Geometrical Isomerism
Cis-Trans Isomerism | Stereoisomerism | Organic Chemistry
1. Definition & Cause
Geometrical Isomerism: A type of stereoisomerism where compounds have the same molecular formula and connectivity but differ in the spatial arrangement of atoms or groups around a rigid structure (like a double bond or ring).
Cause: Restricted Rotation.
- Carbon-Carbon Double Bond ($C=C$): Rotation is restricted due to the presence of the Pi ($\pi$) bond. Twisting breaks the bond overlap.
- Cyclic Structure: Rotation is restricted by the ring structure.
- Carbon-Nitrogen Double Bond ($C=N$): Oximes.
2. Necessary Conditions
For an alkene of type $abC=Cxy$ to show geometrical isomerism:
- There must be restricted rotation (Double bond or Ring).
- Each of the two doubly bonded atoms must be attached to two different groups.
- i.e., $a \neq b$ AND $x \neq y$.
Example: Propene ($CH_3-CH=CH_2$) does not show GI because one carbon has two identical H atoms.
3. Types of Isomers (Cis & Trans)
A. Cis-Isomer
Identical or similar groups are on the same side of the double bond or ring.
B. Trans-Isomer
Identical or similar groups are on opposite sides of the double bond or ring.
C. E-Z Notation (Brief)
Used when all four groups are different. Based on Cahn-Ingold-Prelog (CIP) priority rules (Atomic Number).
- Z (Zusammen/Together): High priority groups on the same side (Like Cis).
- E (Entgegen/Opposite): High priority groups on opposite sides (Like Trans).
4. Physical Properties Comparison
| Property | Cis-Isomer | Trans-Isomer | Reason |
|---|---|---|---|
| Dipole Moment ($\mu$) | Higher | Lower (often 0) | Vectors add up in Cis; cancel out in Trans. |
| Boiling Point | Higher | Lower | Cis is more polar $\rightarrow$ stronger intermolecular forces. |
| Melting Point | Lower | Higher | Trans is more symmetrical $\rightarrow$ fits better in crystal lattice. |
| Stability | Less Stable | More Stable | Cis has steric repulsion between bulky groups on same side. |
5. Important Examples
A. But-2-ene ($CH_3-CH=CH-CH_3$)
- cis-But-2-ene: Methyl groups on same side.
- trans-But-2-ene: Methyl groups on opposite sides.
B. But-2-ene-1,4-dioic Acid
- Maleic Acid (cis): -COOH groups on same side. Forms anhydride easily on heating.
- Fumaric Acid (trans): -COOH groups on opposite side. Does not form anhydride easily.
Practice Quiz
Test your ability to identify Cis-Trans Isomers.
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