Treatments for More Complex Electrochemical Reactions

In more complex organic reactions involving several charge transfer or chemical steps in the reaction sequence, it is often possible for one step to have a smaller rate constant than another. If this situation applies to the second or subsequent steps in a sequence, then the steps prior to the ratecontrolling step can usually be regarded as almost at equilibrium, and the surface concentrations of intermediates, 6, can then be expressed as a function of potential, so that there are two parts of the rate equation where potential-dependent terms are involved. 

The following example may be considered in order to illustrate the principles involved. Acetophenone reduction in acid medium follows the pathway 

In neutral or basic medium, an alternative pathway exists, leading to formation of the alchohol 

Except at high or medium pH, step (a) is a rapid protonation equilibrium. If, however, step (c) were rate controlling, i.e., if the rate constants were such that kc kb, k-b or ka, then step (b), as well as step (a), would be almost at equilibrium (this occurs, e.g., in some polarographically reversible systems see Section 8.2). Under these conditions, the surface coveraget 6k of the ketyl radical -COH CH2 ( iiT ) at the electrode surface could be represented approximately by 

In practice, there are often complications in this kind of approach, e.g., the relation (15) is more complex for intermediate surface coverages, and displacement of adsorbed solvent molecules in step (b) requires a more sophisticated treatment than that given above. It is of interest to note that if the pathway were (a), (b), (d), (e) giving the alcohol, and the second electron transfer step was rate determining [step (d)], the slope dE/d In / would be -RTKX + P)F, since another electron transfer step follows the prior one (b) at equilibrium. 

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