A probe for homolytic reactions in solution. Part VIII. An electron spin resonance study of the rates of fragmentation and spin trapping of t-butoxycarbonyl radicals and acyl radicals

Abstract

From a study of the competition between spin trapping and decarboxylation, the rate constant for addition of the t-butoxycarbonyl radical to 2-methyl-2-nitrosopropane (nitrosobutane) has been determined to be 1·1 × 106 I mol–1s–1 at 40° in di-t-butyl peroxide solvent. The rate constant in benzene must be very similar. Competitive scavenging and decarbonylation of a series of acyl radicals (RĊO) shows that the ease of loss of carbon monoxide increases along the series R = Me < Pri < adamantyl < But < PhCH2. An absolute rate constant (3·9 × 103s–1 at 40°) for the decarbonylation of PriĊO is obtained by assuming that this radical adds to nitrosobutane at the same rate as t-butoxycarbonyl. This rate constant is considerably smaller than that obtained previously in the gas phase and a possible source of error in the gas-phase value is identified. The formation of t-butoxy t-butyl nitroxide by addition of t-butoxyl radicals to nitrosobutane is reversible at ambient temperatures. Fragmentation of this nitroxide to give t-butoxyl radicals occurs with a rate constant of ca. 0·14 s–1 at 40°, whilst fragmentation with loss of t-butyl radicals is shown to occur by N–C, and not O–C, fission with a rate constant of ca. 10–3 s–1 at 40°.