Electronic Effects

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Chapter: Organic Chemistry : Mechanisms of Organic Reactions

Besides bond breaking, another common feature of many reactions is the formation of charged species as intermediates. Carbocations, carbanions, oxonium ions, and so on, are all commonly encountered intermediates formed in the rate-determining step of multistep reactions.


ELECTRONIC EFFECTS

Besides bond breaking, another common feature of many reactions is the formation of charged species as intermediates. Carbocations, carbanions, oxonium ions, and so on, are all commonly encountered intermediates formed in the rate-determining step of multistep reactions. As a consequence, charge development in the activated complex is expected. In terms of the reaction mechanism, it is very important to know the charge type (positive, negative, or none) and the extent of charge development in the activated complex.

The use of rate constants can provide a clue to charge development as well. Changes in the rate constant of a reaction due to changes in structure can be indicative of the charge distributions present in the activated complex. For example, rate constants are much larger for the base-promoted deuterium exchange of phenylacetone than for acetone itself because the phenyl group stabilizes the negative charge on the enolate ion (and the transition state leading to it). Hence the α proton is removed more rapidly and deuterium exchange is speeded up correspondingly. This behavior is entirely consistent with an increase of electron density on the α carbon during the rate-determining step.


The hydration of styrene, α-methylstyrene, and α-trifluoromethylstyrene gives a benzylic alcohol product; however, α-methylstyrene reacts 105 more rapidly than styrene itself, while α-trifluoromethylstyrene reacts 107 less rapidly than styrene. This behavior is consistent with the rate-determining step being pro-tonation of the double bond to give a carbocation. The developing positive charge in the transition state is stabilized by the inductive effect of the methyl group in α-methylstyrene, the transition state is of lower energy, and it thus reacts faster. The developing positive charge in the transition state is destabi-lized by the electron-withdrawing inductive effect of the trifluoromethyl group in α-trifluoromethylstyrene, the transition state is of higher energy, and thus it reacts more slowly.

While changes in rate constants in response to changes in structure are extremely valuable for indicating the type of charge development occurring in the activated complex, the actual extent of charge development in the activated complex is an additional structural descriptor that would be very useful.



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