Explaining the antioxidant activity of some common non-phenolic components of essential oils

Limonene, linalool and citral are common non-phenolic terpenoid components of essential oils, with attributed controversial antioxidant properties. The kinetics of their antioxidant activity was investigated using the inhibited autoxidation of a standard model substrate. Results indicate that antioxidant behavior of limonene, linalool and citral occurs by co-oxidation with the substrate, due to very fast self-termination and cross-termination of the oxidative chain. Rate constants k_p and 2k_t, (M^-1s^-1) at 30°C were 4.5 and 3.5×10⁶ for limonene, 2.2 and 9.0×10⁵ for linalool and 39 and 1.0×10⁸ for citral. Behavior is bimodal antioxidant/pro-oxidant depending on the concentration. Calculations at the M05/6-311+g(2df,2p) level indicate that citral reacts selectively at the aldehyde C-H having activation enthalpy and energy respectively lower by 1.3 and 1.8kcal/mol compared to the most activated allyl position. Their termination-enhancing antioxidant chemistry might be relevant in food preservation and could be exploited under appropriate settings.