HOMOLYTIC BOND-DISSOCIATION ENERGIES FOR THE CLEAVAGE OF ALPHA-N-H BONDS IN CARBOXAMIDES, SULFONAMIDES, AND THEIR DERIVATIVES - THE QUESTION OF SYNERGISM IN NITROGEN-CENTERED RADICALS

Estimates of homolytic bond dissociation energies (BDEs) of the N-H bonds of acetamide, benzamide, and benzenesulfonamide by measurements of their equilibrium acidities and the oxidation potentials of their conjugate bases in dimethyl sulfoxide (DMSO) solution place them within experimental error of that of ammonia (107 kcal/mol). The BDE for urea is 4 kcal/mol higher, but those for cyanamide and thiourea are lower by 8 and 18 kcal/mol, respectively. N—Methyl substitution in acetamide fails to affect the BDE, but N-phenyl substitution lowers it by 8 kcal/mol. Introduction of N-methoxyl and N-hydroxyl groups into acetamide lower the N-H BDE by 17 and 19 kcal/mol, respectively. Identical effects are observed for like substitutions into benzamide. The introduction of an N-NH2 group into CH3CONH2, PhCONH2, PhS02NH2, (H2N)2C=0, and (H2N)2C=S lowers the N-H BDE by 25, 26, 24, 23, and 20 kcal/mol, respectively. The results are discussed in terms of the relative stabilization energies of the corresponding radicals. © 1990, American Chemical Society. All rights reserved.