The Thermal Decomposition of Methyl Hydroperoxide

Abstract

The thermal decon~position of methyl hydroperoxide has been studied in solutioll and in the gas phase. The decomposition was found to be partly heterogeneous in solution in dimethyl phthalate and no reliable rate constants were obtained. Use of the toluene carrier method for the gas phase work enabled measurement of the rate constant for the homogeneous decomposition. The first order rate constants obtained range from 0.19 s-1 a t 292 "C to 1.5 s-1 a t 378 "C, leading to log A , 11 f 2, and activation energy, 32 f 5 kcal/mole. These results are compared with the expected values of log A , 13-14, and activation energy, 42 kcal/mole. The significance of these findings is discussed.Although the mechanism of its production has not been unan~biguously established, and indeed inay not be unique, methyl hydroperoxide has now been recognized as a significant product in the low temperature ( I ) , radiolytic (2), and mercury photosensitized oxidation (3) of methane, the photooxidation of azomethane (4, 5), acetone (6), and methyl iodide (7), and the oxidation of methyl radicals (8). Although frequently not found in the products, it seeins likely to be formed in any system involving methyl radicals and oxygen below about 400 "C. The possibility that methyl hydroperoxide acts as a chainbranching agent in the low temperature oxidation of methane has been in dispute and appears to be still uncertain (9). This uncertainty exists largely because the rate of hon~ogeneous decoinposition of the hydroperoxide is not 1-~n o w n .Early ~vork (10) showed that decomposition of methyl hydroperoxide in a stream of nitrogen led to a coinplex mixture of products, almost certainly arising from simultaneous homogeneous and heterogeneous reaction. Recently, Fisher and Tipper (9) have studied the decomposition of the pure vapor in a static system and found the half-life of the peroxide to be up to 30 s a t 395 "C depending on the condition of the reaction vessel surface. The lack of inore complete data may be due to the difficulty of obtaining the h ydroperoxide pure and its fairly ready heterogeneous decomposition. In this work, it was found that pure methyl hydroperoxide could be obtained safely by fractionation a t reduced pressure and could be reliably estimated by iodine liberation and thiosulfate titration. The latter contrasts with other literature reports (11, 9). Details are given in the Experimental section.This research was undertaken to obtain evidence for the hoinogeneous decomposition of methyl hydroperoxide into radicals [I] (M +) CH3OOH -+ CHIO. + OH. (+ M) and to measure the rate constants and Arhennius parameters both in solution and in the gas phase. For the gas phase work, the toluene carrier method was adopted despite the criticisms which may be levelled against it (12). The facile heterogeneous decomposition of methyl hydroperoxide with low activation energy necessitates worlting a t high temperature and short reaction times if the homogeneous reaction is to be favored. In addition, the relative effectivene...

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