Reactions of fatty materials with oxygen. XX. Recent developments in the autoxidation of methyl oleate and other monounsaturated fatty materials

Summary Some significant developments since 1947 in the autoxidation of methyl oleate and other monounsaturated fatty materials have been reviewed and critically evaluated. Subjects discussed are preparation and characterization of hydroperoxides, and mechanism, kinetics, and secondary products of autoxidation. Major developments in the field have resulted largely from the use of newer instruments (polarograph, infrared spectrophotometer) and separation techniques (urea complexes, molecular distillation, countercurrent distribution). Direct experimental evidence is now available which demonstrates that a) hydroperoxides are the predominating, but not the exclusive, primary products of autoxidation; b) the hydroperoxides obtained from methyl oleate are mostly, if not entirely,trans; c) substantially all the methyl oleate undergoes single attack in the chain before any significant amount of multiple attack occurs, and d) α,β-unsaturated carbonyl compounds are among the most important secondary products of autoxidation. Paper XIX is reference 66a. Presented at the Fall Meeting of the American Oil Chemists' Society, Philadelphia, Pa., October 10–12, 1955. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Reactions of fatty materials with oxygen. XVI. Relation of hydroperoxide and chemical peroxide content to total oxygen absorbed in autoxidation of methyl oleate

Summary Methyl oleate has been autoxidized at 100°, 80°, and 60° in the Barcroft-Warburg apparatus. Samples have been analyzed for total peroxide by the iodometric method and for hydroperoxide by the polarographic method. These peroxide values have been compared with each other and with total oxygen absorbed. The relation of chemical peroxide (y_c) to oxygen uptake (x) is expressed by y_c=1.09x^0.936. This equation is equally valid at the three temperatures for the first 150 millimoles (15%) of oxygen absorbed per mole of methyl oleate. Similarly, the hydroperoxide content (y_h) for the first 150 millimoles of oxygen absorbed at 80° and 100° is given by the equation y_h=1.02x^0.936. The ratio of hydroperoxide to chemically determined peroxide was, on the whole, constant throughout the entire range of oxidation (15–300 millimoles of oxygen absorbed per mole), and averaged about 95%. It has been shown unequivocally that the major portion of the peroxides formed in the autoxidation of methyl oleate are hydroperoxides, confirming conclusions of recent investigators. A small but significant amount of non-hydroperoxidic peroxide appears to be formed concurrently. Paper XV is reference 6. Presented at the Spring meeting of the American Oil Chemists' Society, San Antonio, Tex., April 11–14, 1954. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Reactions of fatty materials with oxygen. XVIII. Catalytic hydrogenation of autoxidized methyl oleate and oleic acid. Preparation of monohydroxystearic acids

Summary Autoxidation of methyl oleate and oleic acid beyond the peak peroxide values followed by catalytic hydrogenation gave mixed monohydroxystearic acids in high yield. The complicated autoxidation mixture which contains peroxides, hydroxy, carbonyl, and oxirane compounds was simplified considerably in composition by this procedure. For complete reduction of the double bond, and the carbonyl and oxirane groups, hydrogenation was conducted at about 150° and 150 lbs.. Peroxides were reduced at room temperature. Catalysts used were palladium on carbon and Raney nickel. The selective reduction of peroxides in autoxidation mixtures has been studied by chemical and catalytic means. Peroxides were converted largely to carbonyl compounds rather than to the anticipated hydroxy compounds. Palladium-lead on calcium carbonate is an excellent catalyst for reducing peroxides with hydrogen. tert-Butyl hydroperoxide, 12- ketostearic acid, stearone,cis-9,10-epoxystearic acid and methyl oleate peroxide concentrate were employed as model substances in determining hydrogenation conditions. Paper XVII. is reference 5. Presented at the fall meeting of the American Oil Chemists' Society, Minneapolis, Minn., Oct. 11–13, 1954. A laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture.     

Reactions of fatty materials with oxygen. XV. Formation of 9,10-dihydroxystearic acid and cleavage products in the oxidation of oleic acid and methyl oleate in acetic acid

Summary The autoxidation of oleic acid and methyl oleate in acetic acid solution at 25–30°, 65°, and 115–120° with a cobalt salt as catalyst has been studied. Samples were withdrawn at intervals and the oxidation products were analyzed and then separated into high-melting 9,10-dihydroxystearic acid, cleavage products, unoxidized and hydroxy materials, and polymers. The best yields of desired oxidation products were obtained at 65°. Yields of pure 9,10-dihydroxystearic acid, m.p. ≧128°, were 12–17% and cleavage products 64–68%, thus accounting for about 80% of the starting material. At 25–30° and 115–120°, yields of the above-mentioned products were low. A mechanism is proposed which accounts for the formation oftrans-9,10-epoxystearic acid from both oleic and elaidic acid autoxidized in the absence of solvent, and the consequent isolation of high-melting 9,10-dihydroxystearic acid when acetic acid is the solvent. Paper XIV is reference 10. Presented at the Fall Meeting of the American Oil Chemists’ Society, Chicago, Ill., Nov. 2–4, 1953. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Service, U. S. Department of Agriculture.     

Autoxidation of fatty materials in emulsion. III. Application of GLC and TLC to studies of the histidine-catalyzed autoxidation of methyl oleate

Methyl oleate, emulsified with sodium dodecyl-sulfate, was extensively oxidized, and the amount of oleate which was reacted was determined by GLC using methyl palmitate as an internal standard. The effects of histidine, iron, and histidine plus iron were compare with the uncatalyzed reaction in the presence and absence of ultraviolet light. Results in the absence of ultraviolet light confirmed previous findings that histidine and histidine plus iron are prooxidants at the concentration studied. In the presence of ultraviolet light the rate of oleate oxidation was about 100× faster than that of the nonirradiated reaction, and the effect of the catalysts was almost negligible. The principal products, determined by GLC and TLC, were epoxide and α,β-unsaturated carbonyl. Hydroperoxy, hydroxy, acid and aldehyde compounds were also present. Epoxy stearate, determined by GLC, compared favorably with epoxy stearate as determined by use of the Durbetaki method. Presented at the AOCS Meeting, Chicago, October 1964. E. Utiliz. Res. Dev. Div., ARS, USDA.     

Polymerization of drying oils. III. Some observations on reaction of maleic anhydride with methyl oleate and methyl linoleate

Summary Adducts of maleic anhydride with methyl oleate, methyl linoleate, and monomeric distillate have been prepared. Reaction of the adducts with ethylene diamine gave a linear, soluble polyamide in the case of the oleate adduct, but gelation occurred at a low degree of reaction with other adducts. The linoleate adduct was separated into monomeric and polymeric components. It is suggested that this polymeric component is responsible for the rapid gelation of polyamides from the adducts of methyl linoleate and monomeric distillate. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

Autoxidation of saturated fatty acids. I. The initial products of autoxidation of methyl palmitate

A highly purified methyl palmitate free of all detectable impurities was oxidized by aeration at 150C. Monohydroperoxide was shown by thin-layer chromatography (TLC), spot and spray test, and polarography to be the initial autoxidation product.     

Autoxidation of fatty materials in emulsion. II. Factors affecting the histidine-catalyzed autoxidation of emulsified methyl linoleate

Several factors which affect autoxidation of methyl linoleate in emulsion have been examined. Data are presented which indicate: 1) In the presence of histidine, the ionic (anionic) emulsifiers examined promote autoxidation of emulsified methyl linoleate, but nonionic emulsifiers do not. 2) The concentration of an emulsifier affects the rate of oxygen absorption. 3) Inorganic salts (0.1 M or less) such as sodium chloride, sodium acetate and sodium sulfate affect oxygen absorption of emulsified methyl linoleate prepared with either ionic or nonionic emulsifiers. In histidine-catalyzed autoxidation there is a suppressing effect in the case of the ionic and a promotional effect in the case of the nonionic. In uncatalyzed autoxidation, these salts have a promotional effect in ionic emulsions and none in nonionic emulsions. 4) Sodium phosphate buffers completely suppress autoxidation due to histidine catalysis, but do not suppress the normal uncatalyzed autoxidation of emulsified methyl linoleate. 5) The pro-oxidative effects of histidine and histidine-metal ion complexes on emulsified unsaturated materials is not limited to polyolefins but also includes mono-olefinic compounds. Presented at the AOCS meeting in Toronto, October, 1962. E. Utiliz. Res. and Dev. Div., ARS, USDA.     

Reactions of methoxy hydroperoxides derived from methyl oleate. Catalytic hydrogenation

Yield improvements in carbonyl compounds obtained by catalytic hydrogenation of methyl oleate ozonolysis products have been achieved by use of catalyst poisons and by the proper choice of catalyst support. Byproduct dimethyl azelate formation with palladium on charcoal was about 20% in the absence of a poison; 10%, with sodium acetate present in the support; 8%, with triethyl amine in solution. Palladium on calcium carbonate-lead acetate gave about 7%, palladium on zine oxide with pyridine or lead acetate present gave 9%. The literature is reviewed on the catalytic hydrogenation of methoxy hydroperoxides derived from ozonolyses in methanol. Presented in part at the Ozone Symposium, Southwest Regional Meeting of the American Chemical Society, Houston, Texas, December, 1963. A laboratory of the No. Utiliz. Res. and Dev. Div. ARS. USDA.     

Some effects of pressure on the oxidation of methyl oleate

The oxidation of methyl oleate under gas pressures up to 800 p.s.i.a. has been investigated. At 120°C. the samples oxidized under pressure built up hydroperoxides rapidly and exploded. There is no necessary connection between hydroperoxide concentration and explosiveness. Oxidations at 100°C are stable, but peroxide formation is drastically reduced. The presence of cobalt acetate increases the instability of the oxidizing oleate, causing it to explode even at 100°C.     

Inhibition of bacterial urease by autoxidation of furan C-18 fatty acid methyl ester products

Autoxidation products of synthetic methyl 9,12-epoxy-octadeca-9,11-dienoate (MEFA) were investigated by gas chromatography-mass spectrometry analysis and tested for bacterial urease inhibition. A suspension of oxidized MEFA in 10% Tween 80 was an effective inhibitor for bacterial urease extract fromHelicobacter pylori (I₅₀=1.3 mM) and for commercial urease fromBacillus pasteurii (I₅₀=0.06 mM). The urease inhibitory effect was cancelled by adding cysteine to the reaction mixture. The total content of biologically active oxidized products in the mixture was found to be 6.2%. Dioxo-ene derivatives of MEFA on the thin-layer chromatography plate surface were converted into more stable compounds, whose formation in the mixture reduced inhibition ofB. pasteurii to about 2% of the former level. The mechanism of urease inhibition is supposed to involve the interaction of the thiol groups of the enzyme’s active center with the inhibitor molecules.     

Epoxidation of methyl oleate with molecular oxygen in the presence of aldehydes

The epoxidation of methyl oleate with molecular oxygen in the presence of aldehydes was investigated and optimized. Epoxide yields up to 99% were observed in organic solvents. The preponderant radical reaction was started by a single organic radical chain initiator, without using a metal catalyst. The radical character of the reaction was revealed by the concurrent occurrence of trans and cis epoxides and by the prevention of epoxide formation through a radical scavenger. Both branched aldehydes and the linear aldehyde n‐hexanal were well suited in organic solvents. The linear n‐hexanal enabled also a superior epoxide yield of 78% in the green solvent water. The oxidation of high‐oleic sunflower oil under the same conditions with n‐hexanal yielded 39% epoxide groups.     

Further observations on the dicarbonyl compounds formed via autoxidation of methyl linoleate

a-Dicarbonyls isolated from oxidized methyl linoleate and conclusively identified as DNP-osazones³ were glyoxal, methyl glyoxal,a-keto hexanal,a-keto heptanal, anda-keto octanal. Technical Paper No. 2012, Oregon Agricultural Experiment Station.     

Reduction of methyl oleate ozonolysis products to aldehydes with activated zinc

Reduction of methyl oleate ozonolysis products with active zinc in the absence of acid or water gave an 88% yield of methyl azelaaldehydate. Under these neutral conditions, side reactions such as acetalization and aldehyde condensations could be more conveniently avoided than when zinc-acetic acid reduction is used. The activated zinc was prepared by treatment with acetic acid followed by washing to remove the acid. No. Utiliz. Res. and Dev. Div., ARS, USDA.     

Studies in autoxidation Part I. The volatile by-products resulting from the autoxidation of unsaturated fatty acid methyl esters

The by-products produced during the autoxidation of the methyl esters of three unsaturated fatty acids, under the influence of a number of promoters, have been studied. The components of the by-products have been separated and identified using GC-MS methods. Mechanisms are proposed to account for their formation.     

Effect of browning reaction products of phospholipids on autoxidation of methyl linoleate

Methyl linoleate was allowed to autoxidize in bulk phase at 50 C in the presence of either synthetic phospholipids consisting of saturated fatty acids or egg yolk phospholipids to estimate the effect of base group and fatty acid moiety of phospholipids on oil autoxidation. Dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) exhibited poor antioxidant activity at 50 C and showed no synergistic effect with α-tocopherol. The addition of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of egg yolk accelerated the oxidation of methyl linoleate. However, egg yolk PC and PE collected at various stages of heating at 180 C inhibited hydroperoxide formation at the initial stage of oxidation. This effect could be attributed to the browning products formed during heating reaction. Thus, browning color products formed from unsaturated phospholipids at high temperatures may influence oil stability, although the base group of phospholipids did not exert any significant effect.     

Uptake of secondary autoxidation products of linoleic acid by the rat

Incorporation of secondary autoxidation products (SP) of linoleic acid into the rat body was investigated. Radioactive SP was administered orally to a group of 5 rats, and excretions of radioactive substances in feces, urine and respiration were measured and compared with excretions from rats fed linoleic acid and its hydroperoxides. The SP-fed group excreted 45% and the other groups about 10% of the administered radioactivity through feces. Urinary excretion accounted for 52% of activity ingested in the SP group and less than 30% in the other groups. The¹⁴CO₂ produced in each group was about 25% of the ingested activity. Incorporation of the radioactive substances of SP into tissues and organs was measured periodically after administration of a single dose. The radioactive substances accumulated in the liver between 12–24 hr after administration and accounted for 2.6% of the total amount given, the highest level of all tissues and organs. This accumulation led to an elevation of serum transaminase activities, an increase in hepatic lipid peroxide, as determined by thiobarbituric acid test, and a slight hypertrophy of liver (1.5-fold). Therefore, absorbed SP appeared to contribute to the deleterious condition of the liver.     

Studies on the ozonization of methyl oleate

The ozonides of methyl 9-octadecenoate, 9-octadecene and methyl 9-octadecendioate (along with pelargonyl aldehyde, methyl azelaaldehydate and a fraction which appears to consist mainly of hydroperoxy-ozonides) were isolated from ozonizations of methyl oleate in pentane and methylene chloride at −65C. The relative amounts of these compounds were determined in ozonizations of methyl oleate in pentane, methylene chloride and ethyl acetate. Present concepts of the mechanism of the reaction are elaborated on the light of new information on the products of the ozonization of methyl oleate and related compounds under various conditions. Supported in part by Grant no. AM-05018, U. S. Public Health Service, National Institutes of Health. Presented at the AOCS meeting in Atlanta, 1963.     

A possible role for singlet oxygen in the initiation of fatty acid autoxidation

A mechanism for the initiation of autoxidation in fatty acids is proposed which involves singlet state oxygen, formed through a photosensitization reaction, as the reactive intermediate. Both singlet oxygen generated in a radio-frequency gasdischarge, and photosensitization by natural pigments, were shown to catalyze the oxidation of methyl linoleate. The involvement of singlet oxygen was shown by the identification of nonconjugated hydroperoxides as products common to both photooxidation and singlet O₂ oxidation. Nonconjugated hydroperoxides could not be detected among the free radical autoxidation products. Further proof for the above mechanism was gained by showing that compounds known to react strongly with singlet oxygen, inhibited the photooxidation. With the exception of chlorophyll, all sensitizers could be completely inhibited. Although singlet oxygen formation can account for approximately 80% of the observed chlorophyll photooxidation, at least one other mechanism must be involved. It is postulated that proton abstraction by the photoactivated carbonyl group of chorophyll could account for the remaining 20% of the observed photooxidation. The conclusion is drawn that oxygen, excited to its singlet state by a photosensitization process, plays the important role of forming the original hydroperoxides whose presence is necessary before the normal free radical autoxidation process can begin.