Optical isomers 3.3.7

Question 1

EQ : The aldehyde CH3CH2CH2CH2CHO reacts with KCN followed by dilute acid to form a racemic mixture of two stereoisomers of CH3CH2CH2CH2CH(OH)CN.
Describe how you would distinguish between separate samples of the two stereoisomers of CH3CH2CH2CH2CH(OH)CN.(2)

Answer 1

• Plane polarised light
• Enantiomers would rotate light in opposite directions

Question 2

EQ : The aldehyde CH3CH2CH2CH2CHO reacts with KCN followed by dilute acid to form a racemic mixture of two stereoisomers of CH3CH2CH2CH2CH(OH)CN.
Explain why this reaction produces a racemic mixture (3)

Answer 2

• planar carbonyl group
• attack from either side
• produces equal amounts of the two enantiomers

Question 3

EQ : 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)

Answer 3

Stage 1 : formation of product
- nucleophilic attack
- planar carbonyl group
- H- attacks from either side

Stage 2 : nature of product
- products of step 1 shown
- this exists in two chiral forms
- equal amounts of each enantiomer / racemic mixture formed

Stage 3 : optical activity
- optical isomers rotate the plane of polarised light equally in
- With a racemic mixture the effects cancel