Organic 2 Section 3 - Unit 25: Nomenclature and Isomerism

Question 1

State the meaning of the term racemic mixture and explain how racemic mixtures occur (3 marks)

Answer 1

• Optically inactive/equal mixture of enantiomers
• Occur when a planar carbonyl group
• Attack from above or below (either side)

Question 2

Explain how you could distinguish between optical isomers (2 marks)

Answer 2

• Plane polarised light

- Rotated in opposite/different directions

Question 3

Butanone is reduced in a two-step reaction using NaBH4 followed by dilute hydrochloric acid. By considering the mechanism of the reaction, explain why the product has no effect on plane polarised light (6 marks)

Answer 3

Stage 1: Formation of product
- Nucleophilic attack
- Planar carbonyl group
- H – attacks from either side
Stage 2: Nature of product
• Product exists in two chiral forms
• Equal amounts of each enantiomer/racemic mixture formed
Stage 3: Optical activity
- Optical isomers/enantiomers rotate the plane of polarised light equally in
- With a racemic mixture the effects cancel