S3.2.7 Stereoisomers (Higher Level Only)

Stereoisomers – Understanding Molecules in 3D

Imagine you’re holding a pair of gloves: one for your left hand and one for your right.
They look almost identical, but no matter how you rotate one glove, it won’t fit on the opposite hand.

This simple analogy captures the essence of stereoisomers—molecules that share the same chemical formula and connectivity of atoms but differ in the spatial arrangement of those atoms.

Cis-Trans Isomerism: A Matter of Rigidity

What Gives Rise to Cis-Trans Isomerism?

Definition

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.

Example

Consider but-2-ene ( $C_{4} H_{8}$ ):

In the cis-isomer, the two methyl groups ( $C H_{3}$ ) are on the same side of the double bond.
In the trans-isomer, the two methyl groups are on opposite sides.

Example

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 isomers of 1,3-dichlorocyclobutane.

Tip

Cis-trans isomerism only occurs if the molecule has restricted rotation and substituents that differ on each side of the double bond or ring.

Common Mistake

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 ( $C H_{3} - C H = C H_{2}$ ) 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?

Definition

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.

Example

2-Aminopropanoic Acid (Alanine)

In alanine ( $C H_{3} C H (N H_{2}) C O O H$ ), the central carbon is chiral because it is bonded to:

A methyl group ( $C H_{3}$ ),
An amino group ( $N H_{2}$ ),
A carboxyl group ( $C O O H$ ), 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.

Example

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 isomers of 2-aminopropanoic acid.

Self review

Can you identify the chiral carbon in a molecule? How would you draw its enantiomers?

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.

Note

You do not have to know d/l and +/- system by heart.

Example

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.

Tip

To distinguish between enantiomers experimentally, use a polarimeter to measure the direction and degree of rotation of plane-polarized light.

Schematic drawing of polarizer's functionality.

Schematic drawing how enantiomers rotate 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

Reflection and Practice

Self review

Cis-Trans Isomerism:
- Draw the cis- and trans-isomers of hex-2-ene.
- Why can’t propene exhibit cis-trans isomerism?
Optical Isomerism:
- Identify the chiral carbon in 2-chlorobutane and draw its enantiomers.
- Explain why a racemic mixture is optically inactive.
Real-World Connection:
- Research a drug that exists as enantiomers. How do the enantiomers differ in their biological effects?

Theory of Knowledge

How does the concept of stereoisomerism challenge the idea that "structure determines function"? Can two molecules with the same structure (but different spatial arrangements) have drastically different effects?

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S3.2.7 Stereoisomers (Higher Level Only)

Stereoisomers – Understanding Molecules in 3D

Cis-Trans Isomerism: A Matter of Rigidity

What Gives Rise to Cis-Trans Isomerism?

Recognizing Cis-Trans Isomers

Chirality and Optical Isomerism: The Mirror Image Phenomenon

What is a Chiral Carbon?

2-Aminopropanoic Acid (Alanine)

Drawing and Identifying Enantiomers

2-Aminopropanoic Acid

Optical Activity and Racemic Mixtures

Summary Table: Cis-Trans vs. Optical Isomerism

Reflection and Practice

You've read 2/2 free chapters this week.

Introduction to Stereoisomers