ON THE IMPORTANCE OF CARBOCATION INTERMEDIATES IN BIMOLECULAR NUCLEOPHILIC-SUBSTITUTION REACTIONS IN AQUEOUS-SOLUTION

The effect of nucleophilic anions on the rate constants for reaction of 1-(4-methoxyphenyl)-2,2,2-trifluoroethyl iodide (1-I) and bromide (1-Br) in water at 25-degrees-C and a constant ionic strength of 6.00 maintained with perchlorate ion has been determined. These substrates react by a D(N) + A(N) (S(N)1) mechanism through the 1-(4-methoxyphenyl)-2,2,2-trifluoroethyl carbocation intermediate (2), which is captured by I-, N3-, and SCN- in diffusion-limited reactions. There are also reactions of 1-I in the presence of the strong nucleophiles N3- and SCN- that are kinetically bimolecular at low nucleophile concentrations (0-1.00 M), but higher nucleophile concentrations (2.00-4.00 M) cause decreases in the velocity of the reaction. Decreases in velocity are observed for the reaction of 1-I in the presence of the weak nucleophiles Cl-, AcO-, and SO42- and for the reaction of 1-Br in the presence of N3-. These data are consistent with a stepwise preassociation mechanism for the bimolecular substitution reactions of N3- and SCN- with 1-I because this pathway will be significant only when the reactions of both the leaving group ion (I-) and the nucleophile (N3- or SCN-) with the free carbocation 2 are diffusion limited. The fit of the data for the reaction of N3- with 1-I to a rate equation derived for the stepwise preassociation mechanism gives K(as) = 0.67 M-1 for the formation of the [N3-.1-I] preassociation complex and the rate constant ratio k(Nu)/k(solv) = 2 for reaction of 1-I with N3- within this complex (k(Na)) and for reaction of 1-I in the presence of solvent alone (k(solv)). The largest rate increase observed at [Nu-] = 1.00 M is only 40% for the reaction of 1-I in the presence of SCN-, so the rate accelerations resulting from bimolecular nucleophilic substitution reactions through a carbocation intermediate are small even when this reaction is favored by the choice of nucleophile and leaving group.