Isomerism: Optical (Cambridge (CIE) A Level Chemistry): Exam Questions

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

Draw the displayed formula of leucine.

Leucine exists as stereoisomers.

i) Explain what is meant by the term stereoisomerism.

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ii) Describe how the two optical isomers of leucine can be distinguished.

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Draw three-dimensional diagrams to show the two optical isomers of leucine.

State the systematic (IUPAC) name of leucine.

Propranolol is an optically active drug. Its structure is shown in Fig. 2.1.

Fig. 2.1

i) Deduce the molecular formula of propranolol.

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ii) On Fig. 2.1, indicate the chiral centre with an asterisk (*).

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

Explain why it is often important that chiral drugs, such as propranolol, are administered as a single pure enantiomer rather than as a racemic mixture.

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

i) State the purpose of using a chiral catalyst in drug synthesis.

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

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

Fig. 3.1

i) State the systematic (IUPAC) name of compound A and of compound B.

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ii) State the name of the mechanism for the reaction between compound A and HCN.

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iii) Describe the mechanism for the reaction between compound A and HCN. Include all relevant curly arrows, lone pairs, and partial charges in your diagram.

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iv) State the systematic name of the organic product formed in this reaction.

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Compound B also reacts with HCN.

i) State the systematic name of the organic product formed.

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ii) Explain why the organic product in b(i) does not exhibit optical isomerism.

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Explain why the reaction between compound A and HCN produces a racemic mixture.

Explain why the racemic mixture formed in this reaction has no effect on plane-polarised light.

This question is about the chemistry of limonene.

Limonene is a naturally occurring hydrocarbon found in citrus fruits, 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 three-dimensional diagrams to show the two optical isomers of limonene.

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

Limonene can undergo full hydrogenation to form menthane as shown.

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, showing your working, the percentage purity of limonene.

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

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

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

Reagent: ..............................................................

Type of reaction: ..............................................................

Explain why all aldehydes, other than methanal, react with HCN to form a product that exhibits optical isomerism, whereas only some ketones do. Use the general formulae RCHO and R'COR'' in your answer. Include equations where appropriate.

The β-hydroxyaldehyde shown in Fig. 2.1 is heated under reflux with acidified aqueous potassium dichromate(VI).

Fig. 2.1

State whether the organic product of this reaction exhibits optical isomerism. Explain your answer.

Pent-1-en-3-ol is reacted with hydrogen gas in the presence of a nickel catalyst.

State whether the organic product formed in this reaction exhibits optical isomerism. Explain your answer.

An isomer of pent-1-en-3-ol, C₅H₁₀O, is a secondary alcohol that exhibits both optical isomerism and geometrical (E/Z) isomerism.

i) Draw the skeletal formula of this isomer and indicate the chiral centre with an asterisk (*).

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ii) State the systematic name of this isomer.

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