ON THE IMPORTANCE OF REACTIONS OF CARBOCATION ION-PAIRS IN WATER - COMMON ION INHIBITION OF SOLVOLYSIS OF 1-(4-METHOXYPHENYL)-2,2,2-TRIFLUOROETHYL BROMIDE AND TRAPPING OF AN ION-PAIR INTERMEDIATE BY SOLVENT

The effect of added bromide ion on the reactivity of 1-(4-methoxyphenyl)-2,2,2-trifluoroethyl bromide in water at 25-degrees-C and I = 6.00 (NaClO4) has been determined. The pseudo-first-order rate constant for this reaction decreases 300-fold as [NaBr] is increased from 0-5.00 M, which shows that greater-than-or-equal-to 99.7% of the solvolysis reaction proceeds through the free carbocation intermediate, whose concentration is reduced by mass action of added bromide ion. At high [NaBr], the observed rate constants show a significant positive deviation from the rate law for a reaction in which solvolysis products are derived solely from capture of the free carbocation. This deviation is consistent with a second pathway for the formation of solvolysis products, by direct attack of solvent on an ion-pair intermediate. The limiting velocity through this pathway, approached at high concentrations of bromide ion, is estimated to be 0.06% of that through the liberated carbocation. It is concluded that the capture of ion-pair intermediates of solvolysis reactions by solvent water is normally an unimportant reaction.