Flash-Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions

Article abstract of DOI:10.1021/ja00193a031

A series of 18 triarylmethyl cations and 10 diarylmethyl cations have been generated by nanosecond laser flash photolysis of cyanide, 4-cyanophenyl ether, and acetate precursors in acetonitrile/water (AN/W) solutions and first-order rate constants for their reaction with the solvent (k_s) have been directly measured following the decay in their optical density.In the standard solvent employed, 1:2 AN/W, the triarylmethyl cations which were studied had k_s values at 20 deg C ranging from 1E1 s^-1 (for the 4,4',4''-(MeO)3-substituted ion) to 9*1E6 s^-1 (4,4'-(CF3)2), while diarylmethyl cations had k_s values ranging from 1E5 s^-1 (4,4'-(MeO)2) to 3*1E7 s^-1 (4,4'-Me2).The parent diphenylmethyl cation and its derivative with one 4-methyl substituent were too short-lived (<15 ns half-life) to be studied with our equipment in 1:2 AN/W, but they were observed in 100percent acetonitrile and in this solvent containing small amounts of water.The effects on k_s of varying the amounts of acetonitrile were investigated for several cations.As water is added to 100percent acetonitrile, k_s increases significantly, but at around 15percent by volume water, there is a leveling and from that point to 100percent water, k_s is almost unchanged, decreasing by about 20percent.A plot of log k_s versus ?⁺ constructed for the triarylmethyl cations shows significant deviations from linearity for the points for the ? donors, in the direction which indicates that ?⁺ is underestimating the stabilizing effect of these substituents for a fully formed cation.A plot versus ?^C+, a parameter obtained from the analysis of NMR spectra of solutions of carbocations, is reasonably linear.A two-parameter correlation indicates that polar andresonance interactions of substituents do not proceed in parallel along the reaction coordinate, the addition of water to cation resulting at the transition state in the loss of 73percent of the equilibrium resonance effect but only a 33percent loss of the polar effect.A rate-equilibrium plot (log k_s versus pK_R) was constructed which covers 23 pK_R units.A single line of slope 0.64 can be drawn to include the entire set of data for both triarylmethyl and diarylmethyl cations.From a small extrapolation the k_s value for the tert-butyl cation in water is obtained as 1E10.5 s^-1.