| Résumé | The rate of the self-initiated autoxidation of hexadecane at 160 °C under 760 Torr of O 2 is reduced by the addition of submolar concentrations of naphthalene or alkylnaphthalene. The kinetics of these (alkyl)naphthalene-modulated, self-initiated hexadecane autoxidations have been examined by monitoring hydroperoxide concentrations. Under similar conditions, the extent of the rate retardation is essentially identical for equal concentrations of naphthalene and 1- and 2-methylnaphthalene (and 2-sec-butylnaphthalene), which proves that retardation is due to chemistry associated with the naphthalene moiety. It is shown that the (alkyl)naphthalene produces both an increase in the rate constant for a bimolecular, peroxyl + peroxyl chain-termination process and a kinetically-first-order chain-termination reaction. Both of these termination processes are attributed to an initial addition of hexadecylperoxyl radicals to the naphthalene ring. Reaction of the resultant peroxycydohexadienyl radical with O 2 produces the HOO • radical which is responsible for the increase in the rate constant for bimolecular termination. Unimolecular decomposition of the peroxycyclohexadienyl radical yields hexadecanol and an (alkyl)naphthoxyl radical, the latter being responsible for the kinetically-first-order termination process. A simplified scheme of 17 elementary reactions yields a kinetic expression which, via computer modeling, is shown to give a very satisfactory agreement between calculated and measured hydroperoxide yields under all experimental conditions surveyed. This simulation yields a rate constant k 19 of ∼70 M -1 s -1 for the addition of hexadecylperoxyl radicals to (alkyl)naphthalenes at 160 °C, a value which may be compared with the estimated rate constant for hydrogen abstraction from 1- or 2-methylnaphthalene under similar conditions, viz., k 15 = 34 M -1 s -1. The simulation also gives a satisfactory agreement between calculated and measured yields of organic hydroperoxides and hydrogen peroxide. That the naphthalene ring was oxidatively cleaved under the experimental conditions was demonstrated by the identification of phthalic acid and phthalic anhydride following the addition of naphthalene or a methylnaphthalene to autoxidizing hexadecane. Both 1- and 2-methylnaphthalene also yielded 2-acetylbenzoic acid. The possible mechanisms by which these naphthalene ring-cleaved products are formed are discussed. |
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