STUDIES OF THE FORMATION AND STABILITY OF PENTADIENYL AND 3-SUBSTITUTED PENTADIENYL RADICALS

Pentadienyl radicals and their 3-methyl and 3-hydroxy derivatives were generated from the corresponding 1,4-pentadienes. The rates of hydrogen abstraction were studied by laser flash photolysis, and the bisallylic C-H bond dissociation energies were determined by photoacoustic calorimetry. Pentadienyl radicals were also generated by monoenergetic electron bombardment of 3-tert-butyl-1,4-pentadiene, and the radical's enthalpy of formation was deduced from the appearance energy of the tert-butyl cation. Evaluation of all recent data leads to recommended values of DELTA-H(f)-degrees (pentadienyl) = 49.8 +/- 1.0 kcal mol-1 and DH-degrees ((CH2 = CH)2CH) = 76.6 +/- 1.0 kcal mol-1. The EPR spectra of 3-methylpentadienyl and several other radicals were also examined. The effect of substituents on the stabilization energy of pentadienyl radicals was assessed, and pentadienyl radicals were compared with other delocalized polyenyl radicals.