ION-PAIRS IN THE SOLVOLYSIS OF SECONDARY SYSTEMS - SALT EFFECT, 18(0)-LABELING, AND POLARIMETRIC STUDIES OF 1-(4'-TOLYL)-2,2,2-TRIFLUOROETHYL TOSYLATE

The effect of added trifluoroacetate and triflate salts on the trifluoroacetolysis of 1-(4'-tolyl)-2,2,2-trifluoroethyl tosylate has been studied, including reactions of optically active and O-18-labeled substrates. With no added salt the polarimetric rate constant k(alpha) exceeds the solvolytic rate constant kw by a factor of 14.7, corresponding to a fraction 0.93 of ion pair return with racemization. On the addition of trifluoroacetate or triflate salts there is a large initial curved acceleration of k(UV), characteristic of the special salt effect, with a smaller linear increase in k(alpha). The O-18 scrambling observed in the unsolvolyzed starting material during trifluoroacetolysis corresponds to fractions of return with scrambling of 0.94 and 0.46 at [NaO2CCF3] = 0.0 and 0.201 M, respectively, as compared to fractions of return of 0.93 +/- 0.01 and 0.37 +/- 0.01 determined from k(alpha), showing that scrambling is the same or modestly more efficient than racemization in the ion pairs. The decrease in the k(alpha)/k(UV) ratio appears to level off to values between 1.07 and 1.48 for [salt] between 0.588 and 0.601 M, consistent with the presence of intimate ion pairs that are not captured by salt. The effect of the salts on k(UV) is quantitatively correlated by kinetic expressions derived for the reversible formation of ion pairs which undergo competitive return to starting material and reaction with solvent or salt leading to product. The addition of 0.1 and 0.3 M NaOTs causes common ion rate depression of k(UV) by factors of 13 and 10%, respectively, consistent with the formation and capture of dissociated ions as reactive intermediates in these reactions. The presence of 0.260 MNaOTs causes an increase in k(alpha) of 17%, consistent with a normal salt effect on racemization of an ion pair.