EQUILIBRIUM ACIDITIES AND HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE H-O BONDS IN OXIMES AND AMIDOXIMES

The equilibrium acidities of 14 aldoximes, 4 amidoximes, and 19 ketoximes have been measured in DMSO solution. Homolytic bond dissociation energies (BDEs) for the O-H bonds in these oximes have been estimated by combining the pK(HA) values thus obtained with the oxidation potentials of their conjugate bases. The acidities of the corresponding radical cations (pK(HA+.)) were estimated for 32 of these oximes by combining pK(HA), E(ox)(A-), and E(OX)(HA). Homohydrogen bonding was observed to be as strong for oximes in DMSO as for phenols. The pK(HA) values for syn-benzaldoxime and acetaldoxime were found to be 20.2 and 28.5, respectively. Comparison of these values with those reported in water led to the conclusion that the pK(HA) values reported in water for the oximes derived from acetaldehyde, acetone, and 3-pentanone and that for acetamidoxime are merely lower limits. The BDEs of the O-H bonds in acetaldoxime and syn-benzaldoxime were estimated to be 6 and 14 kcal/mol lower, respectively, than those reported for the O-H bond in alcohols. The O-H bonds in most oximes were found to be in the range of 90 +/- 2 kcal/mol; anti-benzaldoximes were found to have BDEs about 2 kcal/mol lower than for syn-benzaldoximes. The presence of alpha-dialkylamino groups in acetophenone oximes or beta-dialkylamino groups in propiophenone oximes failed to evoke through-space electron transfer on oxidation of their anions, contrary to expectations based on a literature report.