Stereoisomer
Stereoisomers are molecules that have the same molecular formula and the same connectivity (sequence of bonds) between atoms but differ in the three-dimensional arrangement of their atoms in space. There are two main types: enantiomers (non-superimposable mirror images, related by chirality) and diastereomers (stereoisomers that are not mirror images, including cis/trans and E/Z isomers). Stereoisomers can have profoundly different biological activities — the drug thalidomide is a classic example, where one enantiomer is therapeutic and the other teratogenic.
Types of Stereoisomers
| Type | Relationship | Optical Activity | Physical Properties | Example |
|---|---|---|---|---|
| Enantiomers | Non-superimposable mirror images | Rotate plane-polarised light in opposite directions | Identical (except optical rotation) | (R)- and (S)-lactic acid |
| Diastereomers | Stereoisomers, not mirror images | May or may not be optically active | Different (mp, bp, solubility) | cis- and trans-2-butene |
| Meso compounds | Achiral despite stereocentres | Optically inactive (internal mirror plane) | Same as diastereomers | meso-Tartaric acid |
| Conformational isomers | Interconvert by bond rotation | N/A (not isolable at room temp) | Same constitution | Chair/boat cyclohexane |
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Related Terms
Chirality
Chirality (from Greek "cheir", hand) is the geometric property of a molecule that makes it non-superimposable on its mirror image, analogous to left and right hands. A chiral molecule lacks any plane, axis, or centre of symmetry; the most common source is a tetrahedral carbon (chiral centre) bonded to four different substituents. Chirality is of enormous biological importance — enzymes, receptors, and drugs are chiral, so enantiomers often exhibit completely different pharmacological activities, which is why modern drug development rigorously controls stereochemistry.
Nucleophilic Substitution
Nucleophilic substitution is a fundamental class of organic reactions in which an electron-rich species (nucleophile) attacks a carbon atom bearing a leaving group, displacing that leaving group and forming a new bond. There are two main mechanistic pathways: SN2 (concerted, bimolecular, inverts stereochemistry) and SN1 (stepwise via a carbocation intermediate, gives racemic mixture). These reactions are central to the synthesis of alcohols, ethers, amines, and other functional groups from alkyl halides.
Elimination Reaction
An elimination reaction is an organic reaction in which atoms or groups are removed from adjacent carbon atoms, forming a new pi bond (typically a C=C double bond) in the product. The two principal mechanisms are E2 (concerted, bimolecular) and E1 (stepwise via carbocation). Elimination reactions often compete with nucleophilic substitution and are favoured by strong, bulky bases; high temperatures; and tertiary substrates — making them key in the synthesis of alkenes and alkynes in industry and the laboratory.
From Greek "stereos" (solid, three-dimensional) + "isos" (equal) + "meros" (part). The term was introduced in the late 19th century as organic chemists, particularly Emil Fischer and Jacobus van't Hoff, recognised that molecules with the same formula could exist as distinct three-dimensional arrangements.