Stereoisomers – Understanding Molecules in 3D
Cis-Trans Isomerism: A Matter of Rigidity
Cis-trans isomerism
Cis-trans isomerism occurs in molecules where rotation about a bond is restricted.
- This is most commonly observed in:
- Alkenes (double bonds): The $π$-bond in a double bond prevents free rotation.
- Cycloalkanes: The rigid ring structure locks substituents in place.
- For cis-trans isomerism to occur, the two substituents attached to each side of the double bond (or ring) must be different.
- If any carbon in the double bond has two identical substituents, cis-trans isomerism is not possible.
Recognizing Cis-Trans Isomers
- Cis-isomer: Identical groups are on the same side of the double bond or ring.
- Trans-isomer: Identical groups are on opposite sides.
Consider but-2-ene ($C_4H_8$):
- In the cis-isomer, the two methyl groups ($CH_3$) are on the same side of the double bond.
- In the trans-isomer, the two methyl groups are on opposite sides.
- In cycloalkanes, the reference plane is the flat face of the ring.
- For 1,3-dichlorocyclobutane:
- Cis-isomer: Both chlorine atoms are on the same side of the ring.
- Trans-isomer: The chlorine atoms are on opposite sides of the ring.
Cis-trans isomerism only occurs if the molecule has restricted rotation and substituents that differ on each side of the double bond or ring.
- Students often forget that for cis-trans isomerism to occur, the groups attached to each carbon of the double bond must be different.
- For example, propene ($CH_3-CH=CH_2$) cannot exhibit cis-trans isomerism because one carbon has two identical hydrogen atoms.
Chirality and Optical Isomerism: The Mirror Image Phenomenon
What is a Chiral Carbon?
Chiral carbon
A chiral carbon (or stereocenter) is a carbon atom bonded to four different atoms or groups.
Molecules with at least one chiral carbon can exhibit optical isomerism, a type of stereoisomerism where the isomers are non-superimposable mirror images of each other.
These isomers are called enantiomers.
2-Aminopropanoic Acid (Alanine)
In alanine ($CH_3CH(NH_2)COOH$), the central carbon is chiral because it is bonded to:
- A methyl group ($CH_3$),
- An amino group ($NH_2$),
- A carboxyl group ($COOH$), and
- A hydrogen atom.
Drawing and Identifying Enantiomers
- Enantiomers are drawn using wedge-dash representations to show their 3D structure:
- Wedge bond: Points out of the plane of the paper (toward the viewer).
- Dash bond: Points behind the plane of the paper (away from the viewer).
- Line bond: Lies in the plane of the paper.
- To draw the enantiomer of a molecule, reflect the structure across a mirror plane.
2-Aminopropanoic Acid
- Draw the first enantiomer with a specific arrangement of groups around the chiral carbon.
- Reflect the structure to draw the second enantiomer.
(The solution is in the image below)
Optical Activity and Racemic Mixtures
- Enantiomers interact differently with plane-polarized light, a property known as optical activity:
- One enantiomer rotates the plane of polarized light clockwise (dextrorotatory, +).
- The other rotates it counterclockwise (levorotatory, -).
- A 50:50 mixture of two enantiomers is called a racemic mixture.
- Racemic mixtures are optically inactive because the rotations cancel each other out.
In IB Chemistry (First Examination 2025), you do not have to know d/l and +/- system by heart.
- A solution of pure (+)-lactic acid rotates plane-polarized light by +3°.
- A racemic mixture of (+)- and (-)-lactic acid will not rotate the light because the effects of the two enantiomers cancel each other out.
To distinguish between enantiomers experimentally, use a polarimeter to measure the direction and degree of rotation of plane-polarized light.
Summary Table: Cis-Trans vs. Optical Isomerism
| Feature | Cis-Trans Isomerism | Optical Isomerism |
|---|---|---|
| Key Requirement | Restricted rotation (e.g., double bond or ring) | Presence of a chiral carbon |
| Type of Isomers | Cis (same side) and Trans (opposite side) | Enantiomers (non-superimposable mirror images) |
| 3D Representation | Not required | Wedge-dash diagrams |
| Optical Activity | Not applicable | Yes |
| Example | Cis- and trans-but-2-ene | (+)- and (-)-lactic acid |
