Chirality and optical isomerismEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student examines four alcohols: propan-2-ol, butan-2-ol, 2-methylpropan-2-ol and pentan-3-ol. Only one of them exists as a pair of optical isomers.

   (a)

   Which alcohol exists as a pair of optical isomers?

   [1 mark]

   • AButan-2-ol
   • BPropan-2-ol
   • C2-Methylpropan-2-ol
   • DPentan-3-ol

   (b)

   Which statement about the two optical isomers of butan-2-ol is correct?

   [1 mark]

   • AThey rotate plane-polarised light in the same direction by different angles
   • BThey have different boiling points
   • CThey rotate plane-polarised light in opposite directions by equal angles
   • DOnly one of them rotates plane-polarised light

   (c)

   Explain why butan-2-ol shows optical isomerism.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A polarimeter measures the rotation of plane-polarised monochromatic light by solutions of 2-chlorobutane. Sample P is a pure single enantiomer and rotates the plane of polarised light by +20°. Sample Q is the other enantiomer of 2-chlorobutane. Sample R is made by mixing equal amounts of P and Q, with all solutions at the same total concentration.

   (a)

   What is the rotation of plane-polarised light by Sample Q?

   [1 mark]

   • A+20°
   • B−20°
   • C0°
   • D+40°

   (b)

   What is the observed rotation by Sample R, and what is R called?

   [1 mark]

   • A+20°, an optically active mixture
   • B−20°, an optically active mixture
   • C+10°, a diastereoisomeric mixture
   • D0°, a racemic mixture

   (c)

   Explain why Sample R shows no optical activity.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Lactic acid, CH₃CH(OH)COOH, exists as two optical isomers. In a polarimeter, a solution of one pure enantiomer of lactic acid at a concentration of 0.10 mol dm⁻³ rotates plane-polarised monochromatic light by +6.0°. The other enantiomer rotates it by the same angle in the opposite direction. A student prepares a second solution, also with total concentration 0.10 mol dm⁻³, containing 75% of the (+) enantiomer and 25% of the (−) enantiomer by amount.

   (a)

   Identify the chiral centre in lactic acid and explain why this molecule has two enantiomers. State how the two enantiomers differ in their effect on plane-polarised light.

   [3 marks]

   (b)

   Calculate the observed rotation for the second solution, showing your working. Explain why this solution is optically active but not as much as the pure enantiomer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Two single enantiomers of halogenoalkanes, X and Y, are hydrolysed separately with aqueous sodium hydroxide. X is 3-bromo-3-methylhexane, a tertiary halogenoalkane, and the rate equation for its hydrolysis is rate = k[X]. The product, 3-methylhexan-3-ol, is found to be optically inactive. Y is 2-bromooctane, a secondary halogenoalkane, and the rate equation for its hydrolysis is rate = k[Y][OH⁻]. The product, octan-2-ol, is found to be optically active.

   (a)

   Explain how the kinetic data and the optical activity of the product both provide evidence that X reacts by an SN1 mechanism.

   [6 marks]

   (b)

   Explain how the kinetic data and the optical activity of the product both provide evidence that Y reacts by an SN2 mechanism.

   [6 marks]

   Total for question 4: 12 marks

End of questions