Reactivity of polyenes in solid‐state autoxidation

Abstract

The reactivity of polyenes (b-carotene, canthaxanthin, lycopene and some retinyl polyenes as related compounds) in the solid state towards molecular oxygen at various temperatures and oxygen pressures was studied. The specimens were thin (ca 0.1 mm) amorphous films prepared by evaporation of one or two drops of appropriate solution on quickly rotating supports. The autoxidation was monitored by recording the changes in the electronic and infrared spectra of oxidizing films and was demonstrated to be a chain free-radical process with extremely high chaininitiation rates (10 À5 -10 À6 mol l À1 s À1 ). For various polyenes the kinetic parameters of the initiation reaction were measured. In the films of several polyenes (retinyl acetate, retinal, methyl retinoate, b-carotene), the formation of free radicals proceeds in the absence of oxygen and the mechanism of the process was suggested. The observed ratios of the propagation and termination rate constants depend on the oxygen pressure, indicating the involvement of polyenyl radicals in the chain termination process. The observed trends are explained using the concepts of the stabilization energy of radicals, the reversibility of oxygen addition to polyenyls and morphological features of polyene films.

Effect of carbonyls on the reactivity of polyenes in autoxidation

“…The experimental procedures for the preparation and purification of the polyenes, excluding diketoretinal (6, Scheme 1), were described previously. 1,2 Diketoretinal (3,7,11,11-tetramethyl-10,15-dioxo-2,4,6,8-hexadecatetraenal) (6) was prepared by oxidation of retinal with Scheme 1. pyridinium chlorochromate in CH 2 Cl 2 . After filtration, dissolution of the residue in diethyl ether and removal of the solvent under reduced pressure, 6 was separated from other products by column chromatography on SiO 2 with hexane-diethyl ether as eluent.…”

“…It was shown previously that the reactivity of the polyenic compounds studied exceeds the reactivity of other compounds by orders of magnitude. 1,2 In this connection, it was considered of interest to establish the dependence of the reactivity of polyenes in autoxidation on the presence of functional groups. In this paper, we report the effect of carbonyls on the autoxidation of polyenes with various conjugated chain lengths.…”