Chirality and optical isomerismEdexcel International A Level Chemistry: Subtopic test

1. 1
   Lactic acid (2-hydroxypropanoic acid), CH₃CH(OH)COOH, is made by muscle cells during hard exercise and by bacteria in yoghurt production.

   (a)

   Which carbon atom in lactic acid is the chiral centre?

   [1 mark]

   • Athe carbon of the CH₃ group
   • Bthe carbon of the COOH group
   • Cthe carbon bonded to the OH group
   • Dthere is no chiral centre

   (b)

   Which of these compounds also shows optical isomerism?

   [1 mark]

   • Apropan-2-ol
   • Bbutan-1-ol
   • C2-methylpropan-2-ol
   • Dbutan-2-ol

   (c)

   Explain why lactic acid shows optical isomerism and what is meant by its optical isomers.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ibuprofen, (CH₃)₂CHCH₂C₆H₄CH(CH₃)COOH, is a painkiller that is sold as an equal mixture of its two optical isomers, although only one of them is pharmacologically active. Optical activity is measured in a polarimeter using plane-polarised monochromatic light.

   (a)

   How many chiral centres (asymmetric carbon atoms) are there in a molecule of ibuprofen?

   [1 mark]

   • A0
   • B1
   • C2
   • D3

   (b)

   A solution of one optical isomer of ibuprofen rotates the plane of plane-polarised light by +12.0°. What would be the rotation for a solution of the other isomer at the same concentration, in the same polarimeter?

   [1 mark]

   • A−12.0°
   • B+12.0°
   • C0°
   • D−6.0°

   (c)

   Explain why the equal mixture of ibuprofen isomers has no effect on plane-polarised light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Two samples of halogenoalkane, each consisting of a single optical isomer, are hydrolysed by heating with aqueous sodium hydroxide. Compound X, 3-bromo-3-methylhexane, CH₃CH₂C(CH₃)(Br)CH₂CH₂CH₃, gives an alcohol product that does not rotate the plane of plane-polarised light. Compound Y, 2-bromobutane, gives butan-2-ol that does rotate the plane of plane-polarised light.

   (a)

   Explain what the result for compound X indicates about the mechanism of its hydrolysis.

   [3 marks]

   (b)

   Explain what the result for compound Y indicates about the mechanism of its hydrolysis.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Ethanal reacts with hydrogen cyanide in the presence of a little potassium cyanide to form 2-hydroxypropanenitrile, which is then hydrolysed by heating with dilute hydrochloric acid to give lactic acid, CH₃CH(OH)COOH. The lactic acid formed does not rotate the plane of plane-polarised light.

   (a)

   Describe the mechanism of the reaction of ethanal with hydrogen cyanide, and explain why the product shows no optical activity.

   [6 marks]

   (b)

   Explain how the two enantiomers of lactic acid compare in their properties, and how a polarimeter could be used to show whether a sample is a single enantiomer or a racemic mixture.

   [6 marks]

   Total for question 4: 12 marks

End of questions