Stereoisomers are a major part of IB Chemistry Topic 10 (Organic Chemistry). They explain why molecules with the same structural formula can behave differently simply because their atoms are arranged differently in space. Stereochemistry appears in naming, reaction mechanisms, pharmaceutical applications, and exam questions involving chirality and geometric isomerism. Understanding stereoisomers makes organic chemistry far more intuitive.
What Are Stereoisomers?
Stereoisomers are molecules with the same molecular formula and the same connectivity of atoms, but different three-dimensional spatial arrangements.
This means:
- The atoms are connected in the same order
- The difference comes from shape, geometry, or orientation
- These small spatial differences can dramatically affect properties
Stereoisomerism arises when rotation is restricted or when a molecule contains a chiral center.
Two Main Types of Stereoisomers
IB Chemistry divides stereoisomers into two major categories:
- Geometric (cis–trans) isomers
- Optical isomers (enantiomers)
Each type depends on different structural features.
1. Geometric Isomers (Cis–Trans)
Geometric isomers occur when rotation around a bond is restricted.
This most commonly happens in:
- Alkenes, due to the C=C double bond
- Cycloalkanes, because ring structure prevents free rotation
Cis isomer
Substituents are on the same side of the double bond or ring.
Trans isomer
Substituents are on opposite sides.
Example:
cis-1,2-dichloroethene vs trans-1,2-dichloroethene
These differ in:
- Polarity
- Boiling point
- Dipole moment
- Stability
Cis is usually more polar; trans is often more stable.
Geometric isomerism only occurs when each carbon of the double bond has two different groups attached.
2. Optical Isomers (Enantiomers)
Optical isomers arise from chirality.
A molecule is chiral if it contains a chiral center—a carbon atom attached to four different groups.
Characteristics of enantiomers:
- They are non-superimposable mirror images
- They rotate plane-polarized light in opposite directions
- They often have identical physical properties (melting point, boiling point)
- They can have drastically different biological activity
A classic example is lactic acid, which has two enantiomers (L and D forms).
Why Optical Isomerism Matters
In biological systems, enzymes and receptors are highly shape-specific.
Two enantiomers of a drug can:
- Act differently
- Have different strengths
- Show different side effects
- Bind differently to enzymes
This is why stereochemistry is essential in pharmaceutical chemistry.
How to Identify a Chiral Centre
A carbon is chiral if:
- It has four different substituents
- None of the substituents repeat
- Its mirror image cannot be superimposed
IB tip:
Hydrogen counts as one substituent, so replacing it with another group often creates chirality.
Physical and Chemical Differences Between Stereoisomers
Geometric Isomers:
- Different dipole moments
- Different boiling points
- Different shapes
- Different stabilities
Example:
cis-1,2-dichloroethene is polar and has a higher boiling point than the non-polar trans form.
Optical Isomers:
- Same boiling/melting points
- Same mass
- Same formula
- Only differ in direction of optical rotation
- May react differently with chiral environments
The chemical difference only appears in reactions involving stereospecific mechanisms or biological systems.
Stereoisomerism in IB Exams
You will be asked to:
- Identify chiral centers
- Draw enantiomers
- Distinguish cis from trans isomers
- Explain differences in physical properties
- Predict whether geometric isomerism is possible
- Classify compounds as structural or stereoisomers
Being able to visualize molecules is key to scoring well.
Common IB Misunderstandings
“All alkenes have cis–trans isomers.”
False. Both carbons must have two different groups.
“Enantiomers always have different boiling points.”
They usually do not; their properties are nearly identical.
“A molecule with no chiral center cannot be optically active.”
Some rare molecules without chiral centers can be optically active (not required in IB).
“Geometric isomers are caused by chirality.”
Geometric and optical isomerism come from entirely different causes.
FAQs
Why can’t geometric isomers rotate around the double bond?
The π bond locks the structure in place, preventing free rotation.
Why do enantiomers rotate light differently?
Their 3D structures interact differently with plane-polarized light.
Can molecules have both geometric and optical isomers?
Yes. Many compounds exhibit both types simultaneously.
Conclusion
Stereoisomers have the same connectivity but differ in their 3D arrangement. Geometric isomers arise from restricted rotation, while optical isomers result from chirality. These differences affect physical properties, polarity, reactivity, and even biological function. Mastering stereoisomers is essential for understanding organic chemistry mechanisms and interpreting IB exam questions.
