ESR OF TRANSIENT RADICALS DURING PYROLYSIS OF FLUIDS

ESR equipment has been developed for directly observing labile free radicals that are present at high temperatures. A pressurized fluid is heated as it slowly flows through the microwave cavity of the spectrometer. Temperatures to 566°C and pressures to 140 kg/cm2have been used. Radicals are identified from well-resolved hyperfine structure. Spectra of cyanoiso-propyl from the decomposition of azobisisobutyronitrile in solution at 155-185°C and hydroxyisopropyl from 2% di-terr-butyl peroxide in isopropyl alcohol at 173-258°C are reported. Indenyl is reported from indene in benzene at 533°C and in tetralin at 471°C. 1,2-Diphenylethyl has been observed from solutions of 1,2-diphenylethane in benzene from 460 to 560°C. For dilute solutions of 1,2-diphenylethane in toluene the benzyl radical is obtained, while for more concentrated solutions a mixture of radicals is present in the following equilibrium for which K = 10±3 at 560°C and 105 kg/cm2: C6H5CH2CH2C6H5+ C6H5CH2=C6H5CHCH2C6H5+ C6H5CH3. Diphenylmethyl is obtained from the dissociation of tetraphenylethane as a dilute solution in benzene and in 50:50 benzene-diphenylmethane. ESR parameters are given in the latter solvent at 420°C. The formation of diphenylmethyl from diphenylmethane as a consequence of impurities has been examined, and diphenylmethane that has been initially air oxidized will form diphenylmethyl upon heating. Cumyl is obtained from cumene at 560°C. a-Methylstyrene is a final product but not dicumyl. Dicumyl in benzene solution at 560°C decomposes to give disproportionation products cumene and α-methylstyrene. The addition of α-methylstyrene or 1,2-diphenylethane to cumene at 560°C markedly enhances the production of cumyl. Cumyl is seen from air-saturated cumene at 350°C. p-Cymyl from p-cymene, phenylethyl from ethyibenzene containing styrene, and o-xylyl from o-xylene containing 1,2-diphenylethane have been observed at 560°C. Hyperfine couplings and g values are given and compared with room temperature values, and a discussion involving kinetic parameters is presented. © 1979, American Chemical Society. All rights reserved.