Optical isomerism and chiralityAQA A-Level Chemistry: Subtopic test

1. 1
   Lactic acid, CH₃CH(OH)COOH, builds up in muscle tissue during hard exercise and is also the acid that makes milk taste sour. Its molecules show optical isomerism.

   (a)

   Which statement explains why lactic acid is chiral?

   [1 mark]

   • AThe second carbon atom is bonded to four different groups
   • BThe molecule contains a carboxyl group
   • CThe molecule contains both an OH group and a C=O group
   • DThe molecule contains a carbon atom bonded to two hydrogen atoms

   (b)

   Which of these alcohols also shows optical isomerism?

   [1 mark]

   • APropan-2-ol
   • BPentan-3-ol
   • CPentan-2-ol
   • D2-Methylpropan-2-ol

   (c)

   Explain why lactic acid exists as a pair of optical isomers.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist has two samples, X and Y, of the same chiral compound. When each is placed separately in a polarimeter, X rotates the plane of plane-polarised light by 12° clockwise and Y rotates it by 12° anticlockwise. When equal masses of X and Y are dissolved together in the same volume of solvent, the solution produces no rotation at all.

   (a)

   What is the relationship between X and Y?

   [1 mark]

   • AThey are structural isomers
   • BThey are enantiomers
   • CThey are E/Z isomers
   • DThey are identical molecules

   (b)

   What name is given to the mixture of equal amounts of X and Y?

   [1 mark]

   • AA mixture of structural isomers
   • BA buffer solution
   • CAn equilibrium mixture
   • DA racemic mixture

   (c)

   Explain why the mixture of X and Y produces no rotation of plane-polarised light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The amino acid alanine, CH₃CH(NH₂)COOH, is found in proteins. Glycine, H₂NCH₂COOH, is the simplest amino acid. A student synthesises alanine in the laboratory from starting materials that are not chiral.

   (a)

   Alanine is chiral. Identify the chiral centre in alanine and the four groups bonded to it, and state how its two enantiomers differ in their effect on plane-polarised light.

   [3 marks]

   (b)

   Explain why glycine is not chiral. The student finds that the alanine made in the laboratory is optically inactive. Explain this observation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician is investigating the four alcohols that have the molecular formula C₄H₁₀O: butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. Only one of them has molecules that exist as a pair of optical isomers.

   (a)

   Butan-2-ol, CH₃CH(OH)CH₂CH₃, shows optical isomerism. Explain why, and describe how its two optical isomers are related and how they could be distinguished experimentally.

   [6 marks]

   (b)

   Deduce whether each of the other three isomers, butan-1-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol, is chiral. Give a reason for each answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions