AbstractDecay 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.
摘要 研究了 2,6-二叔丁基-4-甲基苯氧基自由基[丁基羟基甲苯(BHT)自由基]在丁基羟基苯甲醚(BHA)存在下,在含有或不含有三乙胺的 1,2-二甲氧基乙烷中的衰变情况。BHT 自由基是由其不稳定的二聚体在溶液中解离生成的。产物为 BHT、3,3′-二叔丁基-5,5′-二甲氧基-2,2′-二羟基联苯(BHA-二聚体)、2,6-二叔丁基对醌甲酰胺(QM)、1,2-双(3,5-二叔丁基-4-羟基苯基)乙烷和 3,3′,5,5′-四叔丁基-4,4′-二苯醌。在不添加三乙胺的反应中,后两种产物和 BHA(回收)的数量较多,而在添加三乙胺的反应中,前两种产物的数量较多。产物分布的明显差异可以通过一系列反应来解释,包括 BHA 与 BHT-自由基的可逆脱氢反应,生成 2-叔丁基-4-甲氧基苯氧基自由基(BHA-自由基)和 BHT;BHA-自由基的可逆二聚反应,生成中间体双(环己二烯酮);以及中间体自发和碱催化原向重排为 BHA-二聚体。BHT-radical 和 BHA-radical 之间的偶联产物没有得到。研究发现,BHA 可与 QM 发生简单的酸催化加成反应,生成两种异构体双(羟基苯基)甲烷。这些结果有助于阐明 BHA 和 BHT 的抗氧化协同作用机制,并可能表明协同作用会受到碱或酸的影响。