p-Sulfonic acid calix[n]arenes: the most active and water tolerant organocatalysts in esterification reactions
This work reports the novel results obtained from an in situ kinetic study, which was conducted via1H NMR, of the palmitic acid esterification reaction with deuterated methanol in the presence of organocatalysts: p-sulfonic acid calix[6]arene, p-hydroxybenzenesulfonic acid and p-toluenesulfonic acid. The kinetic order of reaction relative to the catalyst concentration and the main thermodynamic properties, such as the activation energy (ΔE), entropy variation (ΔS), enthalpy variation (ΔH) and Gibbs free energy variation (ΔG), were obtained. The results indicated that p-sulfonic acid calix[6]arene was the most active organocatalyst in esterification reactions, regardless of temperature or catalyst concentration employed. Additionally, by performing the reactions in the presence of increasing amounts of water, it was found that p-sulfonic acid calix[n]arenes are more tolerant to water than the other catalysts investigated. This result, when combined with the kinetic and thermodynamic measurements, may explain their highest catalytic activity.