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Incoming links: Phosphorus stereocenter
Past exam of the chemistry B course of the University of Cambridge 2021 Paper 1 B3 a Solution 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 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]Enantiopure spiro[3.3]heptane-2,6-dicarboxylic acid by Hong-Zhi Tang and others. Abstract, isolation of enantiopure spiro derivatives
- 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 , , and . 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.
Past exam of the chemistry B course of the University of Cambridge 2023 Paper 1 B3 a i Solution 2026-10-20
A and C are chiral; B is achiral. In A, a quaternary ammonium stereocenter occurs because nitrogen has four different substituents: phenyl, methyl, ethyl, and the sulfinylethyl chain. It has no nitrogen lone pair available for ordinary pyramidal inversion of an amine. The sulfoxide sulfur is also stereogenic: its pyramidal arrangement distinguishes oxygen, methyl, the nitrogen-containing chain, and the lone-pair direction. There are two independent stereocenters and no internal symmetry, giving four configurations in two pairs of enantiomers. This is chirality of the cation; a bromide counterion does not remove it.
B can adopt a trans-1,4-dimethylcyclohexane chair with both methyl groups equatorial. An inversion centre at the chair centre exchanges its opposite carbon atoms and methyl groups. Its mirror image is therefore superposable: trans substituents in a flat ring drawing do not by themselves prove chirality. ring inversion gives the less favourable diaxial conformer without changing the trans configuration.
In C, tetrahedral phosphorus has four different substituent directions: oxo, methoxy, ethoxy, and phenoxy. C has a phosphorus stereocenter. Delocalization in the P–O bonds does not make these four ligand environments identical. A tetrahedral drawing and its mirror cannot be superposed without exchanging two ligands.
Original three-dimensional models of the two stereocenters in A, the inversion symmetry of trans-1,4-dimethylcyclohexane, and tetrahedral phosphorus in C
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