Solvent Ionizing Power. Comparisons of Solvolyses of 1-Adamantyl Chlorides, Bromides, Iodides and Tosylates in Protic Solvents

Article abstract of DOI:10.1021/jo00200a034

Solvolytic rate constants for 1-adamantyl iodide (1-AdI) in binary aqueous mixtures of ethanol, methanol, acetone, trifluoroethanol, and hexafluoroisopropyl alcohol and in acetic and formic acids are reported.Additional kinetic data for solvolyses of 1-adamantyl halides in 97percent w/w hexafluoroisopropyl alcohol/water were obtained by using a microconductivity cell (volume, ca. 0.4 mL).Kinetic data for iodine-catalyzed solvolyses of 1-AdI in methanol/water mixtures are also reported.A scale of solvent ionizing power for iodides (Y_I) is defined by log(k/k₀)_1-AdI=Y_I, where k is the rate constant for solvolysis of 1-AdI in any solvent at 25 deg C relative to 80percent v/v ethanol/water (k₀).Correlations of Y_I and similarly defined scales for tosylates (Y_OTs) and bromides (Y_Br) with data for chlorides (Y_Cl) show variations in slopes attributed to charge delocalization in the leaving group (slopes, Cl>Br>I>OTs); acidic solvents show significant deviations from the correlation lines.The effect of iodine catalysis increases as solvent ionizing power decreases, consistent with formation of the charge delocalized leaving group I3^-.Y_I does not correlate satisfactorily with Kosower's Z values for aqueous and alcohol solvents, and the range of Z values is substantially greater in energy terms than the corresponding range of Y_I values.Our data provide qualified independent support for a recent proposal by Swain et al. that only two solvent properties correlate the major solvent effect on rates, equilibria and spectra.