SUBSTITUENT EFFECTS ON THE CC BOND STRENGTH .18. STABILIZATION OF ALKYL RADICALS BY 4-PYRIDYL, 1-NAPHTHYL OR 2-NAPHTHYL SUBSTITUENTS IN ALPHA-POSITRON

The formation of the a-substituted radicals 7-9 with the 4-pyridyl (7) , 1-naphthyl (8), and the 2-naphthyl (9) substituent by thermolysis of their dimers 1-3 in solution was investigated. The activation paramet ers of the homolysis of the central CC bond were obtained from kinetic measurements of the thermal cleavage of 1-3 in the presence of radica l scavengers. These data were compared with those obtained from the th ermolysis of alkanes which cleave into alpha-alkyl branched radicals, and which lose the same amount of strain during the dissociation. The ''strain-free'' radical stabilization enthalpies of the alpha-substitu ted radicals [RSE = 7.9 +/- 1.8 (7), 12.0 +/- 1.4 (8), and 8.7 +/- 1.9 kcal/mol (9)] were obtained in this way.