Oxidative Dimerization of t-Butylated Hydroxyanisole (BHA) during Autoxidation of Methyl Linoleate and the Influences of Phospholipids and Tri-n-octylamine on the Dimerization

Abstract

The oxidative decomposition of t-butylated hydroxyanisole (BHA) and formation of its dimers were investigated during the autoxidation of methyl linoleate (ML). The effects of phospholipids (PL) and tri-n-octylamine (TOA) on oxidative dimerization were also examined.ML, in which BHA had been dissolved, was oxidized by air at 60°C in the dark. The specific surface area was 16.9 cm2/g. The extent of oxidation of ML was determined from weight increase resulting from oxygen uptake. The stability of ML toward autoxidation was estimated by the induction period before acceleration of oxygen uptake. After oxidation for a fixed time, an aliquot of the reaction mixture was subjected to HPLC analysis to determine the amount of the BHA monomer and its dimers.At an earlier stage of autoxidation, decrease in the amount of BHA monomer and increase in dimers were seen, followed by a decrease in the dimers, and the amounts of both the BHA monomer and its dimers were zero at the end of the induction period. Dimer quantities (ratio of the dimers to the initial BHA concentration) in the reaction mixture with a high initial concentration of BHA were larger than those with low initial concentration. A diphenyl ether type dimer was predominantly formed during autoxidation, and the amount of a diphenyl ether type dimer was about ten times that of a biphenyl type dimer.The oxidative dimerization of BHA during the autoxidation of ML containing phosphatidyl ethanolamine (PE) or phosphatidyl choline (PC) in various concentrations was followed so as to investigate synergism between BHA and PL. A remarkable increase in the stability of ML containing BHA was observed by the addition of PE at a low concentration. The efficiency of PC as a synergist was essentially the same as that of PE. The velocity of decomposition of BHA was lowered and the amounts of the dimers formed decreased by the addition of PL.Synergism between TOA and BHA was quite strong and the effect of TOA on the oxidative dimerization of BHA was unique. A large amount of a biphenyl type dimer, i. e. the amount corresponding to BHA monomer disappeared, was formed within the first 5 d of autoxidation, followed by a small change in the ratio of BHA monomer to its dimers.