Acidities and homolytic bond dissociation energies of the αC—H bonds in ketones in DMSO

Abstract

Equilibrium acidities in DMSO are reported for nine cycloalkanones, acetone, acetophenone, and 19 of their α-substituted derivatives. Oxidation potentials in DMSO for the conjugate bases of most of these ketones are also reported. Combination of these E OX (A − ) and pK HA values gives estimates of the homolytic bond dissociation energies (BDEs) of the acidic C—H bonds in the ketones. The ΔBDEs, relative to the BDE of CH 3 -H, or a parent ketone, provide a measure of the radical stabilization energies (RSEs) of the corresponding radicals. The effects of successive α-Me and α-Ph substitutions on RSEs, relative to those of CH 3 COCH 2 -H or PhCOCH 2 -H, are similar to those reported in the gas phase for methane. The RSE for the MeĊHCOPh radical, relative to CH 3 • is 17 kcal/mol, which is smaller than the sum of the RSEs of the MeCH 2 • and PhCOCH 2 • radicals relative to CH 3 • (7 + 12 = 19), contrary to the prediction of the captodative postulate. When G in PhCOCH 2 G is PhCO, CH 3 CO, or CN the ΔBDEs (relative to PhCOCH 2 -H) are 0, 1, and 3 respectively; for MeCOCH 2 SO 2 Ph, PhCOCH 2 SO 2 Ph, and PhCOCH 2 NMe 3 + the ΔBDEs are −5, −2, and −4, respectively. The BDEs in C 5 , C 6 , C 7 , C 8 , C 10 , and C 12 cycloalkanones are within ±2.5 kcal/mol of that of 3-pentanone. Acetophenones bearing meta or para substituents all have BDEs of 93-94 kcal/mol. Ketone radical cations, [RCOR′] +• , appear to be superacids with estimated [Formula: see text] values below −25. Keywords: acidities, bond dissociation energies, ketones.