Reaction Pathways and Cyclic Chain Model of Free Radicals in the Early Stage of Natural Ester Autoxidation: A DFT and Experimental Study

Free radical reactions are crucial in the thermal oxidation of natural esters, yet the underlying elementary reaction pathways and oxidation mechanisms remain unclear. This study uses density functional theory to systematically investigate these mechanisms. The interaction between triglyceride free radicals and oxygen during the initial thermal oxidation stage was examined. Structural and thermodynamic data for 16 reactions were evaluated, proposing a cyclic model for the involved free radicals. Most reactions are exothermic, releasing 35.06–166.59 kJ/mol. Initial reactions (R1 and R2) are thermodynamically favourable for heat accumulation and peroxyl radical formation. Peroxyl radicals initiate cyclic pathways and react with C–C double bonds, forming new active sites and alkoxy radicals (RO·), depleting reactive groups and oxygen. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) data corroborate the computational predictions, confirming the approach’s precision. This study suggests eliminating key active groups (R·, ROO·, RO· and ·OH) to prevent premature oxidative failure of natural esters by terminating the cyclic chain reaction. This insight could guide the development of more stable natural ester-based products.