Radical-scavenging activity of butylated hydroxytoluene (BHT) and its metabolites

To clarify the radical-scavenging activity of butylated hydroxytoluene (BHT), a food additive, stoichiometric factors ( n) and inhibition rate constants ( k inh) were determined for 2,6-di- tert-butyl-4-methylphenol (BHT) and its metabolites 2,6-di- tert-butyl- p-benzoquinone (BHT-Q), 3,5-di- tert-butyl-4-hydroxybenzaldehyde (BHA-CHO) and 3,5-di- tert-butyl-4-hydroperoxy-4-methyl-2,5-cyclohexadiene-1-one (BHT-OOH). Values of n and k inh were determined from differential scanning calorimetry (DSC) monitoring of the polymerization of methyl methacrylate (MMA) initiated by 2,2′-azobis(isobutyronitrile) (AIBN) or benzoyl peroxide (BPO) at 70 °C in the presence or absence of antioxidants (BHT-related compounds). The n values declined in the order BHT (1–2) > BHT-CHO, BHT-OOH (0.1–0.3) > BHT-Q (∼0). The n value for BHT with AIBN was approximately 1.0, suggesting dimerization of BHT. The k inh values declined in the order BHT-Q ((3.5–4.6)×10 4 M −1 s −1) > BHT-OOH (0.7–1.9×10 4 M −1 s −1) > BHT-CHO ((0.4–1.7)×10 4 M −1 s −1) > BHT ((0.1–0.2)×10 4 M −1 s −1). The k inh for metabolites was greater than that for the parent BHT. Growing MMA radicals initiated by BPO were suppressed much more efficiently by BHT or BHT-Q compared with those initiated by AIBN. BHT was effective as a chain-breaking antioxidant.