Kinetic Study of Aquo-Ethylene Glycol on Alkali Catalyzed Hydrolysis of Heterocyclic Acid Ester Methylpicolinate and Thermodynamic Activation Parameters

Abstract

A kinetic investigation was conducted to analyze the thermodynamic parameters of the hydrolysis of Methylpicolinate in water-Ethylene glycol solvent system,using different concentration of Ethylene glycol (v/V) at four different temperature 200C, 250C, 300C, and 350C.The influence of the solvent on the reaction rate was analyzed in terms of activation parameters.The decreasing trend in ?H* and ?S* values, coupled with the rising ?G* with increasing concentration (mole %) of the organic co -solvent in the reaction media,inferred an enthalpy-dominated and entropy-controlled reaction.The evaluated value of Iso-kinetic temperature isvery close to 300, suggesting the presence of weak but positive interaction between solvent and solute present in the reaction media.

Introduction

Objective

The study investigates how solvent composition, particularly the proportion of ethylene glycol (EG) in water, affects the rate and thermodynamic parameters of the alkali-catalyzed hydrolysis of methyl picolinate. It aims to understand how solvent polarity and solvation influence reaction kinetics and mechanisms.

Experimental Approach

• Aqueous solutions with 10% to 60% v/v ethylene glycol were prepared.

• The reaction mixture included ethylene glycol-water solvent, NaOH, and methyl picolinate.

• Reactions were conducted at 20°C, 25°C, 30°C, and 35°C.

• Reaction progress was monitored using conductivity measurements.

• Second-order rate constants (k) and thermodynamic activation parameters (?H*, ?G*, ?S*) were calculated.

Key Findings

1. Solvent Effect on Reaction Rate

• Reaction rate decreases with increasing ethylene glycol content.

• This is due to:

  • Lower dielectric constant with EG addition.

  • Reduced polarity of the medium, which weakens solvation of the transition state.

• These trends align with Hughes-Ingold theory and prior findings by Singh et al.

2. Thermodynamic Activation Parameters

• As ethylene glycol content increases:

  • ?H* and ?S* decrease significantly.

  • ?G* increases slightly.

• This suggests:

  • The reaction is enthalpy-promoting (favorable enthalpic pathway).

  • It is entropy-deactivating (more ordered transition state).

• Nonlinear trends in ?H*, ?G*, and ?S* indicate specific solvation behavior in water-EG mixtures.

3. Iso-Kinetic Temperature and Solvent-Solute Interaction

• A plot of ?H* vs. ?S* gives an iso-kinetic temperature (β) of 286 K, indicating:

  • Weak but positive solvent-solute interactions.

• Supports conclusions by Barclay-Butler and others, including Lefler and Singh et al.

Conclusion

Based on the analysis of the results, the following findings have been observed: 1) The specific rate constant values exhibit a decreasing trend with an increasing proportion of the co-solvent across all temperatures. 2) The nonlinear plots of ?G*, ?H*, and ?S* against the mole percentage of the solvent suggest the occurrence of specific solvation in the reaction medium. 3) The simultaneous decrease in ?H* and ?S* values, along with an increase in ?G*, indicates that the reaction is enthalpy-stimulated and entropy-inhibited. 4) The iso-kinetic temperature, though slightly below 300, suggests significant interactions between the solvent and solute within the reaction medium.

References

[1] Singh A K., ArjumanBano: Kinetics and Solvent effect on Aquo-dipolarprotic organic-solvent system of Hexanoate ester.” Asian journal of Research in Chemistry, 12(6) Nov 2019, pp 341-344. [2] Sharma Sangita et al.:Kinetic study of specific base catalysed hydrolysis of Ethyl Acrylate in water-Ethanol binary system. Russian Journal of Physical Chemistry A. Vol. 87, No. 5, 2013. Pp730-736 [3] Singh A K.:Kinetics and solvent effect on activation parameter of aquo-propanol solvent system for acid catalyzed solvolysis of propyl formate. International Journal of Chemical Science, vol-3, issue-4, July, 2019 pp85-88.

Copyright

Copyright © 2025 Shalini Kumari, D. K. Verma, Prahlad Kumar, Rajesh Kumar, Rachana Rani. 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.