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

Article abstract of DOI:10.1021/jo00028a041

The effect of added bromide ion on the reactivity of 1-(4-methoxyphenyl)-2,2,2-trifluoroethyl bromide in water at 25 deg C and I = 6.00 (NaClO4) has been determined.The pseudo-first-order rate constant for this reaction decreases 300-fold as is increased from 0 - 5.00 M, which shows that >/= 99.7percent of the solvolysis reaction proceeds through the free carbocation intermediate, whose concentration is reduced by mass action of added bromide ion.At high , 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 0.06percent 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.