Stereoisomers
Definition and meaning of Stereoisomers in chemistry.
Stereoisomers are compounds with the same molecular formula and atom connectivity but different spatial arrangements of atoms in three-dimensional space.
In more detail
Stereoisomers differ only in their three-dimensional orientation; the bonds between atoms are identical. They arise from restricted rotation around double bonds or rings (cis-trans isomerism) or from the presence of chiral centers (typically sp3 carbon atoms bonded to four different groups). The two main categories are enantiomers (non-superimposable mirror images) and diastereomers (stereoisomers that are not mirror images). Diastereomers, including cis-trans isomers, commonly have distinct physical properties such as different boiling points, melting points, and densities, along with different chemical reactivity. Enantiomers, by contrast, share identical physical properties in an achiral (non-chiral) environment and differ mainly in optical rotation and in how they interact with other chiral molecules, which is why they often show different biological activities.
Key facts
| Definition characteristic | Same molecular formula and connectivity, different 3D arrangement |
|---|---|
| Main types | Enantiomers and diastereomers |
| Causes | Restricted rotation (double bonds, rings) or chiral centers |
| Field | Organic Chemistry |
Cis-but-2-ene and trans-but-2-ene both have the formula C4H8. The methyl groups attached to the C=C double bond point toward the same side in the cis isomer but toward opposite sides in the trans isomer. This spatial difference results in different boiling points (cis-but-2-ene boils at about 3.7 degrees C, trans-but-2-ene at about 0.9 degrees C) and different chemical reactivity, despite their identical molecular composition.
Frequently asked questions
What is the difference between stereoisomers and structural isomers?
Structural isomers have different atom connectivity (different bonding arrangements). Stereoisomers have identical connectivity but differ in spatial 3D arrangement.
Are all stereoisomers enantiomers?
No. Enantiomers are stereoisomers that are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images.