Kinetic study of the reaction between tocopheroxyl radical and unsaturated fatty acid esters in benzene

Abstract

Abstract A kinetic study of the reaction between a tocopheroxyl radical and unsaturated fatty acid esters has been undertaken. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl oleate 2 , ethyl linoleate 3 , ethyl linolenate 4 , and ethyl arachidonate 5 ) by the tocopheroxyl radical (5,7‐diisopropyltocopheroxyl 6 ) in benzene have been determined spectrophotometrically. The second‐order rate constants, k 3 , obtained are 1.04×10 −5 M −1 s −1 for 2 , 1.82×10 −2 M −1 s −1 for 3 , 3.84×10 −2 M −1 s −1 for 4 , and 4.83×10 −2 M −1 s −1 for 5 at 25.0°C. Thus, the rate constants, k abstr /H, given on an available hydrogen basis are k 3 /4=2.60×10 −6 M −1 s −1 for 2 , k 3 /2=9.10×10 −3 M −1 s −1 for 3 , k 3 /4=9.60×10 −3 M −1 s −1 for 4 , and k 3 /6=8.05×10 −3 M −1 s −1 for 5 . The k abstr /H values obtained for the polyunsaturated fatty acid esters 3,4 , and 5 containing H‐atoms activated by two π‐electron systems are similar to each other, and are about three orders of magnitude higher than that for the ethyl oleate 2 containing H‐atoms activated by a single π‐system. From these results, it is suggested that the prooxidant effect of α‐tocopherol in edible oils and fats may be induced by the above hydrogen abstraction reaction.