A Kinetic Study of the Solvent Effect of Aquo-Dipolar Aprotic Organic-Solvent Systems on the Biochemical Efficiency of Caprate Ester

Abstract

For studying the Solvent-effect of a dipolar aprotic solvent, DMSO on the biochemical efficiency of Caprate ester, the kinetics of alkali catalysed hydrolysis of Propyl caprate was carried out in aquo-DMSO solvent systems having different concentration of the organic component (DMSO) varying from 30 to 80% (v/v) at different temperatures ranging from 20 to 40°C The mumber of water molecules associated with the activated complex of the reaction were found to decrease from 1.339 to 0.324 and from this, it is inferred that DMSO acts as structure reformer of water converting its bulky form into dense form and it changes the mechanistic path way of the reaction from unimolecular to bimolecular. With gradual addition of DMSO to the reaction media, enthalpy of activation (?H*) and entropy of activation (?S*) were found increasing with simultaneous increase in free energy of activation (?G*). On the basis of this observation, it has been inferred that the enthalpy of activation enhances to greater extent than entropy of activation and initial state of the reaction is solvated and the transition state is desolvated, The Iso-composition activation energy (EC) values of the reaction were found to be enhancing from 102.11 kJ/mol to 130.53 kJ/mol with increasing concentration of DMSO in the reaction media. From this enhancement in EC it has been confirmed that the initial state of the reaction is solvated and its transition state is desolvated. From the depletion observed in iso-dielectric activation energy (ED) of the reaction with increase in D values of the reaction media, it is concluded that EC and ED values of the reaction are complementary to each other. From the numerical value of the iso-kinetic temperature of the reaction, which is evaluated to be 331.44, it is opined that there is strong and appreciable interaction between solvent and solute in water-DMSO reaction media.

Introduction

The study investigates the effect of solvent composition on the solvolysis of Propyl caprate in water–DMSO mixtures, focusing on both reaction kinetics and mechanisms and its biochemical and industrial relevance (e.g., perfumes, cosmetics, and food additives).

Experimental Approach

• Reaction system: Alkali-catalyzed solvolysis of Propyl caprate in water–DMSO mixtures (30–80% v/v DMSO).

• Temperatures studied: 20, 25, 30, 35, and 40°C.

• Methods:

  • Second-order kinetics to calculate specific rate constants.

  • Plots of log?k vs log?[H?O] to determine solvation number.

  • Arrhenius plots to calculate iso-composition (EC) and iso-dielectric (ED) activation energies.

  • Thermodynamic activation parameters (ΔG*, ΔH*, ΔS*) determined via Wynne-Jones and Eyring equations.

Key Findings

1. Mechanistic Insights

• Solvation number:

  • Below 47.9% water, water molecules in the activated complex decrease from 0.860 to 0.324.

  • Above 47.9% water, they decrease from 1.339 to 0.584.

• Mechanistic shift:

  • DMSO acts as a structure reformer, converting water from bulky to dense forms.

  • Reaction mechanism shifts from unimolecular to bimolecular in the presence of DMSO.

2. Thermodynamic Activation Parameters

• ΔG (free energy of activation):* Increases with DMSO content (e.g., at 30°C: 78.85 → 81.16 kJ/mol).

• ΔH and ΔS:** Both decrease with increasing DMSO, indicating enthalpy-dominated but entropy-controlled reaction.

• Interpretation: Non-linear variations of ΔH* and ΔS* suggest specific solvation in water–DMSO mixtures; transition state is desolvated while initial state is solvated.

3. Iso-composition Activation Energy (EC)

• EC increases from 102.11 to 130.64 kJ/mol with DMSO concentration.

• Cause: The initial state is solvated while the transition state is desolvated, consistent with ΔH* and ΔS* trends.

4. Iso-dielectric Activation Energy (ED)

• ED decreases from 142.18 to 128.08 kJ/mol as the dielectric constant (D) increases (opposite trend of DMSO concentration).

• EC and ED are complementary; both reflect solvent effects on reaction energetics.

Conclusion

• DMSO modifies the solvolysis of Propyl caprate by:

  1. Changing the mechanistic pathway (unimolecular → bimolecular).

  2. Affecting thermodynamic and activation parameters, indicating desolvation at the transition state.

  3. Highlighting the importance of solvent composition in industrial and biochemical applications such as perfumes, cosmetics, and food additives.

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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.