Figure 1. Trans-esterification of triacylglycerol (triolein) to its fatty acid methyl ester (biodiesel) and glycerol.
Oxidation of culinary oils has been studied for decades, detailing the numerous minor species that form (e.g. hydroperoxides, epoxides, aldehydes, acids, ketones) . Multitudes of reports confirm that oxidation and oligomerization occur more rapidly for oils with increased olefinic character. But the oligomers formed during thermal oxidation lack elucidation. Crosslinks in heated oils are usually attributed to C-C bonds without oxygen atmosphere and to ether C-O bonds open to air . Clearly, oxidative polymerization of oil is inconsistent with a Diels-Alder type reaction to give C-C crosslinks . Supposedly, poly-unsaturated “drying oils” (e.g., linseed), used for centuries, polymerize spontaneously in air by ether cross-links .
Clearly, the situation is complex for unsaturated triacylglycerols. Along with ester hydrolysis, glycerol and fatty acids form reactive alkoxyl and peroxyl radicals which undergo polymerization and scission reactions simultaneously . Molecular size increase during lipid oxidation is easily measured by chromatographic methods. But this gives no cross-link identity
Recently evidence was presented for dispelling ether cross-links in thermally oxidized vegetable oils. Model reactions between carboxylic acids and alcohols (90°C with aeration) showed facile ester formation . NMR spectroscopy found ester bond formation in thermally oxidized oleic acid and soybean oil . Although poorly sensitive, NMR spectroscopy is the most widely utilized spectroscopic technique. It can identify the chemical groups providing the first cross-links and measure molecular size (diffusion rate) of the major products. Information indispensable for unravelling the polymerization reaction.
Due to NMR spectral degeneracy, ester cross-links are difficult to detect starting with tri-acyl glycerides. Hwang et al. had to chromatographically separated the polar fraction of oxidized soybean oil (potatoes frying, 175°C, 24h) to observe 13C NMR signals of the new ester species. Also, they observed primary alcohols by 1H NMR, from fragmentation reactions, in common with our oleic acid oxidation .
We continue simplified stepwise approach to study oil polymerization. This work investigates mono-unsaturated acyl chains (e.g. oleyl) with our standard treatment, 210°C for 3h exposed to air. Oleic acid represents vegetable oil since hydrolysis is an early reaction step. In addition, without NMR signals from acylglycerides and glycerol, spectra are more intense, more resolved, and easier to interpret. Since focus is on characterizing the first larger-sized species formed in polymerization (oligomers), high frying (210°C) above normal frying (175°C), was used for a short time (3h) in open-air containers.
This standard oxidation process is applied to three other “model” molecules (stearic acid, trans-7-tetradecene and methyl oleate) closely related to oleic acid, to evidence the roles of olefinic and carboxylate species in oligomerization. NMR studies provided information on both structures (1H, 13C) and sizes (DOSY) of the non-volatile molecules produced by auto-oxidation and polymerization. One important structural feature provided by NMR spectroscopy is 3-bond C-H connections which identify the nuclei near the reactive centers (olefin, acid, alcohol) of triacylglycerides.