Benzene-Based Carboxylic Acids: A DFT Study of the Effect of Alkyl-Chain Length on Structural, Thermochemical and Electronic Properties

A systematic density functional theory (DFT) investigation was performed to examine the influence of alkyl-chain length on the structural, energetic, thermochemical, and electronic properties of benzene-based alkyl carboxylic acids. A homologous series of 4-alkylbenzoic acids, ranging from 4-methylbenzoic acid to 4-sexylbenzoic acid, was studied at the B3LYP/6-311G(d,p) level using Gaussian 09. The optimized molecular structures were analyzed in terms of total electronic energy, dipole moment, thermal energy, heat capacity, and entropy. The results reveal a systematic variation in molecular properties with increasing alkyl-chain length. The total electronic energy becomes progressively more negative, while the dipole moment shows an overall increasing tendency. The thermochemical parameters, including thermal energy, heat capacity and entropy, increase consistently with chain extension, indicating enhanced molecular complexity and conformational flexibility. Frontier molecular orbital analysis shows a gradual reduction in the HOMO-LUMO energy gap from 5.6159 to 5.5824 eV, accompanied by systematic changes in the global reactivity descriptors. These results demonstrate that progressive alkyl-chain extension produces measurable changes in the stability, thermochemical behavior and electronic reactivity of benzene-based alkyl carboxylic acids. The study provides useful theoretical insights into the structure–property relationship of this class of substituted aromatic carboxylic acids.

DFT; B3LYP; HOMO-LUMO; Alkyl chain

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