Ionization of 9-cyanofluorene in Me2SO–water mixtures. The role of solvent reorientation in proton tranfers

Abstract

Rate constants of the reversible deprotonation of 9-cyanofluorene by primary aliphatic amines were measured in 10%, 50%, and 90% aqueous Me2SO (v/v) by the stopped-flow method. Intrinsic rate constants defined as k0 = k1B/q = k−1BH+/p at ΔpK + log (p/q) = 0 were obtained from Brønsted plots. The values for log k0 are 3.60 ± 0.15 in 10%, 3.81 ± 0.10 in 50%, and 3.61 ± 0.05 in 90% Me2SO. The constancy of k0 with increasing Me2SO content of the solvent contrasts with results obtained by Ritchie (C. D. Ritchie, J. Am. Chem. Soc. 91, 6749 (1969)) who found that k0 for the ionization of 9-carbomethoxyfluorene by carboxylate ions in Me2SO is much higher than for the ionization of the same carbon acid by methoxide ion in methanol. Our findings suggest that it is mainly early desolvation of the oxyanions in the hydroxylic solvent which accounts for Ritchie's results.