Chemistry Reference
In-Depth Information
5.6 Reactive Electrodes
In order to avoid the decomposition of product
or intermediate once it has been formed at the
counter electrode, a divided cell is required.
However, the divided cell increases the cell
voltage (the voltage between anode and
cathode) and maintenance is also troublesome.
When the desired reaction is cathodic
reduction, anodically dissolving metal
electrodes are used as anodes to avoid both the
decomposition of product or intermediate at the
anode and increase the cell voltage. Such
electrodes are called sacrificial electrodes,
sacrificial anodes or sacrificial metal anodes
[60]. For instance, as shown in
Eq. 5.18
,
cathodic carboxylation of organic halides using
a magnesium anode provides carboxylic acid
selectively in high yield [61]. In this case, the
carboxylate anion formed is trapped with
magnesium ion to precipitate. Both the anodic
oxidation of the formed carboxylate ion and its
esterification with unreacted organic halide can
therefore be avoided. In such cathodic
carboxylation, aluminium and zinc anodes, as
well as magnesium anodes, are often used.
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