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Rate constants for solvolyses of 4-chloropent-2-ene (a,y-dimethylallyl chloride, 1) and 4-chloro-4-methylpent-2-yne (2) are reported for aqueous binary mixtures with acetonitrile, acetone, dioxane, ethanol, and methanol. Additional kinetic data are reported for solvolyses of 2-chloro-2-methylpentane (3) in acetonefwater and ethanolfwater and for tert-butyl chloride in acetonitrile/water. The response to changes in solvent ionizing power (Grunwald-Winstein m value) is almost identical for solvolyses of 1,2, and p-methoxybenzyl chloride (4). Comparison of data for solvolyses of 1 and 2 with published rate constants for solvolyses of tert-butyl chloride and of 4 show deviations from the ethanolfwater correlation line (dispersion) in the order: aryl >> propargyl > allyl > alkyl; these trends are attributed to differences in solvation of the part of the alkyl group (R) adjacent to the reaction site. Allowance for specific solvation of R groups guides the choice of “similarity models” to improve the precision and reliability of predictions using the Grunwald-Winstein and related equations for aqueous binary mixtures; prediction of solvent effects on solvolytic reactivity can be achieved using Ysim values. The Grunwald-Winstein equation (l)l* relates logarithms of solvolysis rate constants (k) to the solvent ionizing power Y, defined relative to the rate constant for solvolyses of tert-butyl chloride in 80% v/v ethanol/water at 25 OC (k,) with m - 1. For substrates other than tertbutyl chloride, both m and log ko are adjustable parameters,’ so ko has a dual role. For solvolyses of tert-butyl chloride, ko is an experimental rate constant, whereas for other substrates it approximates to the rate constant in 80% ethanol but it is actually an adjustable parameter. To avoid this duality, there is an alternative, more usual? procedure in which ko always refers to observed rate constants for solvolyses in 80% ethanol/water and m and c are then the two adjustable parameters (eq 2). log k = mY + log ko

the journal of organic chemistry

Solvation effects adjacent to the reaction site. Differences in solvation between alkyl, alkenyl or alkynyl, and aryl groups in binary aqueous mixtures