Distinguishing between Solvation Effects and Mechanistic Changes. Effects Due to Differences in Solvation of Aromatic Rings and Alkyl Groups

Article abstract of DOI:10.1021/jo00004a048

Solvolytic rate constants at 25 deg C are reported for p-methoxybenzylchloride (1) in aqueous binary mixtures with acetone, acetonitrile, dioxane, dimethyl sulfoxide, ethanol, methanol, and 2,2,2-trifluoroethanol and for benzoyl chloride and p-methoxybenzoyl chloride (2) in aqueous acetonitrile and aqueous dioxane.Product selectivities are reported for solvolyses of 1 in aqueous ethanol and methanol.Logarithms of rate constants for solvolyses of 2 correlate linearly with solvolyses of 1 (unit slope and only a small "dispersion" - the tendency for the various binary mix tures to show seperate correlations), showing that the solvation requirements of S_N1 reactions of aromatic carboxylic acid chlorides are very similar to those of benzylic chlorides.Similar good correlations are reported vs solvolyses of α-phenylethyl chloride and vs Z values.For S_N1 reactions of chlorides in binary mixtures having similar electrophilicities, the phenomenon of dispersion in Grunwald-Winstein plots (vs Y) and the poor correlation vs Y_Cl (reported previously) appears to be due to the differences in solvation between aromatic rings and alkyl groups; this difference is largest for binary aqueous mixtures with methanol > ethanol > aprotic solvents.The complex rate-rate profile for similar solvolytic reactions of benzoyl chloride is dissected quantitatively into contributions from two competing reaction channels, one of which shows only a small "dispersion" in a correlation with solvolyses of 2; these results are consistent with a mainly dissociative reaction channel for solvolyses of benzoyl chloride in highly aqueous media.