HOMOLYTIC DECOMPOSITIONS OF HYDROPEROXIDES .V. THERMAL DECOMPOSITIONS

Solutions of 0.01-0.26 M t-butyl hydroperoxide in toluene have been thermally decomposed at 100-215°. Products included t-butyl alcohol, acetone, methane, CO, CO2, bibenzyl, benzaldehyde, and benzyl alcohol in yields varying with temperature and peroxide concentration. At initial concentrations below 0.02 M, decompositions of hydroperoxide were first order; nevertheless, even at 0.001 M peroxide, at least 30% of the total decomposition at 180° was induced decomposition. Acetone is a mild catalyst of thermal decomposition. The rate constant for homolytic cleavage of the O-O bond at 170-215° is about 1015.8 e-43,000/RT. At 100°, extraneous unexplained factors caused a thermal decomposition 20 times as fast as expected from this relation. The absence of methanol in the products excludes a previously postulated rearrangement of t-butylperoxy radicals at 180°. Less detailed studies in benzene, cumene, n-heptane, and cyclohexane showed that these decompositions are largely radical induced in the nonaromatic solvents and are mostly homolytic in aromatic ones. Some decompositions of dilute solutions of n-BuO2H, sec-BuO2H, and α-cumyl hydroperoxide in toluene at 170-180° gave rates two to three times as fast as those found for t-BuO2H. From the products of decomposition, 21% of the n-BuO· and 60% of the α-cumyl-O· radicals are estimated to cleave in toluene at 182°. About 50% of the t-BuO· radicals cleave under these conditions. The absence of cumene in the products from decompositions of 1.4 M α-cumyl hydroperoxide in toluene at 125 and 139° shows that a previously proposed reaction, PhCH2· + α-cumyl O2H → cumene + PhCH2O2H, does not occur. This research probably pushes to its limits the technique of increased dilution as a means for measuring the uncomplicated homolytic decomposition of t-BuO2H. There are methods for monitoring hydroperoxide concentration at much higher dilution, but they, like the iodometric method of analysis, do not distinguish between the hydroperoxide originally present and that possibly produced from the solvent. Analyses for products, on which a postulated mechanism must ultimately rest, become exceedingly tenuous at 0.001 M hydroperoxide. © 1968, American Chemical Society. All rights reserved.