The kinetic regularities of the autooxidation of R(+) limonene (LH) were experimentally studied at various temperatures. A kinetic model including the main elementary reactions and the corresponding rate constants was developed. The degenerate chain branching during li monene oxidation proceeds via bimolecular reactions and involves hydroperoxide LOOH + LH Limonene (4 isopropenyl 1 methylcyclohexene) is a natural olefin of the terpene series containing two unsat urated bonds. Two optical isomers exist in nature: R(+) with the orange odor and S(-) with the lemon odor. The R(+) isomer is more abundant and accessible. Interest in oxidation transformations of limonene is caused by its wide use in composing perfume compounds, for soap scents in the production of fragrants, and as an ecologi cally safe solvent for vanishes and resins. Limonene is a component for nearly all turpentine essences and ethereal oils of conifers. Recently, limonene has been used as the solvent for the synthesis of chiral compounds. The oxida tion of limonene with oxygen (both 3 О 2 and 1 О 2 ) touches only the cyclohexene ring, and the exocyclic isopropenyl group remains unchanged in the transformation prod ucts. 1-4 The composition of the oxidation products of limonene depends on the reaction conditions. 5,6 For ex ample, for the liquid phase oxidation of pure limonene with molecular oxygen in the presence of a catalyst (pal ladium or copper supported on hydrotalcites), the com position of the products depends on the metal nature. 6 Palladium is involved in the formation of allyl radicals by the activation of the C-H bonds of limonene and in the formation of adducts via double C=C bond cleavage. Cop per is a more efficient catalyst for the decomposition of hydroperoxides leading to the formation of radical inter mediates. The reaction products are various oxygen con taining limonene derivatives (1,2 and 8,9 epoxyli monenes, cis and trans carveol and carvone). Epoxyli monenes are formed by the oxidation of limonene with hydroperoxides and hydrogen peroxide in the presence of heteropolyacids. 7The oxidation of LH with molecular О 2 in organic solvents proceeds via the mechanism of free radical chain reactions, and the oxidation rate W O2 is described by the known equation of liquid phase chain radical hydrocar bon oxidation 3,4where W i is the rate of radical initiation, k p and k t are the apparent rate constants of oxidation chain propagation and termination, respectively. The value of the oxidability parameter a = k p /(2k t ) 0.5 depends on the type of the sol vent.The purpose of this work is the experimental study of the kinetic regularities of the autooxidation of R(+) li monene (LH) at various temperatures and the develop ment of the corresponding kinetic model.
ExperimentalR(+) Limonene (Fluka) was additionally purified on a chro matographic column packed with alumina (Fluka, 0.05-0.15 nm, pH = 9.5) followed by distillation in an argon flow under reduced pressure. Limonene was oxidized in a temperature con trolled magnetically stirred ...
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