4.6.2Unit 4. Rates, Equilibria and Further Organic Chemistry
Racemates and Optical Activity — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 43 of 56 Unit quiz
Part 1 of 4Racemic Mixtures
A racemate contains the two enantiomers in equal amounts.
Definition
- A racemic mixture, or racemate, contains equal amounts of the two enantiomers of a chiral compound.
No net rotation
- The rotation caused by one enantiomer is cancelled by the equal and opposite rotation from the other.
Optically inactive
- A racemate has no effect on plane-polarised light overall, even though each molecule is chiral.
Naming
- Racemates are sometimes written with the prefix (±) to show an equimolar mixture.
Pharmaceutical use
- Some chiral medicines are sold as racemates even though only one of the two enantiomers may give the useful biological effect.
Part 2 of 4Why Racemates Form
Ordinary chemical syntheses have no preference for one mirror-image form.
1
A reaction that creates a chiral centre from a flat, planar arrangement allows attack from either face of the molecule.
2
Each face is equally likely to be attacked, so both enantiomers are produced at the same rate.
3
The product mixture is therefore racemic and has no optical activity.
4
Producing a single enantiomer needs a chiral catalyst, an enzyme or separation of the mixture afterwards.
Part 3 of 4Importance in Medicine
Importance in Medicine
Drug action
- Receptors and enzymes in the body are chiral, so one enantiomer of a drug may fit while the other does not.
Different effects
- Sometimes one enantiomer is the active drug and the other is inactive or causes unwanted side effects.
Cost
- Separating or selectively synthesising a single enantiomer is expensive, which raises the price of a medicine.
Regulation
- Pharmaceutical companies must study each enantiomer separately before a chiral drug is approved for use.
Part 4 of 4Exam Checklist & Tips
4.6.2 – Key Points for the Exam
- Racemate Equimolar mixture of two enantiomers.
- Optically inactive Does not rotate plane-polarised light.
- Enantiomerically pure Containing only a single enantiomer.
- Planar Having all atoms in one flat plane.
Common Mistakes
- Saying a racemate is optically inactive because the molecules are not chiral.
- Forgetting that the mixture must have exactly equal amounts of each enantiomer.
- Claiming enantiomers react differently with all reagents.
- Assuming synthesis always gives a single optical isomer.
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