ECE Mechanism p-chlorobenzonitrile

An ECE mechanism is similar to that in Problem 7.3, except the product of the chemical step is electroactive and is able to undergo a further electron 

CI-C6H4-CN + e CI-C6H4-CN-CI-C6H4-CN- + H CgHs-CN + Cl-CgHs-CN + e CeHs-CN- 

Sketch the voltammetry for the reduction of CI-C6H4-CN at high, medium and low scan rates (relative to the rate of the chemical step). 

In contrast, at relatively low scan rates (Fig. 7.7(c)) the forward reductive scan exhibits two reductive peaks situated at -1.96 V and -2.32 V. The first voltammetric wave corresponds to the reduction of the CI-C6H4-CN species to C1-C6H4-CN . Due to the relatively low scan rate this species is able to further react within the voltammetric scan, to form CeHs-CN. Hence the CeHs-CN now present adjacent to the electrode may be further reduced to CeHs-CN at —2.32 V. On the reverse scan the CeHs-CN is reoxidised leading to a peak. 

No peak is observed on the reverse scan for the oxidation of C1-C6H4-CN . This is because the chemical step is both irreversible and fast (on the timescale of the experiment), and hence any C1-C6H4-CN present has been consumed to form CeHs-CN. 

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