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 l-(4-methoxyphenyl)-2,2,2-trifluoroethyl iodide (1-I) and bromide (1-Br) in water at 25 °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_N1) mechanism through the l-(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⁻·l-I] preassociation complex and the rate constant ratio k_Nu/k_solv = 2 for reaction of 1-I with N₃⁻ within this complex (k_Nu) 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.