Analysis of Methyl Substitution in Phenolic Compounds as Potential Biofuel Additives: From Solid‐State Description to Antioxidant Potential

ABSTRACT Background: The growing demand for renewable energy sources has positioned biodiesel as a promising alternative to fossil fuels. However, its limited oxidative stability poses significant challenges for storage and performance, particularly at elevated temperatures. Objectives: This study aimed to investigate the influence of a methyl substituent group on the structural, electronic, and antioxidant properties of hydroquinone‐derived phenolic compounds to evaluate their potential as biofuel additives. Methods: A combination of supramolecular and theoretical approaches was employed to examine the intermolecular interaction patterns in the crystals of 2,5‐dimethylhybenzene‐1,4‐diol and 2‐methylhybenzene‐1,4‐diol through QTAIM and natural bond orbital (NBO) analyses. Additionally, density functional theory (DFT) calculations at the M06‐2X/6‐311++G(d,p) level of theory were performed to optimize the molecular structures and evaluate the corresponding chemical reactivity descriptors. The antioxidant mechanisms and the stability of the radicals generated during free‐radical scavenging were further investigated using thermodynamic descriptors. Results: Electronic‐structure analysis showed that methyl substitution slightly lowers the bond‐dissociation enthalpy and ionization potential of the phenolic compounds, thereby enhancing their antioxidant capacity. Compared with commercial additives, the studied compounds exhibited competitive thermodynamic antioxidant profiles. Conclusions: Both compounds demonstrated antioxidant potential as multifunctional biofuel additives, with dimethyl substituted hydroquinone standing out as a promising candidate.