| Résumé | The kinetics of the azobis(isobutyronitrile) initiated autoxidation of 1- and 2-methylnaphthalene have been studied from 30 to 60 °C in the pretence of sufficient tert-butyl hydroperoxide (1.0 M) to ensure that the tert-butylperoxyl radical totally dominates chain propagation and bimolecular peroxyl/peroxyl chain termination. Initial rate measurements demonstrate that these oxidations do not follow the kinetic rate law which applies to alkylbenzenes and other hydrocarbons. Specifically, in addition to the usual second-order peroxyl/peroxyl termination there is a kinetically first-order chain termination reaction, the relative importance of which increases as the oxygen partial pressure is reduced. As a consequence, these autoxidations are self-inhibiting. The major reaction between tert-butylperoxyl and the methylnapbthalenes is hydrogen abstraction from the methyl group (rate constant, K p BR). Self-inhibition is a consequence of a competing, peroxyl radical addition to the aromatic ring, reaction 8. This is a very minor process, e.g., K 8 BR/K p BR = 0.0036 at 30 °C, and it does not entirely lead to chain termination. That is, the adduct radical formed in reaction 8 may react with O 2, a process which leads to chain propagation, or it may decompose to yield tert-butyl alcohol and a methylnaphthoxyl radical (reaction 9). It is the methylnaphthoxyl radical that is responsible for the kinetically first-order termination process (via its reaction with a second peroxyl). Reaction 9 is also responsible for the existence of a maximum chain length, ν max, in these autoxidations. That is, at a given temperature and oxygen partial pressure ν cannot be increased indefinitely by reducing the rate of chain initiation as is the case for most hydrocarbons. Thus, at 60 °C in the presence of 1.0 M tert-butyl hydroperoxide ν max ≈ 88 ± 4 and 161 ± 23 at 160 and 760 Torr of O 2, respectively, while in the absence of the hydroperoxide ν maxis only ca. 12 and does not depend on the oxygen pressure. Our detailed kinetic results and mechanistic conclusions explain why alkylnaphthalenes are so much more resistant to autoxidation than the corresponding alkylbenzenes. |
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