1H NMR Spectroscopy for Identification of Oxidation Products and for Elucidation of Reaction Mechanisms

Abstract

One of the most significant contributions of the 1H NMR technique is that it made major advances in elucidation of molecular structures of oxidation products. Since oxidation mechanisms and consequent oxidation products are different at different temperatures, this chapter is divided into two parts: 1) 1H NMR to indentify oxidation products during storage of oil and 2) 1H NMR to indentify oxidation products at frying temperatures. The study on oxidation products using 1H NMR was conducted as early as 1966 when Zimmerman utilized a very early model of the NMR instrument (60 MHz) to verify the isomerization of unsaturated hydroperoxide to a ketohydroxy compound. Since then, many oxidation products including hydroperoxides, aldehydes, ketones, alcohols, and epoxides were identified by the NMR method. One major difference between the oxidation products formed at relatively lower temperatures and at frying temperatures is that intermediate oxidation products such as hydroperoxides are not observed at frying temperatures because these intermediate oxidation products easily react with other compounds or decompose to produce secondary oxidation products. These intermediate products can be detected only when the oil was heated for a relatively short time at a frying temperature.