A Kinetic Study of the Solvent Effect of Aquo-Dipolar Protic Solvent Systems on the Catalysed Solvolysis of Higher Hexanoates

Abstract

The Kinetics of alkali catalysed hydrolysis of propyl hexanoate (a higher hexanotes) in aquo-propan-2-ol media was studies with a view to highlight the effect of a dipolar protic solvent (propan-2-ol) on the medicinal efficiency of the hydrolytic product of the ester (hexanote acid or caproic acid). The initial sharp decrease followed by slow and smooth depletion in the rate with gradual addition of the organic co solvent in the reaction media and with increasing temperature of the reaction has been explained in the light of solvation or desolvation of initial and transition state to the different extent. The increase and decrease respectively in the value of iso-composition and iso-dielectric activation energies of the reaction respectively have been observed and explained on the basis of solvation and desolvation to different extent of initial and transition states by the added solvent. Enhancement in the values of free energy activation ?G* with simultaneous increase in enthalpy of activation ?H* and entropy of activation ?S* reveals that the specific solvation is taking place in the reaction media and propnal-2-ol acts like entropy controller and enthalpy stimulator solvent.

Introduction

The study investigates the solvent effect of propan-2-ol (a dipolar protic solvent) on the alkali-catalyzed hydrolysis of propyl hexanoate, addressing a research gap where such solvents and higher-carbon aliphatic esters have received little attention compared to dipolar aprotic solvents. Kinetic experiments were conducted in water–propan-2-ol mixtures using NaOH, and rate constants along with activation and thermodynamic parameters were evaluated.

Results show that increasing the proportion of propan-2-ol reduces the reaction rate, with a marked change in slope around 17.6 mol % propan-2-ol. The rate depletion decreases with rising temperature and is attributed to lower medium polarity and dielectric constant, reflecting combined dielectric and solvation effects. Iso-composition activation energy increases with higher propan-2-ol content, indicating greater solvation of the initial state and desolvation of the transition state, supported by rising activation entropy. Conversely, iso-dielectric activation energy decreases with increasing dielectric constant.

Analysis of solvation numbers reveals that the number of water molecules associated with the transition state decreases with higher propan-2-ol content and temperature, implying a shift from bulk to dense water structures. Mechanistically, the reaction pathway transitions from unimolecular to bimolecular with increasing propan-2-ol concentration and temperature.

Thermodynamic analysis shows simultaneous increases in ΔH*, ΔS*, and ΔG*, with the process being enthalpy-dominated and entropy-controlled. Evaluation of the iso-kinetic temperature (~320 K) indicates strong solvent–solute interactions and structural changes in the reaction system. Overall, propan-2-ol significantly influences kinetics, mechanism, and energetics through solvation and dielectric effects.

References

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Copyright

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.