THERMAL AND PHOTOCHEMICAL FRAGMENTATION OF ALPHA-ALPHA-DIALKOXYBENZYL RADICALS - A COMPARISON OF THE THERMAL-REACTIONS WITH LASER-INDUCED FRAGMENTATIONS BY USING LASER FLASH AND LASER-JET PHOTOLYSES

The thermal and photochemical cleavage of alpha,alpha-dialkoxybenzyl radicals has been examined using a combination of techniques, including two-laser two-color laser flash photolysis and the laser-jet technique. For the parent alpha,alpha-dimethoxybenzyl radical photofragmentation occurs with a quantum yield of 0.80. The study of several unsymmetrically substituted radicals (e.g., methoxyisopropoxy) leads to the conclusion that the photoinduced fragmentation shows no selectivity. In contrast, the thermal decomposition of the radicals shows that fragmentation follows the expected radical stabilities, i.e., isopropyl > ethyl > methyl, the differences being almost exclusively due to changes in the activation energy. By comparing with literature data for methyl elimination it is possible to estimate the rate constants for fragmentation at 56-degrees-C as 213, 1380, and 16 600 s-1 for methyl, ethyl, and isopropyl elimination.