Further spectroscopic and kinetic studies on carbonyloxyl radicals

UV-visible absorption spectra and kinetic data obtained at room temperature are reported for three (alkenylcarbonyl)oxyls: trans-(CH 3) 3CCH=CHC(O)O •, 1a; trans-C 6H 5CH=CHC(O)O •, 1b; (CH 3) 2C=CHC(O)O •, 1c; and two (alkynylcarbonyl)oxyls: (CH 3) 3CC≡CC(O)O •, 2a; C 6H 5C≡CC(O)O •, 2b. Rate constants for decarboxylation of 1a, 1b, 2a, and 2b are estimated to be ≤1.1 × 10 7, 1 × 10 6, 5 × 10 5 and 2 × 10 5 s -1, respectively. The first-order decay of 1c would appear to occur primarily by an intramolecular H atom abstraction, k ∼ (2 ± 1) × 10 7 s -1. The (alkynylcarbonyl)oxyls are more reactive than the (alkenylcarbonyl)oxyls in a variety of H atom abstraction reactions, e.g., with c-C 6H 12, and addition reactions, e.g., with C 6H 5CH=CH 2. Combining the present and earlier 5-7 kinetic data for carbonyloxyls yields the following order of decreasing reactivity for hydrogen abstraction and addition reactions: RC≡CC(O)O • ≥ ROC(O)O • > RR′C=CHC(O)O • ≥ C 6H 5C(O)O •. The reactivities of meta- and para-substituted aroyloxyls can be correlated with the intrinsic acidities and with the pK a's of the corresponding benzoic acids, reactivity increasing with acid strength.