Solvent and substituent effects in the solvolysis of 9-aryl-9-bromofluorenes

The solvolysis of eight 9-aryl-9-bromofluorenes (6b similar to 6i) in a variety of solvents were studied. Correlation analysis using single-parameter Grunwald-Winstein equation (Eqn. 1) with different Y scales showed good linearity (R greater than or equal to 0.98) for most cases if Y-xBnBr was employed. Linear relationships were observed from Hammett-type analysis of logarithm of rate constants using Brown-Okamoto sigma(+) constants (Eqn. 3), although inverse order of k(p-CF3)/k(m-CF3) was realized in a number of cases. The rho values were found to vary slightly with different solvent systems. Calculated atomic charge reveals the similarity between 9-phenyl-9-fluorenyl cation (7) and triphenylmethyl cation (8). An extended charge delocalization throughout the fluorenyl ring led to the conclusion of the insignificance of antiaromaticity, which was in harmony with that obtained in previous studies. The variation of relative k(Br)/k(Cl) rate ratios was attributed to the electrophilic pull by solvents in solvolysis.