Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section B / 10 / ii / c / Solution

The drawn aryl dichloride is expected to undergo no analogous substitution under ordinary ethoxide conditions. Its chlorines are attached directly to sp2 aromatic carbons, rather than to the benzylic sp3 carbon of the previous substrate.

A normal SN2 reaction cannot follow the accessible backside geometry available at a primary benzylic carbon. A simple SN1 reaction would require a highly unfavorable aryl cation. Addition–elimination aromatic substitution would need suitable ring activation, such as a strong electron-withdrawing group in an appropriate position, which the drawing lacks. Forcing aryne chemistry cannot be inferred from ethoxide alone.

The conclusion addresses the actual E-plus-ethoxide scheme shown. The accompanying reference to replacing product B is inconsistent with the preceding substrate labels; interpreting it literally would not define the intended substitution comparison.