SUBSTITUENT EFFECTS ON HOMOLYTIC BOND-DISSOCIATION FREE-ENERGIES OF OXYGEN-ACETYL BONDS IN PHENYL ACETATES AND NITROGEN-ACETYL BONDS IN ACETANILIDES

The use of thermochemical cycles makes it possible to determine substi tuent effects on the homolytic bond dissociation free energies, BDFE, of the oxygen-acetyl bonds in substituted phenyl acetates and the nitr ogen-acetyl bonds in substituted acetanilides. A linear correlation be tween BDFEs for the phenyl acetates and the BDFEs of the oxygen-hydrog en bonds in correspondingly substituted phenols is found with a slope equal to 0.78. An equivalent linear correlation does not exist between the BDFEs of the nitrogen-acetyl bonds in acetanilides and the nitrog en-hydrogen bonds in anilines. This difference in the two correlations of substituent effects on the BDFEs can be explained by the different degrees of stabilization of the parent molecules by the substituents on the phenyl rings. The acetanilides are in this respect extraordinar y, since stabilization involving pi-resonance plays an important role.