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 CH bond dissociation energies were determined by photoacoustic calorimetry. Pentadienyl radicals were also generated by monoenergetic electron bombardment of 3-tert-butyl-l, 4-pentadiene, and the radical’s enthalpy of formation was deduced from theappearance energy of the tert-butyl cation. Evaluation of all recent data leads to recommended values of (pentadienyl)= 49.8±1.0 kcal mol'1 3and DHe ((CH2= CH) 2CH)= 76.6±1.0 kcal mol'1. TheEPR spectra of 3-methylpentadienyl andseveral 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.

Alkenee, RCH2CH= CH2, frequently undergo scission of the allyl CH bond in homolytic reactions. These bonds are weaker than the secondary CH bonds in alkanes be-cause the allylic radicals RChCHCH2 are thermodynamically stabilized by delocalization of the unpaired electron over three carbon centers. The enthalpy of formation of the allyl radical is known to good precision, andthere is considerable information about the effects of substituents. 1 Hydrogen abstraction from “skipped” dienes, eg, 1, 4-pentadiene, by free radicals leads to the formationof pentadienyl radicals, 1, in which the unpaired electron is delocalized over five carbon centers. These types of intermediate are important in many processes including hydrocarbon combustion and unsaturated fatty acid (and lipid) autoxidation. Because of thegreater extent of electron delocalization, 1 is more thermodynamically sta-bilized than allyl. In spite of several attempts to quantify