ELECTROPHILIC SUBSTITUTION AT SATURATED CARBON .37. A KINETIC -THERMODYNAMIC ACIDITY CORRELATION OF CARBON ACIDS

Rate constants and activation parameters for potassium methoxide catalyzed exchange of fluorene, 9-phenylxanthene, 4-biphenylyldiphenylmethane, triphenylmethane, and diphenylmethane with methanol-O-d-dimethyl-d6 sulfoxide (75:25 by volume) were measured and extrapolated to 75°. These kinetic acidities varied by a rate factor of ca. 107 and were correlated by the Br0nsted relationship with reported thermodynamic acid dissociation constants that varied by a factor of about 1010. Substrate kinetic isotope effects for racemization of 2-(N,N-dimethylcarboxamido)-9-methylfluorene (kH/kD = 6.75 at 25°) and 4-biphenylylmethoxyphenylmethane (kH/kD = 2.5 at 126°) in the same medium were also measured. The curvature in the Br0nsted plot coupled with the variation of the kinetic isotope effect and changes in activation parameters with pKaof the carbon acids are interpreted in terms of changes in mechanism that occur for the isotopic exchange reaction as the ∆pKa's (pka of carbon acid — pKa of methanol) change. © 1968, American Chemical Society. All rights reserved.