Antioxidant synergism between butylated hydroxyanisole and butylated hydroxytoluene

Decay of the 2,6‐di‐tert‐butyl‐4‐methylphenoxy radical [butylated hydroxytoluene (BHT)‐radical] in the presence of butylated hydroxyanisole (BHA) was investigated in 1,2‐dimethoxyethane with or without triethylamine. BHT‐radical was conveniently generated by dissociation of its unstable dimer in solution. The products were BHT, 3,3′‐di‐tert‐butyl‐5,5′‐dimethoxy‐2,2′‐dihydroxybiphenyl (BHA‐dimer), 2,6‐di‐tert‐butyl‐p‐quinone methide (QM), 1,2‐bis(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)ethane, and 3,3′,5,5′‐tetra‐tert‐butyl‐4,4′‐stilbenequinone. The reaction without added triethylamine gave larger quantities of the last two products and BHA (recovery), whereas the reaction with it provided larger quantities of the first two products. The marked difference in the product distribution can be accounted for by a series of reactions including reversible dehydrogenation of BHA with BHT‐radical, which generates 2‐tert‐butyl‐4‐methoxyphenoxy radical (BHA‐radical) and BHT, reversible dimerization of BHA‐radical, which affords an intermediarybis(cyclohexadienone), and spontaneous and base‐catalyzed prototropic rearrangement of the intermediate into BHA‐dimer. Products of coupling between BHT‐radical and BHA‐radical were not obtained. BHA was found to undergo facile acid‐catalyzed addition to QM, providing two isomericbis(hydroxyphenyl)methanes. The results help to elucidate the mechanism of antioxidant synergism between BHA and BHT and may suggest that the synergism can be affected by base or acid.