The Grunwald-Winstein relationship in the solvolysis of crowded tertiary alkyl chlorides. Hindered hydration and hydrophobic effect

Various highly crowded tertiary alkyl chlorides having a neopentyl or a (1‐adamantyl)methyl substituent on the reaction center were subjected to solvolysis rate studies, and the Grunwald–Winstein (GW) type relationship with respect to the YCl scale was examined. Analyses of the plots showed that these bulky substituents efficiently preclude the nucleophilic solvent participation from the rear side and that the data points for non‐aqueous protic solvents give linear GW type plots. On the other hand, increased crowding causes considerable downward dispersions of the data points in aqueous mixtures of ethanol, acetone and 1‐propanol. The magnitude of the downward dispersion increases in this order, giving a curvature with a downward bulge in the GW type relationship. Aqueous mixtures of the smallest alcohol, methanol, on the other hand, give only slight downward dispersions of the data points, which constitute a linear GW type plot. These results can be explained in terms of two causes. First, structural crowding makes the transition state of ionization less susceptible to the Brønsted base‐type hydration to the β‐hydrogens than 1‐chloroadamantane as the standard of the YCl scale. Second, with highly hydrophobic substrates the first solvation shell in aqueous ethanol is expected to become more ethanol rich than the bulk phase, causing less easy ionization of the substrate. The rate data can be semiquantitatively analyzed by using Hansch's hydrophobicity parameters. The present anomalies found in solvolysis reactions are regarded as a kinetic version of Wepster's observations of the solvent effects on the magnitude of Hammett σ constants of bulky alkyl groups. Copyright © 2001 John Wiley & Sons, Ltd.