Stereochemistry & Isomer Counting — formula sheet

Stereoisomers: compounds with the same connectivity — every atom bonded to the same partners — differing only in the spatial arrangement of those bonds.

cis: the two named ligands are adjacent — 90° apart in an octahedron or a square plane.trans: the two named ligands are opposite — 180° apart.

fac (facial): the three identical ligands occupy one triangular face of the octahedron. Every pair among them is mutually cis (90°).mer (meridional): the three lie on a meridian — a great circle passing through both poles. Two of them are mutually trans; the third is cis to both.

Chiral: a molecule that is not superimposable on its mirror image. The two mirror-image forms are enantiomers.

A molecule is chiral if and only if it possesses no improper axis of symmetry S_n. Two special cases of S_n cover almost everything you will meet: S₁ = a mirror plane σ, and S₂ = a centre of inversion i.

δ and λ (lower case): the two conformations of a puckered five-membered chelate ring, corresponding to the two senses of twist about the C–C bond. Unlike Λ/Δ, these interconvert freely at room temperature by ring flipping — no bonds break.

lel (from parallel): the ring’s C–C bond lies roughly parallel to the three-fold axis.ob (from oblique): the C–C bond lies at an angle to it.

To separate enantiomers, first turn them into diastereomers. Combine the racemic mixture with a single enantiomer of some other chiral substance. The two resulting combinations are no longer mirror images of each other — they are diastereomers — and diastereomers can be separated by ordinary means.

Racemisation: the conversion of a single enantiomer into a 50:50 mixture of both, with complete loss of optical activity. It is thermodynamically inevitable — the racemic mixture has higher entropy — so the only question is how fast.