Solvent Effect of Polyhydric Alcohol on the Biochemical Potential of Iso-amyl Methanoate

Abstract

The biochemical potential of Iso-amyl methanoate is due to formation of methanoic acid as its solvolysis product. It is best highlighted by studying the kinetics of acid catalysed solvolysis of Iso-amyl methanoate in aquo-Glycerol reaction media having different concentration of Glycerol (trihydric alcohol) from 20-80% (v v) and at different temperatures ranging from 20 to 40°C. The depletion and enhancement observed respectively in iso-composition and iso-dielectric activation energies reveal that the transition state is solvated and initial state is desolvated with addition of Glycerol in reaction media, Almost unity value of the slope of the plots of log k values against log [H+] values shows that the reaction follows AAC2 mechanism. From the values of iso-kinetic temperature, which comes to be 283.0, it may be concluded that in water-Glycerol reaction media, the reaction follows Barclay-Butler rule and there is weak but acceptable interaction between solvent and solute.

Introduction

The study investigates the effect of polyhydric alcohol (Glycerol) on the acid-catalyzed hydrolysis of Iso-amyl methanoate, which is relevant for its use as a flavoring agent in food and beverages. While the solvent effects of dipolar aprotic solvents (DMSO, DMF, dioxane) on similar reactions have been studied, the impact of polyhydric alcohols had not been previously explored.

Experimental Details:

• High-purity Iso-amyl methanoate and Glycerol were used.

• Hydrolysis was conducted in 0.5 M HCl, with 0.70 mL ester added to 50 mL solution.

• The reaction follows first-order kinetics.

• Rate constants, iso-composition and iso-dielectric activation energies (E_C and E_D), thermodynamic activation parameters (ΔH*, ΔG*, ΔS*), and the effect of H? concentration were evaluated.

• Number of water molecules in the transition state was determined using Robertson’s equation.

Key Findings:

1. Effect of Glycerol on Reaction Rate:

   • Rate constants decrease with increasing Glycerol content.

   • Rapid decrease up to ~17.8 mol% Glycerol, then slower decline.

   • The decrease is attributed to lower dielectric constant and solvation effects, consistent with Hughes-Ingold predictions.

2. Mechanism and Water Molecules:

   • Number of water molecules associated with the transition state increases with temperature and water content, from 0.290 to 1.565.

   • Mechanism shifts from bimolecular to unimolecular with higher water content or higher temperature.

   • Addition of Glycerol shifts water from dense to bulky structure.

3. Activation Energies:

   • Iso-composition activation energy (E_C) decreases from 101.43 to 62.85 kJ/mol with more Glycerol, due to solvation of the transition state and desolvation of the initial state.

   • Iso-dielectric activation energy (E_D) increases from 75.03 to 122.51 kJ/mol with increasing dielectric constant.

4. Thermodynamic Activation Parameters:

   • ΔG* slightly increases (82.73 → 85.10 kJ/mol), while ΔH* and ΔS* decrease, indicating an entropy-controlled but enthalpy-dominated reaction.

   • Nonlinear variation of ΔH* and ΔS* with Glycerol indicates specific solvation effects.

5. Solvent-Solute Interaction:

   • Iso-kinetic temperature (~283 K) suggests weak but significant solvent-solute interactions in aquo-Glycerol media.

6. Effect of H? Concentration:

   • Rate is linearly dependent on [H?] (slope ≈ 1), indicating the reaction follows an AAC2 (acid-assisted unimolecular) mechanism.

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.