Properties of stereoisomers

When life gives you lemons … sniff them! This experiment will see students discovering the differences between two different enantiomers of limonene, using their scientific curiosity, and their noses. 

This experiment should take 5 minutes.

Equipment 

Apparatus

• Plastic bottles x2
• Cotton wool

Chemicals

• (R)-(+)-Limonene
• (S)-(–)-Limonene

Health, safety and technical notes 

• Read our standard health and safety guidance.
• Students must wear eye protection if carrying out steam distillation.
• Not needed for sniffing the stereoisomers.

Procedure

In this experiment, students detect the differences in smell of each enantiomer absorbed on cotton wool inside small sample bottles.

To prepare these;

1. Place a small quantity of cotton wool into each bottle.
2. Add 10 drops of the stereoisomer.
3. The bottles can then be passed around the classroom.

Questions

Would you expect the two stereoisomers of limonene to behave differently in their:

1. Reaction with bromine?
2. Reduction with hydrogen?
3. Melting point?
4. Boiling point?
5. Infrared spectrum?
6. Effect on plane of polarisation of plane-polarised light?
7. Combustion?
8. Mass spectrum?

Explain your answers.

Extension

Students could obtain small quantities of (R)-(+)- limonene in natural fruits by carrying out steam distillation of the peel of citrus fruits such as oranges and lemons and comparing the odours against the standards.

However, the S-(–) isomer is scarce in citrus fruits: pine needles might be a good source, but the presence of other terpenes might make it hard to separate.