THE AUTOXIDATION OF TRIETHYLBORON AND THE DECOMPOSITION OF ITS PEROXIDES
LU Da-Shun; HUANG Pao-Tung; CHAO Shou-Chang Institute of Applied Chemistry, Academia Sinica
This work presents evidence that the autoxidation of Et_3B at different temperatures gives boron peroxides which are different in their composition and decompose along distinctly different paths. Autoxidation of Et_3B at 30°invariably absorbs more than one mole of oxygen per mole of Et_3B (Table 1); at -70°, however, oxygen-absorption per mole of Et_3B never exceeds 1.0 (Table 2). From the autoxidation data and the difference of their oxidation products in oxidizability, heat stability, and decomposition kinetics, it is concluded that the peroxidic products resulting from 30°oxidation is a mixture of diperoxyboronate (II) and peroxyboronate (III), while that from the low-temperature oxidation is the monoperoxyboronite (I). From the mechanistic point of view, II and III can be pictured as behaving similarly in their thermal decomposition; for simplicity, the mixture of 30°oxidation products is termed "diperoxide" in this paper. The monoperoxide (I) is susceptible to further oxidation. On being warmed up to room temperature, the low-temperature oxidation mixture began to pick up more oxygen at about -20°, giving eventually the "diperoxide", as judged from the total oxygenabsorption and the peroxide content (Table 3). Thus, for the first time, the autoxidation of a boron alkyl other than trimethyl boron has been controlled to stop cleanly at the monoperoxide stage. I is more liable to thermal decomposition than the "diperoxide"; for example, at 45°and comparable concentration, the half-life of the low-temperature oxidation product is only less than half an hour, while that of the "diperoxide" is about 4 days. Kinetic data showed that while the "diperoxide" decomposes (at 45, 55, 65 and 82℃) according to the second-order law (Tables 4 and 5), the decomposition of the -70℃ oxidation product (at 0, 22, 32 and 45℃) obeys the first-order law (Table 7). From the Arrhenius plots (Figs. 2 and 4), the activation energies of the two processes are found to be 21 and 12 kcal/mole respectively. That the "diperoxide" decomposes by a non-free radical mechanism is supported by the facts that the decomposition does not consume iodine (Table 6), is accelerated by acids and alkalies (acetic acid, benzoic acid, pyridine, and triethylamine), and does not initiate polymerization of vinyl acetate at either room temperature or 45°(with exclusion of air). On the contrary, under identical conditions, the monoperoxide quickly initiates the polymerization of vinyl acetate (Table 8); thus the free radical nature of its decomposition is fully appreciated. The constitution of the "diperoxide" and I is substantiated by the identification of B(OEt)_3 and EtB(OEt)_2 in the form of their respective ethanolamine derivatives (V) and (VI) from the decomposition products of the corresponding peroxides.