Leaving Group and Solvent Effects on SN1 Reactions of Adamantyl Substrates. Contributions from Electrostatic, Electrophilic, and Lipophilic Effects

Article abstract of DOI:10.1021/jo00224a034

Solvolytic rate constants are reported for reactions of 1-adamantyl picrate, 2-adamantyl trifluoromethanesulfonate (triflate), and 2-adamantyl perchlorate in binary aqueous mixtures of ethanol, methanol, acetone, trifluoroethanol, and hexafluoroisopropyl alcohol.Logarithms of solvolysis rates for various compositions of a binary mixture (aqueous ethanol, methanol, or acetone) are correlated with the solvent ionizing power (e.g., logarithms of solvolysis rates for 2-adamantyl tosylate).Variations in slopes of the correlations for picrates are attributed primarily to electrostatic effects - more extensive delocalization of negative charge in the anionic leaving group reduces the sensitivities of the solvolyses to changes in solvent ionizing power.The discussion includes previous work on chlorides, bromides, and iodides.Deviations from the correlation lines for aqueous alcohol mixtures provide a measure of electrophilic assistance in acidic and fluorinated solvents.Triflates may be the only exceptions to the general pattern that electrophilic assistance by carboxylic acid solvents depends on the electronegativity of the atom(s) of the leaving group on which charge develops.Only triflates show similar deviations for fluorinated alcohol solvents.Perchlorates and picrates show enhanced reactivities in high percentage acetone/water mixtures, but it is suggested from a plot of Y₁ vs.Y_Pic that lipophilic effects may not cause these deviations.