The thiobarbituric acid reaction and the autoxidations of polyunsaturated fatty acid methyl esters

The TBA reaction of autoxidized single methylene-interrupted diene, triene, tetraene, pentaene, and hexaene fatty acid esters has been measured at various stages of autoxidation. Linoleate free of higher polyunsaturates gave no TBA color at early stages of oxidation. At early stages of oxidation TBA values of triene, tetraene, pentaene, and hexaene were found to vary linearly with diene conjugation, peroxide value, and oxygen uptake. TBA values of triene, tetraene, pentaene, and hexaene were found to vary linearly with diene conjugation throughout the course of the oxidation. A mechanism is proposed for TBA reactant formation from methylene-interrupted polyunsaturated compounds with cyclic peroxide as an intermediate in the formation of malonaldehyde. The existence of a cyclic five-membered ring peroxide in the oxidation products of polyunsaturated acids is postulated. The mechanism of oxidation of methylene-interrupted trienes, tetraenes, pentaenes, and hexaenes postulates an additional step in chain propagation involving intramolecular rearrangement of the hydroperoxy free radical to a free radical bearing a closed five-membered ring peroxide. The scope of usefulness and the limitations of the TBA test are discussed.