Studies on the Solvent Effect of Aquo-DMSO Solvent Systems on the [H+] Ion Catalysed Hydrolysis of Butyl Formate

Abstract

The solvent effect of Aquo-dipolar aprotic organic solvent systems was highlighted by studying the kinetics of the acid catalysed solvolysis of an aliphatic formate (Butyl formate) in water-DMSO media (aquo-dipolar aprotic organic solvent systems) of various composition having 20 to 80% of DMSO at five different temperatures ranging from 20 to 40°C. In the beginning, fast decrease followed by slow depletion in the rate with gradual addition of the organic co-solvent in the reaction media and also with increasing temperature of the reaction has been explained in the light of solvation of initial and transition states to different extents. The changes in the values of iso-composition and iso-dielectric activation energies of the reaction have also been explained in the light of solvation and desolvation of initial and transition states to different extent. Increase in the numerical values of free energy of activation (?G*) with simultaneous enhancement in the values of enthalpy of activation (?H*) and entropy of activation (?S*) of the reaction, reveals that DMSO acts as enthalpy accelerator and entropy controller solvent. Effects of ionic strength and [H+] ion on the rate of reaction have also been studied and it is concluded that the acid catalysed hydrolysis of Butyl formate is ion-dipolar reaction and it follows AAC2 mechanistic pathways in water-DMSO reaction media.

Introduction

The study investigates the effect of Dimethyl Sulfoxide (DMSO), a dipolar aprotic solvent with unique solvation properties, on the acid-catalyzed hydrolysis of Butyl formate, a higher formate ester. While the influence of dipolar aprotic solvents like DMF, dioxane, and acetone on lower formates has been widely studied, DMSO’s effect on higher formates had not been thoroughly explored.

Experimental Approach:

• Purified DMSO and Butyl formate were used.

• The hydrolysis was carried out by adding the ester to 0.5 M HCl solution, and reaction kinetics were monitored using first-order rate equations.

• Specific rate constants, activation energies (iso-composition ECE_CEC? and iso-dielectric EDE_DED?), solvation numbers, and thermodynamic parameters were evaluated.

• The influence of water concentration and DMSO content on the reaction rate was systematically studied.

Key Results and Discussion:

1. Effect of DMSO on Reaction Rate:

   • Initially, adding DMSO to water decreased the reaction rate, which slowed further beyond 23.75 mol% DMSO.

   • At higher temperatures, this rate-depleting effect diminished.

   • The decrease in rate is attributed mainly to decreased medium polarity and lower dielectric constant, rather than H?O? depletion, since DMSO is non-basic.

2. Solvent Effect on Activation Energy:

   • Iso-composition activation energy ECE_CEC? increased from 104.19 to 136.82 kJ/mol with 20–80% DMSO.

     • This is explained by simultaneous solvation of the initial state and desolvation of the transition state, supported by increasing activation entropy.

   • Iso-dielectric activation energy EDE_DED? decreased from 147.31 to 115.95 kJ/mol as the dielectric constant of the medium increased.

   • These trends align with prior studies and illustrate the dual role of DMSO in modulating reaction energetics.

3. Participation of Water in Transition State and Mechanism:

   • Using Robertson’s method, the number of water molecules in the activated complex was determined.

   • At low water concentrations (<46.3% v/v), ~1 water molecule participates; at higher concentrations, ~1.5 molecules are involved.

   • This indicates that the mechanism shifts from unimolecular to bimolecular with increasing water content and temperature.

   • DMSO influences water structure, promoting a transition from dense to bulkier water forms, affecting the reaction pathway.

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.