Stereoisomers and Chirality HL Only
Quick Notes
- Stereoisomers have the same structural formula (same atoms and connectivity) but different spatial arrangements.
- Two key types:
- Cis-trans isomerism (geometric isomers)
- Optical isomerism (enantiomers)
- Cis-trans:
- Occurs in non-cyclic alkenes and small cycloalkanes (C3, C4).
- Cis = groups on same side; trans = groups on opposite sides.
- Chiral carbon: bonded to four different groups.
- Produces enantiomers (non-superimposable mirror images).
- Enantiomers rotate plane-polarised light in opposite directions (optical activity).
- A racemic mixture contains equal amounts of both enantiomers and shows no optical activity.
Full Notes
Stereoisomerism
Stereoisomers have the same structural formula, but atoms are arranged differently in 3D space.
Note – Cis and Trans and E and Z isomerism has been covered in more detail
here.
This page is just what you need to know for AHL IB chemistry :)
Geometrical Isomerism (Cis/Trans)
Cis-Trans isomerism occurs in alkenes due to restricted rotation around the C=C double bond, as pi-bonds are unable to rotate without breaking.
To show cis/trans isomerism:
- Each carbon in the double bond must have two different groups.
- Cis = the same groups on each carbon are on the same side of the double bond.
- Trans = the same groups on each carbon are on opposite sides of the double bond.
Example But-2-ene

Cis-but-2-ene: CH3 groups on same side of C=C.
Trans-but-2-ene: CH3 groups on opposite sides of C=C.
This type of isomerism is also common in cyclic compounds when rotation is restricted.
Example 1,2-dimethylcyclopropane

The ring prevents free rotation, so the two methyl groups can be locked in position on the same side (cis) or opposite sides (trans) of the ring plane.
Optical Isomerism
If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule).
There are two possible ways the atoms or groups can be arranged giving two possible stereoisomers.
The isomers are non-superimposable mirror images of one another and are called optical isomers (enantiomers).

Example 2-hydroxypropanoic acid (Lactic Acid)

Chiral carbon: CH3CH(OH)COOH
Two enantiomers that exist as mirror images.
These isomers rotate plane-polarised light in opposite directions and have identical physical properties, except how they interact with light and other chiral substances.
A mixture that contains equal amounts of two enantiomers is called a racemic mixture and has no overall optical activity (doesn't rotate plane-polarised light).
Some molecules can have more than one chiral centre in them, especially biological molecules such as amino acids.

The terms used for describing optical isomerism can be confusing. Make sure you are comfortable with the following:
- Chiral: A carbon is chiral if it is bonded to four unique atoms or groups.
- Optical activity: The ability of a chiral compound to rotate the plane of polarised light.
- Enantiomer: One of two non-superimposable mirror image forms of a chiral molecule.
- Racemic mixture: A 50:50 mixture of two enantiomers that shows no overall optical activity.
Identifying Chiral Centres and Isomer Types
From a structural formula, you should be able to:
- Identify chiral centres (look for carbon with four different groups).

- Identify possible cis/trans pairs (look for a carbon-carbon double bond where each carbon is bonded to two different groups).

Summary
- Cis-trans isomerism results from restricted rotation in alkenes and small rings.
- Chirality arises from carbons bonded to four different groups.
- Enantiomers are optically active mirror images.
- Racemic mixtures contain both enantiomers and show no net optical activity.
- You must be able to draw and identify wedge-dash structures for enantiomers.