Journal of Physical Organic Chemistry p. 368 - 372 (2005)
Update date:2022-09-26
Topics:
Juric, Sandra
Kronja, Olga
Solvolysis rates of chlorides that share the same side-chain comprising two neighboring groups [tertiary chloride 10, 2-chloro-2,6-dimethyl-9-methoxy-(6E)- nonene, and benzyl chlorides 11, 1-chloro-1-aryl-5-methyl-8-methoxy-(5E)octene, with various phenyl substituents (Y=p-OCH3, p-CH3, H, p-Br and m-Br)], were measured in 80% (v/v) aqueous ethanol. Both the tertiary substrate 10 and the benzyl substrates 11 solvolyze with smaller entropy and enthalpy of activation than the corresponding reference analogs with one neighboring group, 6 and 8, respectively (ΔΔH≠ = -34 ± 6kJ mol-1, ΔΔ≠ S = -122 ± 19 JK-1 mol-1 with 10; ΔΔH≠ = -33 ± 6kJ mol -1, ΔΔS≠ = -95 ± 17 J K-1 mol -1 with 11), indicating that in addition to the double bond, the methoxy group also participates in the rate-determining step. Chloride 10 has a significantly reduced secondary β-deuterium kinetic isotope effect (k H/kD= 1.07 ± 0.01 in 80E; kH/k D = 1.05 ± 0.1 in 97T) in comparison with the typical value for the tertiary chlorides (kH/kD = 1.80), as a consequence of the less positive charge on the reaction center in the transition state. The slope of the Hammett plot σ + value obtained with the series of 11 is considerably smaller than that obtained with the reference chlorides 8-Y (σ+ = -1.29 ± 0.11 vs -3.93 ± 0.10), confirming that benzyl substrates also solvolyze with extended π,n-participation. On both types of substrates, 10 and 11, the kinetic parameters indicate that very pronounced assistance of both neighboring groups occurs in the rate-determining step. Copyright
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(2005)