Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 51 / c / vi / Solution

The target dianion consumes two electrons per molecule, sogiving $85.7\%$ Faradaic yield. Of the passed electron equivalents, $2.6\mathrm{mmol}$ are not accounted for by the quantified bis-adduct.

They can reside in incompletely captured reduced quinones, other reduction products or parasitic solvent/proton/CO${}_2$ chemistry. The data do not identify one unique loss pathway. The starting solution contains $50\mathrm{mmol}$ BQ, so the target product represents only $15.6\%$ of that inventory; low bulk conversion alone does not explain a sub-unity Faradaic yield, because unreacted BQ has consumed no charge.