Studies on the Solvent Effect of Aquo-n-propanol Solvent systems on the Kinetic of Catalysed Solvolysis of Aliphatic Caprate Ester

Abstract

The kinetics of the alkali catalysed hydrolysis of aliphatic caproate (Propyl caproate) were studied in aquo-n-propanol solvent systems and the specific rate constants of the reaction were found decreasing with increasing concentration of the organic content (n-propanol) of the media It is inferred that dielectric effect and the solvation changes are the causes of depletion in the rate of the reaction. The iso-composition activation energy (EC) and iso-dielectric activation energy (ED) of the reaction were found to increase and decrease respectively and from this, it is inferred that the transition state of the reaction is desolvated and its initial state is solvated. From the deletion observed in the solvation number of the water content of the aquo-n-propanol solvent systems with increase in the temperature of the reaction it is inferred that unimolecular mechanistic path of the reaction is changed to bimolecular mechanistic path. Increase observed in free energy of activation with simultaneous increase in the values of both the ?H* and ?S*, it is inferred that in the presence of n-propanol with reaction media, the reaction becomes enthalpy dominating and entropy inhibiting. From the evaluated values of iso-kinetic temperature of the reaction which comes to be 328, it is concluded that Barclay-Butler rule is obeyed by the reaction and there is strong solvent-solute interaction in presence of n-propanol in the reaction media.

Introduction

This study investigates the alkali-catalyzed hydrolysis of propyl caproate in water–n-propanol mixtures containing 30–80% (v/v) n-propanol at temperatures ranging from 20°C to 40°C. The reaction was analyzed because of its commercial and pharmaceutical importance and the limited previous research on this ester. Experimental results showed that the hydrolysis follows second-order kinetics. Various kinetic and thermodynamic parameters, including specific rate constants, activation energies, solvation numbers, and activation parameters (ΔH*, ΔG*, and ΔS*), were determined.

The results revealed that the reaction rate decreases as the concentration of n-propanol increases because the solvent mixture's dielectric constant decreases, reducing ion–dipole interactions and altering solvation. The iso-composition activation energy (EC) increased with increasing n-propanol concentration, indicating greater energy is required for the reaction due to enhanced solvation of the reactants and desolvation of the transition state. Similarly, the iso-dielectric activation energy (ED) increased as the dielectric constant decreased, confirming the complementary relationship between EC and ED.

Analysis of solvation numbers showed that the number of water molecules associated with the activated complex decreased with increasing temperature, suggesting a change in the reaction mechanism from unimolecular to bimolecular. This behavior was attributed to structural changes in water from a bulky to a denser form in the presence of n-propanol and higher temperatures.

The thermodynamic activation parameters ΔH*, ΔG*, and ΔS* all increased with increasing n-propanol concentration, indicating specific solvation effects, a desolvated transition state, and a solvated initial state. The larger increase in enthalpy compared to entropy suggests that the reaction is enthalpy-controlled and entropy-suppressing. Furthermore, the reaction obeyed the Barclay–Butler relationship, yielding an iso-kinetic temperature of approximately 327 K, which confirms significant solvent–solute interactions during the hydrolysis process. Overall, the study demonstrates that solvent composition strongly influences the kinetics, mechanism, and thermodynamic behavior of propyl caproate hydrolysis.

Conclusion

The kinetics of alkali catalysed hydrolysis of Propyl caproate (having longer carbon chain) in water-n-propanol media has been carried ut, as this reaction is very useful from commercial as well as medicinal points of views and also that it has not been paid adequate attention by the researcher so far.

References

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Copyright © 2026 Dr. Kumari Priyanka. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.