Auto‐catalytic production of eugenyl acetate and eugenyl butyrate using microwave radiation: a kinetic and mechanism‐related approach

Abstract : BACKGROUND : Applications of chemical and enzymatic methods of esterification have been limited by low selectivity of chemical catalysts and low productivity and high cost of enzymes. The objective of this study was to produce eugenol esters with high productivity and selectivity using a microwave technique without adding any catalyst or solvent. Specific focus was given to reaction temperature and molar ratio to establish the kinetics and mechanism of eugenol auto‐catalytic acylation using microwave radiation. RESULTS : Reaction conversion was observed to increase in response to higher temperatures. The optimum molar ratio of eugenyl/acetic anhydride was 1:6, while the optimum molar ratio of eugenyl/butyric anhydride was 1:4.5. A high degree of selectivity (of over 97%) occurred under all conditions of the reaction. Based on modelling studies, it can be concluded that the intended reaction follows second‐order kinetics for both eugenyl acetate and eugenyl butyrate. Microwave heating at 70 °C led to an increase of rate constant by 3.71‐ and 3.83‐fold for eugenyl acetate and eugenyl butyrate, respectively, as compared to the conventional method. This was associated with a decrease in the activation energy (4.38% for eugenyl acetate and 3.11% for eugenyl butyrate) and an increase in the frequency factor (42.31% for eugenyl acetate and 49.84% for eugenyl butyrate) and entropy (4.71% for eugenyl acetate and 5.22% for eugenyl butyrate) as a result of using microwave radiation. CONCLUSIONS : The microwave technique can be considered as a green, low‐cost and quick process for auto‐catalytic production of eugenol esters. © 2020 Society of Chemical Industry (SCI)