Stereoisomerism
Stereoisomerism
- In stereoisomerism the atoms are joined in the same order but point in different directions in space.
- Two kinds: geometrical and optical.
- Both matter a lot in real molecules.
Practice
In stereoisomerism, the atoms are:
Stereoisomers share the same connectivity but differ in 3-D arrangement.
Geometrical (cis/trans)
- Happens at a C=C double bond.
- The pi bond stops the carbons rotating (restricted rotation), so groups are fixed:
- cis = on the same side; trans = on opposite sides.
Practice
Geometrical (cis/trans) isomerism happens at a C=C bond because:
Restricted rotation about the C=C bond fixes groups cis (same side) or trans (opposite sides).
Optical isomerism
- Happens at a chiral centre — a carbon with four different groups.
- It gives two mirror-image forms, enantiomers, that cannot be superimposed.
- A molecule can have more than one chiral centre.
Practice
A chiral centre is a carbon with:
A carbon bonded to four different groups is chiral and gives two mirror-image forms.
Practice
Enantiomers are:
Optical isomers (enantiomers) are mirror images that cannot be superimposed on each other.
You've got it
Key idea
- stereoisomers = same order of atoms, different directions in space
- geometrical (cis/trans): at a C=C bond, restricted rotation fixes groups on the same (cis) or opposite (trans) sides
- optical: at a chiral centre (carbon with 4 different groups) → non-superimposable mirror images (enantiomers)