AQA A Level Chemistry

Topic Questions

7.1 Optical Isomerism (A-level only)

1a2 marks

Name the two types of stereoisomerism.

1b1 mark

What is a chiral carbon or chiral centre?

1c1 mark

The structure of one optical isomer of a chlorofluorocarbon is shown in Figure 1.

Figure 1

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Draw the structure of the other enantiomer.

1d1 mark

The chemical and physical properties of optical isomers are identical. In terms of properties, state one difference between optical isomers.

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2a1 mark

The skeletal structure of an organic compound is shown in Figure 1.

Figure 1

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Name the organic compound.

2b2 marks

Identify the chiral carbons in Figure 1, part (a).

2c1 mark

Explain why carbon a in Figure 2 cannot be a chiral carbon.

Figure 2

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2d2 marks

Figure 3 identifies a different carbon, b, in the organic compounds structure.

Figure 3

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Complete Figure 4 to show the 3D representations of the optical isomers formed at carbon b

Figure 4

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3a1 mark

Define the term racemic mixture.

3b1 mark

Describe the composition of enantiomers when a reaction mixture is optically active.

3c2 marks

In the 1950’s, thalidomide was a drug that was prescribed as a sedative and for the treatment of morning sickness. The drug used was a racemic mixture rather than being enantiopure (containing only one enantiomer).

i)
Suggest one reason why the drug was used as a racemic mixture.

ii)
Suggest one reason why it would have been better to use an enantiopure version of thalidomide.

3d2 marks

The S enantiomer of ibuprofen has the known non-steroidal anti-inflammatory drug effect, while the R enantiomer has no anti-inflammatory effect.

Based on this information, suggest why ibuprofen is produced as a racemic mixture.

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4a1 mark

Carvone is an optically active molecule which is found widely in plants, mostly in caraway seeds and spearmint leaves. The structure is shown in Figure 1

Figure 1

q4a-7-1-optical-isomerism-aqa-a-level-chemistry

Mark on the diagram using an ‘x’ the chiral carbon which causes this structure to exhibit optical isomerism.

4b1 mark

Draw the structure of the other optical isomer formed by carvone shown in Figure 1.

4c2 marks

One optical isomer of carvone smells of spearmint, the other optical isomer smells of caraway seeds. 

Explain how both isomers can be distinguished from one another.

4d2 marks

Draw the two 3-dimensional optical isomers of 2-chlorobutane.

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5a3 marks

There are two position isomers of hydroxypropanoic acid.

i)
Draw and name the two position isomers of hydroxypropanoic acid.

ii)
Identify which isomer is optically active.
5b1 mark

Nineteen out of the 20 amino acids used in protein synthesis in the cells of the human body are optically active.

Three amino acids are shown in Figure 1.

Figure 1

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Identify the amino acid that is not optically active.

5c2 marks

The structure of serine is shown in Figure 2.

Figure 2

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Draw 3D representations of the two isomers to show the relationship between them.

5d2 marks

The molecular formula of alanine is NH2CHCH3COOH.

Draw 3D representations of the two isomers to show the relationship between them.

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1a6 marks

Cholesterol, shown in Figure 1, is a fatty chemical used by the body to build healthy cells.

Figure 1

1-2

State two chemical tests that could be used to identify functional groups within the cholesterol structure.

Your answer should include any expected observations and diagrams of the product of each test.

1b1 mark

State the number of chiral carbons in the cholesterol structure.

1c1 mark

A student suggested that cholesterol could be tested with plane polarised light to show that it contains chiral centres. 

Is the student correct? Justify your answer.

1d2 marks

There are a large number of naturally occurring biological molecules that contain chiral carbons. These biological molecules are often produced as enantiopure compounds in living organisms. All amino acids, except glycine, are a good example of this.

Suggest why amino acids are able to be produced as enantiopure compounds in living organisms.

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2a2 marks

Limonene, shown in Figure 1, is a naturally occurring hydrocarbon with the molecular formula C10H16 commonly found in the rinds of citrus fruits such as grapefruit, lemon, lime and oranges.

Figure 1

2-1
Limonene exists as a pair of enantiomers; one enantiomer is responsible for a strong orange smell while the other is thought to smell like lemons.

Draw 3D representations of the two enantiomers of limonene. 

2b3 marks

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

2c4 marks

Limonene can undergo full hydrogenation to form menthane as shown in Figure 2.

Figure 2

3-1

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 dm3 at room temperature and pressure)

2d2 marks

Limonene can be used in the multi-step synthesis of menthol. The final step of this reaction is shown in Figure 3.

Figure 3

4-2

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

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3a1 mark

Lactic acid has the molecular formula C3H6O3. The oxidation product of lactic acid does not react with Fehling’s solution.

Draw the skeletal formula of lactic acid.

3b4 marks

Illustrate the types of isomerism shown by lactic acid. Your answer should not consider ethers.

3c2 marks

The general structure of polylactic acid is shown in Figure 1.

Figure 1

5-1

Draw two possible structures formed from two repeating units. 

Your answer should keep the main polymer chain in the same plane but show the 3D representation of the chiral carbons.

3d1 mark

State why the polymer formed from the uncontrolled condensation polymerisation of lactic acid is not a racemate.

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4a3 marks

Draw the displayed formula and 3D representations of the smallest aldehyde to form optical isomers.

4b3 marks
i)
Draw the displayed formula of the oxidation product of the aldehyde identified in part (a).

ii)
State a suitable reagent, including observations, for the oxidation of the aldehyde identified in part (a).
4c2 marks
i)
Write a balanced symbol equation, using structural formulae, to show the reduction of the aldehyde identified in part (a). You should use [H] to show the reducing agent.

ii)
State a suitable reducing agent.
4d1 mark

Name the organic compound formed by the reaction of the oxidation product in part b) and the reduction product in part c)of the aldehyde identified in part (a).

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5a4 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.

5b3 marks

A β-hydroxyaldehyde, shown in Figure 1, reacts with acidified potassium dichromate(VI)  solution under reflux.

Figure 1

6-1

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

5c3 marks

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

5d3 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.
ii)
Give the IUPAC name for this isomer.

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1a8 marks

Two isomers of C3H6O are shown below in Figure 1

Figure 1

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

i)
Give the IUPAC names of Compounds A and B.

ii)
Name and outline the mechanism for the reaction of compound A with HCN.

iii)
Name the product formed from this reaction. 
1b3 marks

Name the product of the reaction between compound B and HCN and explain why this compound is optically inactive.

1c3 marks

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

1d2 marks

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

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2a1 mark

The structure of pentanal is shown below in Figure 1

Figure 1

q2a-med-7-1-optical-isomerism-aqa-a-level-chemistry

Draw the structure of a branched isomer of pentanal that exhibits optical isomerism.

2b1 mark

Draw the structure of a branched isomer of pentanal that does not exhibit optical isomerism.

2c3 marks

In terms of optical isomer products, compare the reactions of unbranched aldehydes and unbranched ketones with HCN.

Some examples of unbranched aldehydes and unbranched ketones are shown in Figure 2.

Figure 2

1-1

2d3 marks

One of the ketones, shown in Figure 2, will form an optical isomer when it reacts with HCN.

Figure 2

1-1

i)
Identify the ketone that will form an optical isomer with HCN and name the product.

ii)
Draw 3D representations of the two optical isomers produced.

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3a1 mark

Lactic acid can be synthesised from ethanal. The first step of this synthesis is the reaction with HCN. The mechanism for this reaction is shown in Figure 1.

Figure 1

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State the role of the CN- in this reaction.

3b1 mark

Lactic acid is an optically active organic acid with the molecular formula CH3CH(OH)COOH. 

Draw the displayed formula of lactic acid. 

3c4 marks

Lactic acid exists as stereoisomers. 

i)
Explain the meaning of the term stereoisomerism.

ii)
Explain how you can distinguish between the isomers. 
3d1 mark

Lactic acid can undergo condensation polymerisation to form polylactic acid, shown in Figure 2.

Figure 2

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Label the chiral carbons in the polylactic acid molecule with an ‘x’.

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4a2 marks

Erythrulose is a carbohydrate, which is used in combination with dihydroxyacetone (DHA) in many self-tanning products because of its natural ability to dye the skin. The sugar reacts with the amino acids of the keratin found in the dead skin cells of the upper layer of the skin.

The structure of Erythrulose is shown in Figure 1.

Figure 1

q4a-med-7-1-optical-isomerism-aqa-a-level-chemistry


Identify which carbon in this molecule is chiral. Justify your answer. 

4b3 marks

Dihydroxyacetone has the molecular formula C3H6O3.

i)
Draw the two isomers of dihydroxyacetone which contain the carbonyl group.

ii)
Name the type of isomerism exhibited by these isomers.

4c1 mark

A student incorrectly states that 1,1-dihydroxypropanone exhibits optical isomerism because the central carbon atom has different groups attached.

Explain the student’s error.

4d2 marks

The  keratin found in the dead skin cells of the upper layer of the skin are rich in the amino acid cysteine.

Using your data booklet, draw 3D representations of the two optical isomers of cysteine.

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5a1 mark

The full displayed formula of threonine is shown in Figure 1.

Figure 1

4-1

State the systematic name of threonine.

5b2 marks

Threonine contains two chiral centres.

Draw 3D representations of the pair of enantiomers from one of the chiral centres, showing how the two structures are related to each other.

5c1 mark

State the bond angle around the chiral carbon drawn in part (b).

5d1 mark

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

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