The substituent effects on the pyridinolysis (XC 5 H 4 N) of Y-aryl ethyl chlorophosphates are investigated in acetonitrile at 35.0 o C. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridines, exhibit large positive deviations from the Hammett plots but little positive deviations from the Brönsted plots. The substituent Y effects on the rates are really significant and the Hammett plots for substituent Y variations in the substrates invariably change from biphasic concave

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... ownwards via isokinetic at X = H to biphasic concave upwards with a break point at Y = 3-Me as the pyridine becomes less basic. These are interpreted to indicate a mechanistic change at the break point from a stepwise mechanism with a rate-limiting bond formation (ρ XY = -6.26) for Y = (4-MeO, 4-Me, 3-Me) to with a rate-limiting leaving group expulsion from the intermediate (ρ XY = +5.47) for Y = (4-Me, H, 3-MeO). The exceptionally large magnitudes of ρ XY values imply frontside nucleophilic attack transition state.