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 solvo lysis rate studies, and the Grunwald-Winstein (GW) type relationship with r espect to the Y-Cl scale was examined. Analyses of the plots showed that th ese bulky substituents efficiently preclude the nucleophilic solvent partic ipation 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 c auses considerable downward dispersions of the data points in aqueous mixtu res of ethanol, acetone and 1-propanol. The magnitude of the downward dispe rsion increases in this order, giving a curvature with a downward bulge in the GW type relationship. Aqueous mixtures of the smallest alcohol, methano l, on the other hand, give only slight downward dispersions of the data poi nts, which constitute a linear GW type plot. These results can be explained in terms of two causes. First. structural crowding makes the transition st ate of ionization less susceptible to the Bronsted base-type hydration to t he beta -hydrogens than 1-chloroadamantane as the standard of the Y-Cl scal e. 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 semiq uantitatively analyzed by using Hansch's hydrophobicity parameters. The pre sent anomalies found in solvolysis reactions are regarded as a kinetic vers ion of Wepster's observations of the solvent effects on the magnitude of Ha mmett sigma constants of bulky alkyl groups. Copyright (C) 2001 John Wiley & Sons.