optical isomerism [O2] PAPER 2

Question 1

what must be present to allow a molecule to exhibit optical isomerism?

Answer 1

a chiral centre - an asymmetric carbon atom, so it is attached to four different groups

Question 2

describe how you would distinguish between separate samples of two enantiomers of ____

Answer 2

• use plane polarised light
• the enantiomers would rotate the light in opposite directions (equally)

Question 3

The aldehyde CH3CH2CH2CH2CHO reacts with KCN followed by dilute acid to form a racemic mixture of the two stereoisomers of CH3CH2CH2CH2CH(OH)CN.

Explain why the reaction produces a racemic mixture.

Answer 3

• planar carbonyl group
• CN– ion can attack from below or above the plane with equal probability

Question 4

explain the meaning of the term racemic mixture.

Answer 4

equal mixture of enantiomers

Question 5

State the relationship between two chiral molecules with the same structural formula.

Answer 5

non-superimposable mirror images

Question 6

what is optical isomerism?

Answer 6

• a type of stereoisomerism (same molecular formula, atoms arranged differently in space)
• occurs as a result of chirality in molecules

Question 7

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.

Answer 7

• formation of the product is via nucleophilic substitution
• planar carbonyl group
• H– (the nucleophile) attacks from either side with equal probability
• product of reaction exists in two chiral forms
• racemic mixture
• enantiomers rotate plane polarised light equally in opposite directions
• with a racemic mixture, the effects cancel