Effect of 1‐ethyl‐3‐methylimidazolium acetate on the oxidation of caffeic acid benzyl ester: An electrochemical and theoretical study

Abstract

AbstractThe effect of the ionic liquid (IL), 1‐ethyl‐3‐methylimidazolium acetate ([emim][AcO]) on the oxidation of (E)‐phenylmethyl‐3‐(3,4‐dihydroxyphenyl)‐2‐propenoate (commonly known as caffeic acid benzyl ester [CABE]) was analyzed through experimental and theoretical methods, such as cyclic voltammetry (CV) and density functional theory (DFT) calculations. The obtained results demonstrated that the AcO− anion promotes the oxidation of the caffeic ester; this is because of the basic character of AcO−, which plays an important role to reduce the amount of energy required for the removal of one electron from the IL‐CABE complex. This suggests that a strong hydrogen bond is formed between this anion and the phenolic H─O. Even the presence of water in the mixture of CABE‐IL does not have a significant effect on the electrooxidation of the complex. Hence, it is possible to infer that CABE is more attached to [emim][AcO] than to the water molecules in the solvation sphere under the experimental conditions evaluated here. Other intermolecular interactions between CABE and IL also contribute to stabilize the resulting complex, e.g., van der Waals and cation‐π interactions, which were evidenced through the theoretical noncovalent interactions (NCI) methodology. The relevant role of basic anions in the extraction of phenolic compounds, using ILs, has been documented in the literature; it should be considered that the strength of the hydrogen bond formed between the phenolic H─O and the IL should not contribute to the oxidation of target compounds.