Isomerism: Optical (CIE A Level Chemistry)

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. 

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.

Give the IUPAC name of leucine.

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

Fig. 2.1

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[1]

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

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

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

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[2]

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

Fig. 3.1

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

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

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

This question is about the chemistry of 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.

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 the percentage purity of limonene.

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

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

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

 

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

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.

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

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

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