One of the most important sources of reactive oxygen species (ROS) in biological systems is the Fenton reaction. In this, the Fe 2+ or Fe 3+ reacts with H 2 O 2 to produce ROS as the hydroxyl radical (•OH), superoxide radical (O 2 •-) and singlet oxygen ( 1 O 2 ). The main ROS, responsible for the high oxidizing power of the Fenton reaction, is not clear. Some authors claim that the principal reactive species is •OH, while others propose a ferryl specie (Fe 4+ or [FeO] 2+ ) (1,2) . Recently, have been proposed that the kind of reaction species produced depends mainly of pH and the iron composition of the coordination sphere. This is highlighted for Fe 3+ , because in mono and (some) bis-complexes Fe 3+ is reduced to Fe 2+ and there are some positions occupied by water or hydroxide ligands, readily to be exchanged by H 2 O 2 . On the other hand, in tris-complexes there are not any positions occupied by water or hydroxide, avoiding the formation of peroxo-complexes, necessary for Fenton or Fenton like reaction.The 1,2-dihydroxybenzenes (DHBs) have been described as modulators of Fenton reaction. The DHBs driven Fenton reaction have been used for environmental applications as an advanced oxidation process. Furthermore, these systems participate in different biological process, as the wood biodegradation by fungi and oxidative stress in neurodegenerative diseases.In this review, the effect of 1,2-dihydroxybenzenes on the activated species production by the Fenton and Fenton like reaction will be discussed and its participation in different systems.

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1,2-dihydroxybenzenes (DHBs) are organic compounds which are widely studied as they are applied to advanced oxidation processes (AOPs). These compounds are also related to the development of oxidative stress, wood biodegradation, and neuronal disease in humans. DHBs are metal ligands with pro-oxidant and antioxidant properties. These activities are related to their chelation properties and a consequence of the deprotonation of their hydroxyl groups. In literature, there are several pKa values for the hydroxyl groups of DHBs. These values vary depending on the experimental conditions or the algorithm used for calculation. In this work, an experimentally validated computational method was implemented in aqueous solution for pKa determination of 24 DHBs. The deprotonation order of the hydroxyl groups in DHB was determined observing a selective deprotonation, which depended on the ability of the substituent to donate or withdraw electron density over the ring.

The presence of Fe(III) mono-complex is essential in the ability of catecholamines to increase the oxidative capacity of Fenton systems.

Fenton systems are interesting alternatives to advanced oxidation processes (AOPs) applied in soil or water remediation. 1,2-Dihydroxybenzenes (1,2-DHBs) are able to amplify the reactivity of Fenton systems and have been extensively studied in biological systems and for AOP applications. To develop efficient AOPs based on Fenton systems driven by 1,2-DHBs, the change in reactivity mediated by different 1,2-DHBs must be understood. For this, a systematic study of the reactivity of Fenton-like systems driven by 1,2-DHBs with different substituents at position 4 was performed. The substituent effect was analyzed using the Hammett constant (σ), which has positive values for electron-withdrawing groups (EWGs) and negative values for electron-donating groups (EDGs). The reactivity of each system was determined from the degradation of a recalcitrant azo dye and hydroxyl radical (HO·) production. The relationship between these reactivities and the ability of each 1,2-DHB to reduce Fe(III) was determined. From these results, we propose two pathways for HO· production. The pathway for Fenton-like systems driven by 1,2-DHBs with EDGs depends only on the Fe(III) reduction mediated by 1,2-DHB. In Fenton-like reactions driven by 1,2-DHBs with EWGs, the Fe(III) reduction is not primarily responsible for increasing the HO· production by this system in the early stages.

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