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Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 1 / B3 / a / Solution by

Codex  0 2026-10-20

There are two, two and four stereoisomers, respectively; all eight individual stereoisomers are chiral. The drawings below give the mirror pairs. Solid and hashed wedges distinguish opposite faces; ordinary changes between conformers are not counted again.

The benzyl-substituted alkylidenecyclohexane has an axial stereogenic element. At one end the exocyclic alkene presents Ph and H; at the other end the substituted ring carbon presents benzyl and H. The ring constrains the intervening paths, so exchanging one end pair gives the nonsuperposable mirror image. There is one enantiomeric pair, rather than an independent ordinary $E / Z$ pair multiplied by a carbon-center count.
The diphenylspiro compound also has one axial chirality. The two four-membered rings occupy different, approximately perpendicular planes. Their remote Ph/H substitutions distinguish the two handed arrangements. It gives one enantiomeric pair and no meso form. axial chirality of this spiro framework is experimentally established, for example by resolution of its 2,6-dicarboxylic-acid derivatives.^[ref]
The phospholane has a stereogenic carbon at the COOH-bearing position and a pyramidal phosphorus stereocenter. The phosphorus has three different carbon ligands and a lone pair: its two ring paths differ because only one encounters the substituted carbon immediately. The four configurations are $(R_{P}, R_{C})$ , $(R_{P}, S_{C})$ , $(S_{P}, R_{C})$ and $(S_{P}, S_{C})$ . The first and last form a mirror pair, as do the middle two. carbon and phosphorus cannot be interchanged by a symmetry operation, so there is no meso reduction in the count.

Unlike the usual rapidly inverting trialkylamine, a pyramidal phosphine can retain a phosphorus configuration on an ordinary observational timescale; its two phosphorus configurations must therefore be included.

Figure 1.
Eight stereoisomers grouped into four mirror pairs, with axial and phosphorus stereochemistry shown
. **Details**.

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