Spectroscopic and Kinetic Characteristics of Aroyloxyl Radicals. 1. The 4-Methoxybenzoyloxyl Radical

Article abstract of DOI:10.1021/ja00217a031

A detailed analysis of the time-resolved, UV-visible spectrum obtained by 308-nm laser flash photolysis of bis(4-methoxybenzoyl) peroxide proves that the broad, structureless absorption in the 500-800-nm region is due to the 4-methoxybenzoyloxyl radical.This radical also has an absorption at 320 nm.The long-wavelength absorption, for which there is less interference from other light-absorbing transients, has been used to measure absolute rate constants, k, for the reaction of 4-methoxybenzoyloxyl with a wide variety of organic substrates at ambient temperatures, e.g., cyclohexane, benzene, triethylsilane, cyclohexene, and 1,3-cyclohexadiene for which k in CCl4 = (5.3 +/- 3.0) x 10⁵, (2.3 +/- ).2) x 10⁶, (4.8 +/- 0.1) x 10⁶, (6.4 +/- 0.3) x 10⁷, and (4.8 +/- 0.2) x 10⁸ M^-1s^-1, respectively.Compared with the tert-butoxyl radical the 4-methoxybenzoyloxyl radical is about as reactive in hydrogen atom abstractions but is very much more reactive in additions to multiple bonds.The rate constant for decarboxylation of 4-methoxybenzoyloxyl at 24 deg C is (3.4 +/- 0.1) x 10⁵ s^-1 in CCl4 but is reduced to 4 s^-1 in CH3CN.The 4-methoxybenzoyloxyl radical can also be photodecarboxylated by using 700-nm light from a second laser or by using high power levels in the primary laser.