Gas-phase substituent effects in highly electron-deficient systems .2. Stabilities of 1-aryl-2,2,2-trifluoroethyl cations based on chloride-transfer equilibria

The relative stabilities of 1-aryl-2,2,2-trifluoroethyl cations were determined based on the chloride ion-transfer equilibria in the gas phase. An application of the Yukawa-Tsuno equation to this substituent effect on the equilibrium constants gave a remarkably larger r(+) of 1.53 and a rho of -10.6, supporting our previous conclusion that the highly electron-deficient benzylic carbocation systems are characterized by extremely high resonance demands. This r(+) value, furthermore, conformed a linear relationship between the r(+) value and the relative stability of the unsubstituted member of the respective benzylic carbocations, clearly demonstrating a continuous spectrum of varying resonance demands characteristic of the stabilities of carbocations. The pi-delocalization of the positive charge into the aryl pi-system increases with the destabilization of a carbocation by the alpha-substituent(s) linked to the central carbon. In addition, the r(+) value of 1.53 for 1-aryl-2,2,2-trifluoroethyl cations was found to be in complete agreement with that for the solvolysis of 1-aryl-2,2,2-trifluoroethyl tosylates in 80% aq acetone. This reveals that the r(+) value observed for this solvolysis must be the intrinsic resonance demand of a highly electron-deficient cationic transition state in the S(N)1 ionizing process. The identity of the r(+) value was consistent with our previous observation for other benzylic carbocation systems, indicating that the degree of the pi-delocalization of the positive charge is identical between the cationic transition state and an intermediate cation for all benzylic systems, which cover a wide range of reactivity and stability of the carbocation. This leads us to the conclusion that the geometry of the transition state in the ionizing process of the S(N)1 solvolysis, which is a highly endothermic reaction, closely resembles the high-energy product, an intermediate cation.