Evidence for extended π,n‐participation in solvolysis of some benzyl chlorides

Chlorides 3 (1‐aryl‐1‐chloro‐4‐methyl‐7‐methoxy‐4‐heptene) and 4 (1‐aryl‐1‐chloro‐4‐methyl‐4‐hexene) with various phenyl substituents were prepared (Y= p ‐OCH 3 , p ‐CH 3 , H and m ‐Br) and the solvolysis rates were measured in 80% (v/v) aqueous ethanol. The rate constants of 3 correlate well with σ + , and the ϱ + value obtained is −1.45±0.15, whereas with 4 breakdown of the Hammett plot occurs, and the ϱ + value without the p ‐anisyl group is −2.55±0.20, indicating extended π,n‐participation in 3 and simple π‐participation in 4 . The drastically smaller activation parameters obtained with 3 than with 4 are consistent with the proposed mechanism in which the high degree of order required in the transition state (large negative Δ S ‡) is overcompensated by a small Δ H ‡. Copyright © 2003 John Wiley & Sons, Ltd.