Rate constants for hydrogen abstraction by resonance stabilized radicals in high temperature liquids

Benzylic H-atom abstraction rates by diphenylmethyl radicals from a series of donors were determined in nonpolar liquids at elevated temperatures. Relative rates were converted to absolute rates via available equilibrium constant data for the dimerization of diphenylmethyl radicals. Abstraction by diphenylmethyl from 1, 2, 3, 4-tetrahydronaphthalene (tetralin) was studied over the temperature range 489–573 K. Its Arrhenius expression is 10^9.9±0.3 exp{?(10183 ± 373)/T} M^?1 s^?1. Abstraction from other donors was studied at 548 K. Rate constant values ranged from a low of 3.6 M^?1 s^?1 for toluene to a high of 3000 M^?1 s^?1 for 9, 10-dihydroanthracene. Similar reactions with the fluorenyl radical were also studied. In this case, relative rates were converted to absolute rates with an equilibrium constant for fluorenyl dimerization determined from the observed homolysis rate of the dimer and an assumed recombination rate. In addition, forward and reverse rate measurements yielded the equilibrium constant for hydrogen transfer between fluorenyl and diphenylmethyl. At 548 K, fluorenyl is favored by a factor of 13 over diphenylmethyl.