethyl malonate in presence of KCl, NaCl, Li2SO4 and MgSO4 in water and in 50% by weight of dioxane—water mixture were measured. The concentration of the added salts covered the range 0–1M. A negative salt effect on the rate of the half-ester hydrolysis was observed. The applicability ofBronsted's equation was tested and deviations found were explained as due among other reasons, to the operation of an iondipole type of interaction. The correlation between the hydration numbers as well as the radii of the cations of the added electrolytes and the reaction rate were discussed. The activation energy was found to increase with an increase of the concentration of electrolyte due to the relative changes in solvation of the transition state as a result of hydration of the electrolytes added. The thermodynamic properties of the activated complex were calculated and discussed as functions of the salt concentration. A remarkable linear compensation between ΔH≠ and ΔS≠ was found.

The acid-catalysed hydrolysis of potassium ethyl malonate in presence of 0.05N-HCl in water and in dioxane-water as well as acetone-water mixtures containing up to about 90% by weight of the organic solvent was studied over the temperature range 30–55°C. The reaction rate decreases continuously with decreasing water content of the solvent mixtures until it reaches a minimum at about 80% dioxane and increases again. The isocomposition activation energy shows only slight changes with solvent composition. The kinetics of the reaction was investigated from the viewpoints of solvent composition, water concentration and dielectric constant. The radii and thermodynamic parameters of the activated complex were calculated and discussed. The conclusions drawn show that the reaction is better treated as an ion-dipole rather than an ion-ion interaction.