How do enantiomers and diastereomers differ in properties?

Enantiomers have identical physical properties except toward chiral influences, so they are hard to separate; diastereomers have different melting points, solubilities, and spectra, and can be separated by standard techniques such as crystallization or chromatography.

What makes a compound meso?

A meso compound contains stereocenters but also an internal mirror plane, so the two halves cancel and the molecule is superimposable on its mirror image — and therefore achiral and optically inactive.

Stereoisomerism and CIP Nomenclature

Stereoisomers share the same connectivity but differ in spatial arrangement; the Cahn–Ingold–Prelog system assigns each stereocenter an unambiguous descriptor.

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Definition

Stereoisomerism is the relationship between isomers that have identical bonding connectivity but differ in the three-dimensional orientation of their atoms; CIP nomenclature is the rule-based system for naming their configurations.

Scope

This topic covers the classification of stereoisomers into enantiomers and diastereomers, meso compounds, cis/trans and E/Z geometric isomerism, the CIP priority rules, and the assignment of R/S configuration to stereogenic centers.

Core questions

What distinguishes enantiomers from diastereomers?
How are CIP priorities assigned and used to label a stereocenter R or S?
Why is a meso compound achiral despite containing stereocenters?

Key theories

Enantiomer/diastereomer classification: Stereoisomers that are mirror images are enantiomers; all other stereoisomers (non-mirror-image) are diastereomers, which differ in physical properties and can be separated by ordinary means.
Cahn–Ingold–Prelog priority rules: Substituents around a stereocenter are ranked by decreasing atomic number; viewing with the lowest priority pointing away, a clockwise arrangement of the remaining three is R and counterclockwise is S.

Clinical relevance

Unambiguous stereochemical nomenclature is essential for specifying the active enantiomer of a drug, for regulatory documentation, and for communicating the precise structure of biologically active natural products.

History

Cahn, Ingold, and Prelog formalized the priority system in stages culminating in their 1966 specification of molecular chirality, replacing the ambiguous earlier D/L conventions with a rigorous, generally applicable descriptor scheme.

Key figures

Robert Sidney Cahn
Christopher Kelk Ingold
Vladimir Prelog

Seminal works

cahn1966
elielwilen1994

Frequently asked questions

How do enantiomers and diastereomers differ in properties?: Enantiomers have identical physical properties except toward chiral influences, so they are hard to separate; diastereomers have different melting points, solubilities, and spectra, and can be separated by standard techniques such as crystallization or chromatography.
What makes a compound meso?: A meso compound contains stereocenters but also an internal mirror plane, so the two halves cancel and the molecule is superimposable on its mirror image — and therefore achiral and optically inactive.