LARGE SALT EFFECTS AND ION-PAIRING IN TRIFLUOROACETOLYSIS FORMING DESTABILIZED DELOCALIZED BENZYLIC CARBOCATIONS - KINETIC-ANALYSIS OF THE WINSTEIN SPECIAL SALT EFFECT

The trifluoroacetolysis rates of benzylic sulfonates substituted with destabilizing fluoroalkyl groups are accelerated by factors of 5.0-14.3 by the addition of 0.2 M NaO2CCF3, while substrates forming less delocalized carbocations show corresponding accelerations by factors of only 1.0-1.7. For PhC(OTs)(CF3)CH3 (1) the rates as a function of [NaO2CCF3] Or of [KO2CCF3] show an initial sharp nonlinear increase up to 0.3 M salt, and the rate dependences on [salt] are quantitatively correlated by kinetic expressions based on the competitive capture of solvent-separated ion pairs by solvent and by salt. This behavior is consistent with the accepted interpretation of the special salt effect, with the added feature that scavenging of the ion pair by trifluoroacetate salts can lead to products either directly or through carbocation trifluoroacetate ion pairs. Even greater initial increases are observed with added LiClO4, but NaOTs gives common ion rate depression. The large salt effects are particularly important in highly delocalized destablized benzylic carbocations which are resistant to solvent capture by trifluoroacetic acid but prone to ion pair return. Alkene formation from 1 is reduced by the addition of NaO2CCF3, and elimination in the absence of salt is proposed to occur from an ion pair, in agreement with other recent results, but contrary to a recent proposal of concerted elimination during solvolysis.