The coumarin synthesis: a combined experimental and theoretical study

Coumarin, which is an important material class of substrate in agricultural and pharmaceutical industries, can be obtained by the Pechmann condensation reaction. In this work, coumarin was synthesized from ethyl acetoacetate and resorcinol with sulfuric acid as the catalyst with 86% yield under a mild condition. The product was confirmed by IR, 1H NMR, and 13C NMR spectroscopy. Reaction rates at difference temperatures between 273 and 313 K were used to determine the apparent activation energy of the reaction to be 34.7 kJ/mol. Molecular-level understanding about the mechanism of Pechmann condensation reaction was revealed by the complimentary M06-2X density functional theory. The reaction proceeded via a three-step mechanism; transesterification, intramolecular hydroxylalkylation, and dehydration. The intrinsic activation energies of these three steps were calculated to be 136.7, 76.5, and 134.0 kJ/mol, respectively. The calculated apparent activation energy of 40.0 kJ/mol was in agreement with the experimental data. The transesterification was considered as the rate-determining step of the reaction.