Effect of alpha-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides.

Abstract

alpha-Tocopherol and 1,4-cyclohexadiene were tested for their effect on the thermal decomposition of methyl linoleate hydroperoxide isomers. The volatiles generated by thermolysis in the injector port of a gas chromatograph at 180 degrees C were analyzed by capillary gas chromatography. In the presence of either alpha-tocopherol or 1,4-cyclohexadiene, which are effective donors of hydrogen by radical abstraction, volatile formation decreased in all tests, and significant shifts were observed in the relative distribution of products in certain hydroperoxide samples. When an isomeric mixture of methyl linoleate hydroperoxides (cis, trans, and trans,trans 9- and 13-hydroperoxides) was decomposed by heat, the presence of alpha-tocopherol and 1,4-cyclohexadiene caused the relative amounts of pentane and methyl octanoate to decrease and hexanal and methyl 19-oxononanoate to increase. A similar effect of alpha-tocopherol was observed on the distribution of volatiles formed from a mixture of the trans,trans 9- and 13-hydroperoxides. This effect of alpha-tocopherol was, however, insignificant with pure cis,trans 13-hydroperoxide of methyl linoleate. The decrease in total volatiles with the hydrogen donor compounds, alpha-tocopherol and 1,4-cyclohexadiene, indicates a suppression of homolytic beta-scission of the hydroperoxides, resulting in a change in relative distribution of volatiles. The increase in hexanal and methyl 9-oxononanoate at the expense of pentane and methyl octanoate in the presence of hydrogen donor compounds supports the presence of a heat-catalyzed heterolytic cleavage (also known as Hock cleavage), which seems to mainly affect the trans,trans isomers of linoleate hydroperoxides.