How can molecules with the same formula differ in three dimensions?
E/Z isomerism, optical isomerism and chirality, the rotation of plane-polarised light, racemic mixtures, and the importance of stereochemistry in drugs.
A focused answer to WJEC A-Level Chemistry Unit 4, covering E/Z and optical isomerism, chirality and chiral centres, the rotation of plane-polarised light, racemic mixtures, and the importance of stereochemistry in pharmaceuticals.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC wants you to describe E/Z and optical isomerism, define chirality and a chiral centre, explain how enantiomers rotate plane-polarised light, define a racemic mixture, and appreciate why stereochemistry matters in drugs.
The answer
Types of stereoisomerism
Chirality and optical activity
Racemic mixtures
A racemic mixture contains equal amounts of both enantiomers, so the opposite rotations cancel and there is no net optical activity. Many laboratory syntheses give racemic mixtures because the reagent can attack a planar intermediate from either face with equal probability.
Where racemic mixtures come from
Many reactions that create a chiral centre give a racemic mixture because the reagent attacks a flat (planar) intermediate from either side with equal probability. In the nucleophilic addition of to an aldehyde such as ethanal, the carbonyl carbon is planar, so cyanide can add from above or below to give equal amounts of the two enantiomers of the hydroxynitrile. The product therefore shows no overall optical rotation despite each molecule being chiral. By contrast, enzyme-catalysed reactions in living systems are stereospecific and make a single enantiomer, because the chiral enzyme active site can only present one face.
Distinguishing the two kinds of stereoisomerism
| Feature | E/Z isomerism | Optical isomerism |
|---|---|---|
| Cause | Restricted rotation about | Chiral centre (four different groups) |
| Requirement | Two different groups on each double-bond carbon | No plane of symmetry |
| Effect on polarised light | None | Rotates it (enantiomers oppositely) |
| Example | But-2-ene | Lactic acid |
Recognising which type a molecule shows, and why, is the most common stereochemistry exam task.
Examples in context
Thalidomide. One enantiomer of thalidomide was a useful sedative while the other caused birth defects, a tragic illustration of why drug stereochemistry must be controlled. Single-enantiomer drugs. Modern pharmaceuticals are often made or separated as a single enantiomer because only one form fits the chiral receptor, improving safety and effectiveness.
Try this
Q1. Define a chiral centre. [1 mark]
- Cue. A carbon atom bonded to four different groups.
Q2. State how the two enantiomers of a chiral compound affect plane-polarised light. [1 mark]
- Cue. They rotate it by equal angles in opposite directions.
Q3. State why a racemic mixture shows no overall optical rotation. [1 mark]
- Cue. It contains equal amounts of both enantiomers, whose opposite rotations cancel.
Q4. Explain why the addition of to ethanal gives a racemic product. [2 marks]
- Cue. The carbonyl carbon is planar, so cyanide attacks from either face with equal probability, giving equal amounts of the two enantiomers.
Q5. State the requirement for a molecule to show E/Z isomerism. [1 mark]
- Cue. Restricted rotation about a double bond with two different groups on each double-bond carbon.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20203 marksDefine a chiral centre and explain how the two optical isomers of a chiral molecule differ in their effect on plane-polarised light.Show worked answer →
A chiral centre is a carbon atom bonded to four different groups, which gives the molecule no plane of symmetry.
The two optical isomers (enantiomers) are non-superimposable mirror images of each other.
One enantiomer rotates the plane of plane-polarised light clockwise and the other rotates it anticlockwise by the same angle; otherwise their physical properties are identical.
Markers reward four different groups, non-superimposable mirror images, and the equal but opposite rotation of plane-polarised light.
WJEC 20183 marksA reaction produces a racemic mixture of a chiral product. State what is meant by a racemic mixture and explain why it shows no overall rotation of plane-polarised light.Show worked answer →
A racemic mixture contains equal amounts of the two enantiomers of a chiral compound.
Each enantiomer rotates plane-polarised light by the same angle but in opposite directions.
Because the amounts are equal, the rotations cancel exactly, so the mixture shows no overall optical rotation.
Markers reward the equal mixture of enantiomers, the opposite rotations, and the cancellation giving no net rotation.
Related dot points
- The carbonyl group, nucleophilic addition, reduction and oxidation of aldehydes and ketones, and tests to identify and distinguish them.
A focused answer to WJEC A-Level Chemistry Unit 4, covering the carbonyl group, nucleophilic addition of cyanide and reduction, the oxidation of aldehydes, and the chemical tests that identify carbonyls and distinguish aldehydes from ketones.
- The preparation and basicity of amines, the properties of amino acids and zwitterions, peptide bonds, and the formation of polyamides and proteins.
A focused answer to WJEC A-Level Chemistry Unit 4, covering the preparation and basicity of amines, the zwitterionic behaviour and isoelectric point of amino acids, peptide bonds, and the formation of polyamides and proteins.
- Reaction pathways linking functional groups, planning multi-step syntheses, practical techniques of preparation and purification, and assessing purity.
A focused answer to WJEC A-Level Chemistry Unit 4, covering reaction pathways linking functional groups, planning multi-step syntheses, the practical techniques of preparation and purification, and assessing the purity of a product.
- Carboxylic acids and their acidity, esters, acyl chlorides and acid anhydrides, and the reactions and interconversions of these derivatives.
A focused answer to WJEC A-Level Chemistry Unit 4, covering the acidity and reactions of carboxylic acids, esters, acyl chlorides and acid anhydrides, and how these derivatives interconvert and react with nucleophiles.
- Mass spectrometry, infrared spectroscopy, proton and carbon-13 NMR, and chromatographic separation in structure determination.
A focused answer to WJEC A-Level Chemistry Unit 4, covering mass spectrometry and fragmentation, infrared spectroscopy, proton and carbon-13 NMR, and chromatographic separation in determining the structure of organic molecules.
Sources & how we know this
- WJEC A-level Chemistry specification — WJEC (2015)