Estudo DFT da transesterificação alcalina de glicerídeos derivados do ácido pentanóico

Abstract

Alkaline catalyzed transesterification reaction mechanism of the base-catalyzed of pentylic acid triglycerides was studied employing ab initio Hartree-Fock (HF) and Density Functional Theory (DFT) calculation for small molecules that mimic animal and vegetal fats. It was used alkyl chain of five carbons and there was considered the attack of methoxid and ethoxid, with and without counter-ion (sodium and potassium). For global transesterification reaction the calculation were carried out for methanolisys without counter-ion, using a pentylic acid triglycerides. All the calculation were carried out in both gas phase and solvent. Solvent effects were included by means of the integral equation formalism for the polarizable continuum model (IEFPCM). By considering the counter ion effects, the results suggests a concerted mechanism, similar to second order nucleofilic substitution reaction. For system without counter ion, the results suggests a two steps mechanism with nucleofilic addiction and elimination, which kinetic determinant strep is the formation of the intermediate, with tetrahedral structure on the carbonyl ester. By considering the counter ion effects, the transesterification activation energy for the monoglycerides reaction depends on the alchooxide used, being potassium methoxid the minus energetic, with 7,96 kcal.mol-1 for activation energy and -6,66 kcal.mol-1 for free energy. The global reaction has a exothermic character of 20 kcal.mol-1 and the determinant step is the methanolysis of diglycerides, with 7,78 kcal.mol-1 of activation energy.