HOMOLYTIC BOND-DISSOCIATION ENTHALPIES OF THE ACIDIC H-A BONDS CAUSED BY PROXIMATE SUBSTITUENTS IN SETS OF METHYL KETONES, CARBOXYLIC ESTERS, AND CARBOXAMIDES RELATED TO CHANGES IN-GROUND STATE ENERGIES

Equilibrium acidities in DMSO were measured for the N-H bonds in 15 carboxamides, and the homolytic bond dissociation enthalpies (BDEs) for these bonds were estimated. For the N-H bonds in five aliphatic carboxamides, the average PKHA is 25.5 and the average BDE is 108 +/- 2 kcal/mol. Equilibrium acidities in DMSO of the C-H bonds for seven sets of acids in series of the type GCH(2)COCH(3), GCH(2)CO(2)Et, and GCH(2)CONR(2) and the oxidation potentials of their conjugate bases, E(ox)(A(-)), where G is H, Ph, PhS, 1,3-dithianyl, Me(3)N(+), Ph(3)P(+), or MeCO, are also reported. For every set, there is a progressive increase in PKHA and a smaller progressive cathodic shift of E(ox)(A(-)), which are associated with progressive small increases in the BDEs of the acidic C-H bonds. These increases in BDEs are believed to be associated with progressive decreases in ground state energies in the order ketone > ester > amide. These results conform to a general rule that structural changes in weak acids that decrease ground state energies will tend to increase the BDEs of acidic H-A bonds, and vice versa.