A New Concept on the Mechanism of Autoxidation of Methyl Oleate, Linoleate, and Linolenate

Abstract

In Farmer's modified theory describing "universal initiation" (4) and in Hilditch'stheory (10) of transitory formation of cyclic peroxide, which both advocate an oxidative attack on olefines by addition a t the double bond, there are two major defects: ( a ) neither makes any provision for the energy ( < 64 Itcal.) needed for opening up one bond (20) of the double bond (-C-C-), (b) Farmer's postulation leads to the formation of an unstable diradical system (21) and Hilditch's t o a cyclic four-membered ring uncler a considerable amount of strain (7). In this paper, a suitable theory which eliminates both these defects in Fanner's and Hilditch's theories will be proposed and explained.The recent research in the field of autoxidation supports Farmer's theory. The agreement has arisen, perhaps, because of a n inadequate appreciation of the reaction sequence in autoxidation. T h e reaction patterns in each of the four periods (12) involved in autoxidation are different. These periods ((A) induction period, (B) peroxidation period, (C) period of main autoxidation, and (D) period of decline in autoxidation) are represented in Fig. 1.

Some Specific Features in Autoxidation of Methyl Linoleate

“…This has bee11 confirmed by the excellent work on free radicals attacking such 5-C systenls by Harrison and \hiheeler (7). T h e question of energy to start off the first radical during the interactions of oxygen ,uncl linoledte can not be easily thrown off (10,11).…”

“…Consequently, they dealt with only main autoxidation reactions involving decomposition of these peroxides and the resultant free radicals. Our point is related to only where and how autoxidation begins and, hence, much importa~lce is attributed to the initial products unchanged and their energy of formation (10).…”

Reactions of the Hydrocarbon Chain of Fatty Acids.

“…In the cis,trans hydroperoxy acid the hydroperoxy group is said to be nearest the trans double bond. The autoxidation of linolenic acid follows the same pattern as for linoleic acid (279,280,420). The peroxide obtained by the oxidation of methyl stearolate has been concentrated and partially characterized (284).…”

“…In spite of the criticisms which have been leveled at the work wdiich has just been described and although a contrary view has been expressed (280), it does seem reasonably certain that during the autoxidation of oleic acid attack is at the 8-, 9-, 10-, and 11-positions. Whether the attack occurs equally at these positions remains to be demonstrated.…”

“…However, it appears that structure XL contributes little to the resonance hybrid. During the conjugation process the double bond that migrates undergoes geometrical isomerization and the result is a mixture in which the cis ¡trans conjugated configuration predominates (279,280,281,288,482). As autoxidation proceeds, trans,trans conjugated acid is also formed (288,419).…”