engleski

Evidence for extended pi, n-participation in solvolysis of some benzyl chlorides

ABSTRACT: Chlorides 3 (1-aryl-1-chloro-4-methyl-7-methoxy-4-heptene) and 4 (1-aryl-1-chloro-4-methyl-4-hex-ene) with various phenyl substituents were prepared (p-OCH3, p-CH3, H and m-Br) and the solvolysis rates were measured in 80% (v/v) aqueous ethanol. The rate constants of 3 correlate well with &thorn ; , and the %&thorn ; value obtained is 1.45 0.15, whereas with 4 breakdown of the Hammett plot occurs, and the %&thorn ; value without the p-anisyl group is 2.55 0.20, indicating extended, n-participation in 3 and simple-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 delta S) is overcompensated by a small delta H.

solvolysis; neighbouring participation

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