Substituent Effects. XX. Highly Electron-Deficient Carbocation Solvolyses

Abstract

Abstract The solvolysis rates of 1-aryl-2,2,2-trifluoroethyl tosylates (1-OTs) and m-nitrobenzenesulfonates (1-ONs) were determined conductometrically in aqueous ethanol and aqueous TFE. While the Brown ρ+σ+ equation does not give a simple linear plots, but a remarkably split pattern, there is a linear free energy relationship between the solvolyses of 1-OTs and 1-aryl-1-(trifluoromethyl)ethyl tosylates (2) with a slope of unity over a wide range (108 in reactivity) of substituents from p-MeO to unsubstituted derivatives. The substituent effect on the solvolysis of 1 should be closer to that on the solvolysis of 2, rather than to the σ+ substituent effect. The ρ value for this system was estimated to be identical to the value of −6.3 assigned for 2, and the r value in the LArSR correlation to be comparable with or even higher than the r=1.4 of 2. The enhanced r value of this system must be caused from a strong destabilization of the transition state by the α-CF3 substituent. Extremely high ρ+ values have recently been pointed out for many solvolyses generating highly electron-deficient carbocations; however, in no case is the ρ+ value equally high for the electron-attracting region of the substituent. In fact, they all resulted in significantly concave σ+ plots, suggesting a mechanistic transition. All of the conflicts must be attributed to the inadequacy of the simple application of σ+. The use of the Brown σ+ scale must be the only cause of a curve break in the correlation plot of such a system having a significantly different resonance demand.