The kinetics of thiyl radical-induced reactions of monounsaturated fatty acid esters
The time-dependent isomerizations and thiol additions of several Z- and E-monounsaturated fatty acid methyl esters catalyzed by alkanethiyl radicals during γ-radiolysis of tert-butyl alcohol solutions are analyzed on the basis of the radiation chemical yield of radicals and established rate data. This provides room-temperature rate constants for the reversible thiyl addition. Within experimental errors, they do not depend on the double bond position in the alkyl chains. Particularly noteworthy is the very fast β-elimination of thiyl radicals from alkyl radicals which carry a second β-substituent. It is supported by additional evidence obtained with a radical clock methodology, and the large preference of fragmentation to the E-isomers is attributed to different barriers for the formation of the E- and Z-transition states from the equilibrium radical structure.