Deep-Dive Guides · Stereochemistry

Stereochemistry — the complete deep dive

Stereochemistry is the geometry of molecules in three dimensions and how that geometry controls properties and reactions.

Free extract — the opening section of the guide, in full.

0. The isomerism map

Fix the taxonomy before any detail. Two molecules with the same molecular formula are isomers. If their atom connectivity differs they are constitutional (structural) isomers. If connectivity is identical but the spatial arrangement differs, they are stereoisomers — and stereoisomers split into enantiomers (non-superimposable mirror images) and diastereomers (stereoisomers that are not mirror images: includes cis/trans, most multiple-stereocentre pairs, and E/Z).

Fig 1. The isomerism tree. The single most useful discrimination in exams: are two structures the same, enantiomers, diastereomers, or constitutional isomers? Enantiomers share all scalar properties (m.p., b.p.) and differ only in chiral environments (rotation sign, chiral reagents); diastereomers differ in all physical properties.

The three questions to ask of any stereochemistry problem: (1) Is the molecule chiral (any improper symmetry element S_n — a mirror plane σ or centre i — present ⇒ achiral)? (2) How many stereoisomers (≤2ⁿ, minus meso coincidences)? (3) What is the relationship between the two structures in front of me (same / enantiomer / diastereomer / constitutional)? Answer these three and most questions resolve themselves.

Read the rest of this guide

The remaining 11 sections — Chirality & the elements of stereogenicity, CIP rules — R/S and E/Z done right, Enantiomers, diastereomers & meso, Optical activity & ee/de and more — are part of ChemVidya Full Access, along with the other guides, the books, the study notes and 6,000+ practice questions with worked solutions.

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