Issue in Honor of Prof. D. E. Sunko ARKIVOC 2002 (iv) 12-17
-2 -1 0 1 log(kHA/p) -6 -5 -4 -3-2 -1 log(qK/p)
a
Figure 2. Brønsted plot for the hydrolysis of 4-methoxy-1,2-dihydronaphthalene in aqueous solution at 25 °C.
The Brønsted exponent given by this correlation is a = 0.70 ± 0.06, which is a reasonable value for a substrate of the present reactivity.13 Fulfillment of these expectations, validates identification of the presently studied process as a vinyl ether hydrolysis reaction. Comparison of the hydronium-ion rate constant determined here with that reported for 3-methoxyindene, kH = 55 M-1s-1,7 then shows the present reaction to be slower by a factor of 2.4. This is consistent with the argument made above regarding the nonplanarity of dihydronaphthalene, in contrast to the planarity of indene, and the consequent reduced ability of the benzene ring of dihydronaphthalene to stabilize the positive charge generated by electrophilic addition to its vinyl group double bond. The factor of 2.4 found here is not large, but that is because most of the positive charge stabilization in the hydrolysis of these vinyl ethers is provided by the methoxy group, leaving little to be effected by the benzene rings. For example, comparison of the hydronium-ion catalytic coefficient for 3-methoxyindene with that for methyl vinyl ether, kH =
0.76 M-1s-1,11 gives a phenyl group stabilization of only 76 fold. Acknowledgements
We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work.
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