The substituent effect on the antioxidant capacity of catechols and resorcinols

Abstract The antioxidant capacity of polyphenols is widely recognized; especially, the catechol moiety is considered a key piece in the structure of natural compounds with this characteristic. The resorcinol is also a dihydroxybenzene that is present in natural antioxidants; however, fewer experimen...
Ausführliche Beschreibung

Abstract The antioxidant capacity of polyphenols is widely recognized; especially, the catechol moiety is considered a key piece in the structure of natural compounds with this characteristic. The resorcinol is also a dihydroxybenzene that is present in natural antioxidants; however, fewer experimental and theoretical studies have been addressed to this moiety. This work reports a comparative study of the substituent effect on properties related to the antioxidant capacity, as energy ionization (I), and O–H bond dissociation enthalpy (BDE), to analyze the electron transfer and hydrogen atom transfer (HAT) mechanisms, in derivatives of phenols, catechols and resorcinols. In the set of substituents tested %$({-}\hbox {CH}_3, {-}\hbox {CH}_2\hbox {CH}_3, {-}(\hbox {CH}_2)_2{-}\hbox {CH}_3, {-}\hbox {CH}{=}\hbox {CH}_2, {-}\hbox {CH}{=}\hbox {CHCOOH}, {-}\hbox {COOH}%$ and %${-}\hbox {CO}(\hbox {CH}_2)_4{-}\hbox {CH}_3)%$ electron-withdrawing and electron-donating groups, including alkyl chains, are considered. We found that electron-donating substituents reduce I and O–H BDE of resorcinols and catechols derivatives. In addition, the length of the alkyl chains is not relevant on resorcinols neither on catechols. With regard to the HAT process, a double bond in a substituent favors it, while the –COOH group shows a contrary effect. The combination of a double bond with a –COOH group, in the substituent, increases the BDE and reduces the electron donor capacity, and this result suggests that this combination induces a reduction in the antioxidant capacity in one polyphenol compound. In several reports, an intramolecular hydrogen bond has been proposed within to the most stable structure of catechol. However, by using arguments from the atoms in molecules approach and the electron localization function, we show that such an interaction is not relevant to stabilize this conformation and consequently and this is the reason that explains the unimportance of this interaction in the properties studied here and in previous studies. Ausführliche Beschreibung