Lack of the neighboring group rate effect in solvolytic reactions that proceed via participation

In order to investigate the influence of solvent polarity on the rate effect of double bonds in reactions that proceed via an extended π ‐participation mechanism, the solvolysis rates ( k U ) of the benzyl chloride derivative 1 and tertiary chloride 2 that have doubly unsaturated side chains were measured in absolute ethanol, 80% v/v. aq. ethanol and 97% wt. aq. trifluoroethanol. The rates of the corresponding saturated analogs 1S and 2S ( k S ) were measured in 80% aq. ethanol and 97% wt. aq. trifluoroethanol, while those in pure ethanol were calculated according to LFER equation log k  =  s f ( E f  + N f ). In solvents with moderate ionizing power (ethanol and 80% aq. ethanol) the expected rate effects were obtained ( k U / k S >1), while in solvent with high ionizing power (2,2,2‐trifluoroethanol) absence of the rate effect was observed ( k U / k S ≈1), indicating that in the k S process the solvation of the transition state is very important, while in k Δ process the breaking of the CCl bond is not appreciably developed in the transition state and the solvent effect is marginal. Copyright © 2007 John Wiley & Sons, Ltd.