An electron spin resonance study of fatty acids and esters. Part 1. Hydrogen abstraction from olefinic and acetylenic long-chain esters

Abstract

The radicals generated from unsaturated fatty acid esters by hydrogen abstraction with photochemically formed t-butoxyl radicals have been investigated by e.s.r. spectroscopy. The substituted allyl and pentadienyl radicals generated from monoenic and dienoic esters were conformationally stable under the conditions of the experiments. The main species observed on hydrogen-abstraction from monoynoic esters were substituted propynyl radicals, and the same radical type was generated from diynoic esters with more than one methylene unit separating the triple bonds. Substituted penta-1,4-diynyl and penta-1,3-diynyl radicals were identified on hydrogen-abstraction from methylene-interrupted and conjugated diynoic esters. Hydrogen abstraction from triacylglycerols containing one or more double bonds was also examined. Most of the unsaturated esters also gave rise to secondary radicals produced by hydrogen-abstraction from the chain methylene groups. From the measured concentrations of the secondary and delocalised radicals the following approximate relative rates of hydrogen-abstraction by t-butoxyl radicals from the different sites were obtained: secondary: propynylic: allylic: bisayllic = 1 : 18 : 36 : 116 at 293 K.