Effect of Reactive Grouping Conjugation

The condition for quantitative comparisons of half-wave potentials is that the electrode process follow the same mechanism for all systems to be compared. This condition is rarely fulfilled when substances bearing different electroactive groups are compared. Any comparison of the reactivity based on comparison of half-wave potentials of systems that are not reduced by the same or an analogous mechanism is at best a rough approximation. 

Another possibility for a rigorous comparison of the reactivities of the individual electroactive groups is the comparison (7,110) of potential ranges (based on the application of the Hammett equations, cf. section 3.1) of half-wave potentials of benzene derivatives bearing an electroactive group either in the side-chain or directly attached to the benzene ring, and further one single substituent in the m- or -position (Fig. 30). 

However, the reduction is not made easier by the presence of the double bond in the vicinity of the electroactive group in all cases. For 

The indicated shift of electrons increases the polarity of the reducible bond for the allyl compound XVI and decreases the electron density on the carbon prone to the electron attack4. On the other hand, the shift of electrons for the vinyl compound XVII increases the electron density on carbon, decreases the polarizibility of the C—X bond in a favourable direction, and causes a more difficult reduction than of the saturated analogue. This behaviour parallels the reactivity in some chemical reactions for instance, the solvolysis of the vinyl derivatives XVII is slower than that of the allyl derivative XVI. 

4 The observed difference for allyl halogenides in the reaction scheme indicated by isolation of organomercurials, involves a difference in the subsequent steps, not in the uptake of the first electron. 

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