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 trifluo roacetolysis of 1-(4'-tolyl)-2,2,2-trifluoroethyl tosylate has been st udied, including reactions of optically active and O-18-labeled substr ates. With no added salt the polarimetric rate constant k(alpha) excee ds 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 additi on of trifluoroacetate or triflate salts there is a large initial curv ed acceleration of k(UV), characteristic of the special salt effect, w ith a smaller linear increase in k(alpha). The O-18 scrambling observe d in the unsolvolyzed starting material during trifluoroacetolysis cor responds to fractions of return with scrambling of 0.94 and 0.46 at [N aO2CCF3] = 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), sh owing that scrambling is the same or modestly more efficient than race mization in the ion pairs. The decrease in the k(alpha)/k(UV) ratio ap pears 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 qua ntitatively correlated by kinetic expressions derived for the reversib le formation of ion pairs which undergo competitive return to starting material and reaction with solvent or salt leading to product. The ad dition of 0.1 and 0.3 M NaOTs causes common ion rate depression of k(U V) by factors of 13 and 10%, respectively, consistent with the formati on 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 io n pair.