Stereoisomers
⭐ Definition of Stereoisomers
Stereoisomers have the same molecular formula and connectivity of atoms (structural formula), but different spatial arrangement of atoms.
Two types you'll study:
E/Z isomerism (arises around double bonds)
Optical isomerism (more advanced, seen in a wider range of compounds including alkanes)
🔁 E/Z Isomerism (Geometric Isomerism)
📌 When does E/Z isomerism occur?
There must be a C=C double bond
Each carbon of the double bond must be bonded to two different groups
✅ If both conditions are met, the molecule can show E/Z isomerism.
But2-ene can exhibit E/Z isomerism, whereas but-1-ene cannot.
📖 Worked Example A: But-2-ene
(Z)-but-2-ene: Both CH₃ groups on same side of double bond
(E)-but-2-ene: CH₃ groups on opposite sides
👥 Cis–Trans Isomerism (Special case of E/Z)
Only used when each C in the C=C is bonded to one H
cis = same side (Z)
trans = opposite sides (E)
🔒 Why does rotation not occur?
The π-bond in the double bond lies above and below the σ-bond plane.
This locks the structure, preventing rotation around the double bond.
🔄 Naming E/Z Isomers (Cahn–Ingold–Prelog rules)
Step 1: Look at each carbon in the double bond
Assign priority to the two groups based on atomic number (higher = higher priority)
Step 2: Compare the higher-priority groups on each side
Z (zusammen) = same side
E (entgegen) = opposite sides
As we can see from this table, the atoms with the higher atomic number have the higher priority.
🧠 Worked Example B: 2-Bromo-1-chloropropene
Left-hand C atom: H vs Cl → Cl = higher priority
2. Right-hand C atom: CH₃ vs H → CH₃ = higher priority
3. Priority groups are on opposite sides → (E)-2-bromo-1-chloropropene
📖 Worked Example C : 2-penten-3-ol
(Z)-2-penten-3-ol: Priority groups same side
(E)-2-penten-3-ol: Priority groups opposite sides
🔬 Worked example D: Priority by First Point of Difference
If atoms bonded to the double bond carbon are the same:
Move along the chain to the first point of difference
Higher atomic number = higher priority
In the molecule above, the C atom has priority over the H atom
On the right hand side, carbon atoms are attached to the C=C carbon, so we need to make our way down to the first point of difference. It comes down to Cl vs O. Cl takes priority.