Substituent effects on the solvolysis of 2,2,2-trifluoro-1,1-diphenylethyl tosylates. II. 3-chlorophenyl- and 3,5-dichlorophenyl-fixed systems

The solvolysis rates of 2,2,2-trifluoro-1-(3-chlorophenyl)-1-(substituted phenyl)ethyl and 2,2,2-trifluoro-1-(3,5-dichlorophenyl)-1-(substituted phenyl)ethyl tosylates or bromides were conductimetrically measured at 25.0 degrees C in 80% aqueous ethanol. The 3-chlorophenyl-fixed series showed a bilinear Yukawa-Tsuno correlation with rho=-4.81 and r=1.41 for substituents more deactivating than 3,5-dimethyl, and with rho=-6.19 and r=1.57 for the substituent range more activating than 4-methyl. The bilinear correlation was interpreted in terms of the changing coplanarity of the two aryl rings. The 3,5-dichlorophenyl-fixed substrates gave an excellent linear Yukawa-Tsuno correlation for the substituents ranging from 4-MeO to 4-Cl with rho=-5.95 and r=1.69. The variable aryl rings in this series show the largest extent of resonance interaction with the extraordinary deactivated carbenium ion center by alpha-CF3 and alpha-(3,5-dichlorophenyl) groups in the transition state. The optimization of the geometries of 2,2,2-trifluoro-1,1-diphenylethyl cation and some ring-substituted derivatives was performed at the RHF/6-31G* level. The symmetric diaryl cations have equivalently twisted conformations, while in unsymmetric ones the more stabilizing aryl group is less twisted from the plane of the positive sp(2) carbon. The observed variation of r and rho parameters is accommodated by geometrical changes of the incipient cation.