Optical isomerismAQA A-Level Chemistry: Topic test

1. 1
   2-Bromobutane, CH₃CHBrCH₂CH₃, is a halogenoalkane that exists as a pair of optical isomers. Its structural isomer 1-bromobutane, CH₃CH₂CH₂CH₂Br, does not.

   (a)

   Which statement describes the chiral carbon atom in 2-bromobutane?

   [1 mark]

   • AIt is bonded to four different atoms or groups
   • BIt is bonded to two bromine atoms
   • CIt is the carbon atom at the end of the chain
   • DIt forms a double bond with the bromine atom

   (b)

   Which property is identical for the two optical isomers of 2-bromobutane?

   [1 mark]

   • AThe direction in which they rotate plane-polarised light
   • BThe boiling point
   • CThe arrangement of the groups in space around the chiral carbon
   • DThe sign of the optical rotation measured in a polarimeter

   (c)

   State what is meant by a chiral centre and identify the chiral centre in 2-bromobutane.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ibuprofen, (CH₃)₂CHCH₂C₆H₄CH(CH₃)COOH, is a painkiller. Its molecule contains a single carbon atom bonded to four different groups. Only the S-enantiomer is effective, but tablets sold in many countries contain an equal mixture of the R- and S-forms.

   (a)

   How many chiral centres are there in one molecule of ibuprofen?

   [1 mark]

   • A3
   • B0
   • C1
   • D2

   (b)

   Which statement about pure R-ibuprofen and pure S-ibuprofen is correct?

   [1 mark]

   • AThey have different molecular formulae
   • BThey are superimposable on one another
   • CThey both rotate plane-polarised light in the same direction
   • DThey rotate plane-polarised light in opposite directions

   (c)

   A tablet contains an equal amount of R-ibuprofen and S-ibuprofen. Explain why the tablet, dissolved in a suitable solvent, does not rotate plane-polarised light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Pentan-2-ol, CH₃CH(OH)CH₂CH₂CH₃, and pentan-3-ol, CH₃CH₂CH(OH)CH₂CH₃, are structural isomers of formula C₅H₁₂O. A technician finds that only one of them exists as a pair of optical isomers.

   (a)

   Identify the alcohol that shows optical isomerism. Explain your answer and explain why the other alcohol does not.

   [3 marks]

   (b)

   Describe how the two optical isomers of the alcohol you identified are related to one another, state one physical property that is the same for both, and compare their effects on plane-polarised light.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Naproxen is an anti-inflammatory drug whose molecule has one chiral centre. The S-enantiomer relieves inflammation, while the R-enantiomer is not only inactive but can damage the liver. Naproxen is therefore sold as the pure S-enantiomer rather than as a racemic mixture, although the pure form costs more to manufacture.

   (a)

   Explain what is meant by chirality and how the two enantiomers of naproxen are related, and describe how a racemic mixture of naproxen differs from a sample of pure S-naproxen when tested with plane-polarised light.

   [6 marks]

   (b)

   Evaluate whether the manufacturer is justified in selling naproxen as the pure S-enantiomer rather than as a racemic mixture.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A chemist is given four hydrocarbons that all have the molecular formula C₇H₁₆: heptane, 2-methylhexane, 3-methylhexane and 3-ethylpentane. Only one of them shows optical isomerism.

   (a)

   Which hydrocarbon shows optical isomerism?

   [1 mark]

   • AHeptane
   • B2-Methylhexane
   • C3-Ethylpentane
   • D3-Methylhexane

   (b)

   A pure sample of one enantiomer of 3-methylhexane rotates plane-polarised light by +12°. What would an equal concentration of the other enantiomer do under the same conditions?

   [1 mark]

   • ARotate it by −12°
   • BRotate it by +12°
   • CNot rotate it
   • DRotate it by +6°

   (c)

   Explain why 2-methylhexane does not show optical isomerism.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Carvone, C₁₀H₁₄O, is found in two forms. R-carvone smells of spearmint and S-carvone smells of caraway seeds. The molecule contains one chiral centre.

   (a)

   What is the relationship between R-carvone and S-carvone?

   [1 mark]

   • AStructural isomers
   • BGeometric isomers
   • CEnantiomers
   • DIdentical molecules

   (b)

   Which property is the same for R-carvone and S-carvone?

   [1 mark]

   • AThe direction of rotation of plane-polarised light
   • BThe boiling point
   • CThe spatial arrangement around the chiral centre
   • DThe sign of the optical rotation

   (c)

   A mixture contains 70% R-carvone and 30% S-carvone. Explain whether this mixture rotates plane-polarised light and, if it does, state which direction it rotates compared with pure R-carvone.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Ethanal, CH₃CHO, reacts with hydrogen cyanide to form 2-hydroxypropanenitrile, CH₃CH(OH)CN. Methanal, HCHO, reacts in the same way to form hydroxyethanenitrile, HOCH₂CN. A chemist finds that the product from ethanal does not rotate plane-polarised light.

   (a)

   Explain why 2-hydroxypropanenitrile shows optical isomerism but hydroxyethanenitrile does not.

   [3 marks]

   (b)

   A student obtains 20.0 g of the product from ethanal. The relative molecular mass of 2-hydroxypropanenitrile is 71.0. The product is optically inactive. Calculate the amount, in mol, of each enantiomer present in the sample, and explain why the sample does not rotate plane-polarised light.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A chemist measures the optical rotation of four samples of the same chiral compound, Q, under identical conditions of concentration and path length. Pure R-Q rotates the plane of plane-polarised light by +30°. Sample 1 rotates it by 0°. Sample 2 rotates it by −30°. Sample 3 rotates it by +15°.

   (a)

   Deduce the composition of each of samples 1, 2 and 3. Explain your reasoning.

   [6 marks]

   (b)

   A pharmaceutical company makes Q by adding a nucleophile to a planar carbonyl group, which produces sample 1. Only R-Q is useful. Explain why this route gives sample 1, why the two forms cannot easily be separated by distillation, and evaluate whether a different route would be better.

   [6 marks]

   Total for question 8: 12 marks

End of questions