Structure-Antioxidant Capacity Relationships of the Dihydroxy Derivatives of Benzaldehyde and Benzoic Acid

In this paper, the antioxidant capacities (AOC) of the dihydroxybenzaldehyes (DHB) and dihydroxybenzoic acids (DHBA) are determined by two single electron transfer (SET) assays, namely CUPRAC and e-CUPRAC (CUPric Reducing Antioxidant Capacity), and a SET + hydrogen atom transfer (HAT) assay: 2,2-diphenyl-1-picrylhydrazyl (DPPH). It is found that the oxidation potential (EOX) determined by voltammetry can be used as a structural parameter in the same way as the HOMO energy. From the dependencies of the AOC with EOX, it follows that only the electrondonor derivatives of DHB and DHBA exhibit antioxidant activity, which means that the number of hydroxyl groups, two for all molecules, is not the only factor related to the AOC. A relationship between the position of the hydroxyl groups and the AOC is shown. Moreover, the value of the AOC also has a contribution of dissociation of the hydroxyl groups.