A Kinetic Study of the Solvent Effect of Tertiary Alcohol on the Thermodynamic Extensive Properties of the Catalysed Solvolysis of Heavy Acetate in Aquo-t-butanol Solvent Systems

Abstract

The solvent effect of a tertiary alcohol was studied on the themodynamic extensve properties of solvolysis of a heavy acetate (butyl acetate) in different aquo-t-butanol reaction media containing 20 to 80% (t-butanol) at different temperatures varying from 20 to 40°C. From the enhancement observed in ?G* values with simultaneous decrease in the values of ?H* and AS of the reaction, it is inferred that the organic co-solvent t-butanol acts as entropy controller and enthalpy stimulator solvent for alkali catalysed hydrolysis of butyl acetate From the evaluated values of water moleculex associated with the activated complex of the reaction which are found to increase with increase in the temperature of the reaction, it is inferred that the bimolecular mechanistic path is changed to unimolecular in presence of the organic component (t-butanol) of the reaction media. The numerical value of Iso-kinetic temperature of the reaction which comes to be nearly 287.0 (below 300) indicates that there is weak but considerable solvent-solute interaction in the aquo-t-butanol reaction media

Introduction

The study investigates the effect of tertiary alcohol (t-butanol) as a co-solvent on the kinetics, mechanism, and thermodynamic activation parameters of alkali-catalyzed hydrolysis of butyl acetate in water-t-butanol media. While solvent effects on simple esters have been widely studied, little attention has been given to heavy esters in tertiary alcohol-containing media.

Experimental Setup:

• Hydrolysis was conducted in aquo-t-butanol mixtures (20–80% t-butanol) with 0.1 M NaOH and 0.05 M ester.

• The reaction follows second-order kinetics.

• Rate constants, iso-composition activation energies (E_C), iso-dielectric energies (E_D), and thermodynamic activation parameters (ΔH*, ΔG*, ΔS*) were evaluated using Arrhenius, Wynne-Jones, and Eyring equations.

• Mechanistic insights were obtained by determining the number of water molecules associated with the transition state at different temperatures.

Key Findings:

1. Effect of t-Butanol on Reaction Rate:

   • Specific rate constants decrease with increasing t-butanol content.

   • Two linear regimes intersect at ~18.1 mol% t-butanol, reflecting different solvation and dielectric effects.

   • Rate decrease attributed to lower polarity and dielectric constant of the solvent mixture.

2. Effect on Activation Energy (E_C and E_D):

   • Iso-composition activation energy (E_C) decreases from 97.07 to 62.45 kJ/mol with increasing t-butanol.

   • Iso-dielectric activation energy (E_D) increases with increasing dielectric constant of the medium.

   • ΔH* and ΔS* decrease with more t-butanol, while ΔG* slightly increases, indicating the reaction becomes entropy-controlled and enthalpy-stimulated.

3. Solvent-Solute Interactions:

   • Iso-kinetic temperature (~287 K) indicates weak but significant solvent-solute interactions in aquo-t-butanol media.

4. Mechanistic Insights:

   • The number of water molecules in the transition state increases with temperature and t-butanol content, from 0.236 to 1.145.

   • Suggests a shift from bimolecular to unimolecular reaction mechanism at higher temperatures.

   • Water structure changes from dense to bulky form in presence of t-butanol.

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.