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 °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 ≥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.