REACTIONS OF THERMALLY GENERATED TERT-BUTYL AND DI(TERT-ALKYL)KETYL RADICALS IN TOLUENE - CAGE EFFECTS AND HYDROGEN TRANSFER

Thermolysis of di(1-adamantyl)-tert-butylmethanol (2a) in toluene at 145-185-degrees-C gives mainly bibenzyl, di(1-adamantyl) ketone, di(1-adamantyl)methanol, and the cross-product, 1,1-di(1-adamantyl)-2-phenylethanol. In the presence of benzophenone (BP) or benzenethiols as hydrogen-accepting and hydrogen-donating radical scavengers, respectively, the di(1-adamantyl)methanol/di(1-adamantyl) ketone ratio tends to steady values as the scavenger/2a ratio is increased, while the cross-product disappears. At 165-degrees-C the secondary alcohol minimum is 8% (BP) and the ketone minimum 11% (thiol). These represent the contributions of geminate hydrogen atom transfer reactions to the overall yields, i.e., the cage effects. With BP the major cross-product is 1,1,2-triphenylethanol. Products from the self- and cross-reactions of benzyl and thiyl radicals are found when thiol is present, the diaryl disulfide predominating at high thiol concentration. In both cases, cross-products resulting from reaction of the tert-butyl radical with the scavenger-derived radical are detected in small amounts, being of greater importance in deuteriated toluene. The tert-butyl radical is considered, therefore, to be less reactive in hydrogen atom abstraction than the 1-adamantyl radical. Cage effects for other di(tert-alkyl)-tert-butylmethanols that thermolyze with exclusive t-Bu-C bond fission have also been measured and the product composition of the scavenger-free reaction interpreted by kinetic simulation based on the steady state approximation. Rate constants for hydrogen abstraction by the tert-butyl radical from toluene are not accurately determined by this procedure but seem, nevertheless, to indicate that the literature value (14.4 M-1 s-1 at 48-degrees-C) is an overestimate. Solvent hydrogen abstraction by the ketyl radical shows a small but well-defined steric effect.