Studies on the Solvent Effect of Aquo-n-Propanol Solvent Systems on the Catalysed Solvolysis of Aliphatic Decanoate Ester

Abstract

The solvent effect of a primary alcohol namely n-propanol on the alkali catalysed solvolysis of reaction was studied by carrying out the hydrolysis of the ester having longer carbon chain namely Methyl decanoate in water-n-propanol media of varying composition consisting of 30 to 80% n-propanol (v/v) at different temperatures ranging from 20 to 40°C. The specific rate constant values of the reaction were found to be depleted with increasing concentration of n-propanol in the reaction media. It was found that number of water molecules associated with the activated complex increases from 0.300 to 1.495 with increasing temperature from 20°C to 40°C and this tells about the fact that the bimolecular mechanistic path of the reaction is changed to unimolecular. The decrease and increase observed respectively in the values of iso-composition activation energy (EC) and iso-dielectric activation energy (ED) of the reaction show that the transition state is solvated and initial state is desolvated with addition of n-propanol in reaction media. From enhancement observed in values of ?G* with simultaneous depletion in ?H* and ?S* values of the reaction, it has been concluded that the reaction is enthalpy stimulating and entropy inhibiting and specific solvation is taking place in the water-n-propanol media. From the evaluated values of iso-kinetic temperature, Le 284.80?285 which is less than 300, it is inferred that this reaction in water-n-propanol media obeys Barclay-Butler rule and there is a weak but considerable solvent-solute interaction taking place in the reaction media.

Introduction

The study investigates the alkali-catalyzed hydrolysis of methyl decanoate in water–n-propanol mixed solvents to understand the solvent effect of the higher primary alcohol (n-propanol) on reaction kinetics and mechanism. Since methyl decanoate is widely used in perfumes, fruit flavors, and food additives, understanding how solvent composition affects its biochemical efficiency is important.

The reaction was conducted at five temperatures (20, 25, 30, 35, and 40°C) with fixed concentrations of ester (0.5 M) and alkali (0.1 M). The reaction followed second-order kinetics, and rate constants were determined under varying solvent compositions.

Effect of n-Propanol on Reaction Rate

• The rate constant decreases as the proportion of n-propanol increases.

• Up to 20 mol% n-propanol, the rate drops sharply; beyond that, it decreases more gradually.

• The rate depletion becomes more pronounced at higher temperatures.

• The decrease in rate is attributed to:

  • Dielectric effect: Addition of n-propanol lowers the dielectric constant of the medium.

  • Solvation effect: n-Propanol, being a protic solvent, alters solvation of reactants and transition states.

The findings align with predictions that ion–dipole reactions slow down as dielectric constant decreases, rather than fully matching Hughes and Ingold’s earlier predictions.

Effect on Reaction Mechanism

By analyzing the relationship between log k and log [H?O], the number of water molecules (n) associated with the activated complex was determined.

Key findings:

• Two mechanistic regions were observed, intersecting at 44.2% water concentration.

• The number of water molecules in the activated complex increases with temperature (from 0.300 to 1.495 between 20°C and 40°C).

• Increasing solvation number (n) suggests a mechanistic shift from bimolecular to unimolecular reaction pathway at higher temperatures in the presence of n-propanol.

• Water structure changes from dense to bulkier form as temperature rises.

Thus, solvent composition and temperature significantly influence the reaction pathway.

Effect on Activation Energy

1. Iso-composition Activation Energy (Ec):

   • Ec decreases with increasing n-propanol concentration.

   • This is attributed to solvation changes, particularly greater solvation of the transition state compared to the initial state.

   • Decreases in activation enthalpy (ΔH*) and entropy (ΔS*) further support this interpretation.

2. The transition state (a large anionic species) interacts more strongly with n-propanol, stabilizing it and lowering activation energy.

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.