Substituent Effects. XIX. Solvolysis of 1-Aryl-1-(trifluoromethyl)ethyl Tosylates

The rates of solvolysis of 1-aryl-1-(trifluoromethyl)ethyl tosylates were determined in 80% aqueous ethanol for a series of substituents. The Brown ρ+σ+ treatment fails to give a simple linear plot, but all the p-π-donors deviate upward from the meta-correlation line. On the other hand, the Yukawa–Tsuno LArSR Eq. gives a linear correlation over the whole range of substituents, log(k⁄k0)=−6.287(σ°+1.388ΔσR+), with excellent precision. Strong destabilization of the carbocation intermediate by α-CF3 appears to be reflected only in the extremely high r value, but not in the ρ value of the substituent effect. This conflicts seriously with the conclusion in the literature given for this reaction based on a simple Brown σ+ treatment. This reaction, having an extremely high resonance demand r=1.4, should be far beyond the scope of the simple Brown σ+ analysis. Varying resonance demand should be a basic requirement for adequately describing the substituent effect regarding conjugative carbocation reactions.