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A LevelChemistryCIEExam Questions29. An Introduction to A Level Organic Chemistry29.4 Isomerism: Optical

Isomerism: Optical (CIE A Level Chemistry)

Exam Questions

1 hour6 questions
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Medium
Hard
1a3 marks

This question is about isomerism in different acids.

Hydroxypropanoic acid has two position isomers.

i)
Draw and name the two position isomers of hydroxypropanoic acid.
 
[1]
 
ii)
Identify which isomer is optically active.
 
[1]

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1b1 mark

There are 20 amino acids used, by the cells in the human body, for protein synthesis. There is only one amino acid that is not optically active.

Three amino acids are shown in Fig. 1.1.

gly-cys-asp

Fig. 1.1

Identify the amino acid that does not have any effect on plane polarised light.

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1c2 marks

Serine, shown in Fig. 1.2, exhibits optical activity.

serineFig. 1.2

Draw three-dimensional representations of the two isomers to show the relationship between them.

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1a1 mark

Leucine is an optically active amino acid with the molecular formula (CH3)2CHCH2CH(NH2)COOH. 

Draw the displayed formula of leucine.

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1b4 marks

Leucine exists as stereoisomers. 

i)

Explain the meaning of the term stereoisomers.

[2]

ii)
Explain how you can distinguish between the isomers.
[2]

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1c2 marks

The two optical isomers of leucine taste different. One isomer has a bitter taste, the other is sweet.

Draw the three-dimensional arrangement of the two optical isomers.

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1d1 mark

Give the IUPAC name of leucine.

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2a2 marks

Propranolol is an optically active drug that is used to treat hypertension. Its structure is shown in Fig. 2.1.

7-1-5a-m-propranolol-a

Fig. 2.1

i)
Give the molecular formula of propranolol.

[1]

ii)
Circle the chiral centre of propranolol in Fig 2.1.

[1]

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2b3 marks

Explain why a racemic mixture of propranolol has no effect on plane polarised light.

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2c1 mark

Explain why it is important to separate a racemic mixture of propranolol into pure single enantiomers.

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2d1 mark

Chiral catalysts are often used in the manufacturing process of drugs.

i)
What is the purpose of a chiral catalyst?

[1]

ii)
Give one advantage and one disadvantage of using a chiral catalyst.

[2]

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3a7 marks

Two isomers of C3H6O are shown below in Fig. 3.1.

q1a-med-7-1-optical-isomerism-aqa-a-level-chemistry

Fig. 3.1

 

i)
Give the names of compounds A and B.
[1]
ii)
Name and draw the mechanism to show the reaction of compound A with HCN. Include all relevant charges, partial charges, lone pairs and curly arrows.
[5]
iii)
Name the product formed from this reaction. 
[1]

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3b3 marks

Name the product of the reaction between compound B and HCN and explain why this compound is not optically active.

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3c3 marks

Explain why the reaction of compound A and HCN can produce a racemic mixture.

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3d2 marks

Explain why the racemic mixture produced from the reaction of compound A and HCN has no effect on plane polarised light.

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1a2 marks

This question is about the chemistry of limonene.

limonene

Limonene is commonly found in the rinds of citrus fruits such as grapefruit, lemon, lime and oranges. It is a naturally occurring hydrocarbon with the molecular formula C10H16.

Limonene exists as a pair of enantiomers:

  • One enantiomer is responsible for a strong orange smell
  • The other enantiomer is responsible for a lemon smell.

Draw 3D representations of the two enantiomers of limonene.

 
optical isomer 1 optical isomer 2








 

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1b3 marks

Suggest why receptors in the human nose can distinguish between the orange and lemon enantiomers of limonene.

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1c
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Limonene can undergo full hydrogenation to form menthane as shown.

 limonene-hydrogenation

For the complete hydrogenation of an impure sample of 3.00 g of limonene at room temperature and pressure, 870 cm3 of hydrogen was required. 

Calculate the percentage purity of limonene.

(1 mol of hydrogen occupies 24.0 dm3 at room temperature and pressure)

 
 
 

percentage purity = ........................ %

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1d2 marks

The final step of the multi-step synthetic conversion of limonene to menthol is shown.

 menthol-synthesis-from-limonene
 

State the reagent for this reaction and the type of reaction.

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2a4 marks

Aldehydes and ketones always react with HCN to form optical isomers.

Discuss this statement, using the general formulae RCHO and R’COR’’. Your answer should include equations where appropriate.

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2b3 marks

Acidified K2CrO7 (aq) was heated under reflux with the following β-hydroxyaldehyde shown in Fig. 2.1. 

b-hydroxyaldehyde

Fig. 2.1


Explain the change in chirality, if any, between the β-hydroxyaldehyde and its organic product.

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2c3 marks
Explain the change in chirality, if any, when pent-1-en-3-ol is hydrogenated.
 

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2d3 marks

Other isomers of pent-1-en-3-ol, C5H10O, exist.

i)
Draw the displayed formula of the secondary alcohol isomer of pent-1-en-3-ol that exhibits both stereoisomerism and optical isomerism and identify the chiral centre.
 
[2]
 
ii)
Give the IUPAC name for this isomer.
 
[1]

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