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Questions on optical isomerism

Chiral centres and how to find them, enantiomers as non-superimposable mirror images drawn with wedges, optical activity and the rotation of plane-polarised light, racemic mixtures and why they are optically inactive, why nucleophilic addition to a carbonyl gives a racemate, optical activity as evidence for SN1 and SN2 with partly racemised products, and single-enantiomer drugs including thalidomide and ibuprofen.

17 original questions · 55 marks · the optical isomerism notes · Carbonyls and carboxylic acids · Flashcards for this unit

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• 6 marks · Explain · OP-Q21

  Explain, using 2-hydroxypropanoic acid (lactic acid) as the example, what optical isomerism is, how the two isomers are told apart, and why lactic acid made in muscle rotates plane-polarised light while lactic acid made in the laboratory from ethanal does not.

  Mark scheme

  Indicative content

  1. carbon 2 of CH₃CH(OH)COOH is bonded to four different groups, H, OH, CH₃ and COOH, so it is a chiral centre
  2. the molecule exists as two enantiomers, non-superimposable mirror images that differ only in the arrangement in space
  3. the enantiomers have identical physical and chemical properties with achiral reagents, so ordinary tests do not separate them
  4. they rotate the plane of plane-polarised light by equal amounts in opposite directions, measured with a polarimeter
  5. the laboratory route adds cyanide to the planar carbonyl of ethanal from either face equally, giving a racemate whose rotations cancel
  6. the enzyme in muscle has a chiral active site that exposes only one face of the substrate, so a single enantiomer forms and rotates the light

  Levels

  • Level 3 (5–6 marks) the chiral centre is identified with its four groups, the enantiomers are described as non-superimposable mirror images, the polarimeter test is given, and both sources are explained: attack on either face of the planar carbonyl giving a racemate in the laboratory, and the chiral active site of the enzyme giving one enantiomer
  • Level 2 (3–4 marks) the chiral centre and the enantiomers are described and the polarimeter given; the racemate from ethanal or the single enantiomer from the enzyme is explained only in part
  • Level 1 (1–2 marks) a chiral carbon or mirror-image isomers are mentioned; laboratory lactic acid is commonly said to be inactive because it has no chiral centre

• 4 marks · Explain · OP-Q6

  Explain why the 2-hydroxypropanenitrile formed from ethanal with KCN followed by dilute acid is optically inactive, although its molecules are chiral.

  Mark scheme

  1. B1 the carbonyl group in ethanal is planar, with three groups around the carbon at 120°
  2. B1 the cyanide ion can attack the carbonyl carbon from above or below the plane with equal probability
  3. B1 attack from the two faces gives the two enantiomers, so they form in equal amounts as a racemic mixture
  4. B1 the equal and opposite rotations of the two enantiomers cancel, so the mixture is optically inactive

• 4 marks · Name · OP-Q10

  Name the organic product when butanone is reduced by NaBH₄, and when pentan-3-one reacts with KCN followed by dilute acid, and explain whether a sample of each product rotates plane-polarised light.

  Mark scheme

  1. B1 butanone gives butan-2-ol, CH₃CH(OH)CH₂CH₃
  2. B1 pentan-3-one gives 2-ethyl-2-hydroxybutanenitrile, CH₃CH₂C(OH)(CN)CH₂CH₃
  3. B1 butan-2-ol is chiral, but hydride attacks the planar C=O equally from either face, so the racemate does not rotate the light
  4. B1 the nitrile's central carbon carries two ethyl groups, so it is not chiral and cannot rotate the light

• 4 marks · Evaluate · OP-Q17

  Evaluate the claim that the harm caused by thalidomide would have been avoided if only the sedative enantiomer had been sold.

  Mark scheme

  1. B1 thalidomide was sold as a racemate, and one enantiomer is a sedative while the other causes fetal limb defects
  2. B1 selling one enantiomer would have avoided giving the harmful enantiomer directly
  3. B1 however, the two enantiomers interconvert in the body, so the racemate forms again from either one
  4. B1 so the claim is not supported: the harmful enantiomer would still have been present and the harm would not have been avoided

• 3 marks · State · OP-Q2

  State what is meant by the terms chiral centre and enantiomers, and state how a pair of enantiomers differ in their effect on plane-polarised light.

  Mark scheme

  1. B1 a chiral centre is a carbon atom bonded to four different atoms or groups of atoms
  2. B1 enantiomers are stereoisomers that are non-superimposable mirror images of each other; reject 'mirror images' alone
  3. B1 they rotate the plane of plane-polarised light by equal amounts in opposite directions; reject 'bend' or 'reflect'

• 3 marks · Identify · OP-Q4

  Identify the number of chiral centres in isoleucine, CH₃CH₂CH(CH₃)CH(NH₂)COOH, and the four groups on each.

  Mark scheme

  1. B1 there are two chiral centres
  2. B1 carbon 2 is bonded to H, NH₂, COOH and the CH(CH₃)CH₂CH₃ group
  3. B1 carbon 3 is bonded to H, CH₃, CH₂CH₃ and the CH(NH₂)COOH group

• 3 marks · Draw · OP-Q8

  Draw the two enantiomers of 2-bromobutane in a way that shows they are optical isomers, and mark the chiral centre.

  Mark scheme

  1. B1 each enantiomer drawn as a tetrahedral carbon with two bonds in the plane of the paper, one solid wedge towards the viewer and one dashed wedge away
  2. B1 the four groups on the chiral carbon are H, Br, CH₃ and C₂H₅, with the asterisk on that carbon
  3. B1 the second structure is the mirror image of the first; two flat structures at 90° score zero

• 3 marks · Describe · OP-Q9

  Describe how a polarimeter is used to show whether a sample of lactic acid is a single enantiomer or a racemic mixture.

  Mark scheme

  1. B1 plane-polarised light is passed through a solution of the sample in a tube
  2. B1 the analyser is turned until the light passing through is brightest again, and the angle of rotation is read
  3. B1 a single enantiomer rotates the plane, while a racemic mixture gives no rotation

• 3 marks · Compare · OP-Q11

  Compare the physical and chemical properties of the two enantiomers of 2-hydroxypropanoic acid.

  Mark scheme

  1. B1 they have identical melting points, boiling points and densities, because their bonds are identical
  2. B1 they react identically with achiral reagents such as sodium carbonate or ethanol
  3. B1 they differ in rotating plane-polarised light in opposite directions, and may differ in their reactions with chiral molecules such as enzymes

• 3 marks · Calculate · OP-Q12

  Calculate the angle of rotation given by a mixture containing 75% of the (+) enantiomer and 25% of the (−) enantiomer of a compound, if the pure (+) enantiomer rotates the plane by +12.0° under the same conditions.

  Mark scheme

  1. M1 the 25% of (−) cancels the rotation of 25% of (+), so the racemic part is 50% of the sample
  2. M1 the excess of (+) is 75 − 25 = 50%, so rotation = 0.50 × (+12.0°)
  3. A1 rotation = +6.0°; the sign is required

• 3 marks · Calculate · OP-Q13

  Calculate the percentage of each enantiomer in a sample of a compound that rotates the plane of polarised light by −6.0°, if the pure (−) enantiomer rotates it by −15.0° under the same conditions.

  Mark scheme

  1. M1 the excess of (−) over (+) is 6.0 ÷ 15.0 × 100 = 40%
  2. M1 the remaining 60% is racemic, so it contains 30% of each enantiomer
  3. A1 70% of the (−) enantiomer and 30% of the (+) enantiomer; error carried forward from M1

• 3 marks · Suggest · OP-Q14

  Suggest three reasons why a pharmaceutical company might choose to sell a drug as a single enantiomer rather than as a racemate.

  Mark scheme

  1. B1 only one enantiomer may fit the chiral receptor or enzyme, so the other enantiomer is inactive
  2. B1 the other enantiomer may have harmful side effects, as one enantiomer of thalidomide does
  3. B1 a smaller dose of the single active enantiomer gives the same effect, and it may act faster

• 3 marks · Explain · OP-Q18 Edexcel

  Explain why the hydrolysis of a single enantiomer of 2-bromopentane by an SN2 mechanism gives a product that rotates plane-polarised light.

  Mark scheme

  1. B1 in SN2 the hydroxide ion attacks the carbon from the side opposite the bromine, in a single step
  2. B1 so every molecule undergoes inversion of configuration in the same way and only one enantiomer of the alcohol forms
  3. B1 the product is a single enantiomer, not a racemate, so it is optically active

• 3 marks · Suggest · OP-Q19 Edexcel

  Suggest why warming a single enantiomer of 3-bromo-3-methylhexane with aqueous sodium hydroxide gives a product that does not rotate plane-polarised light.

  Mark scheme

  1. B1 it is a tertiary halogenoalkane, so it reacts by SN1, with the C–Br bond breaking first to give a carbocation
  2. B1 the carbocation is planar, so the hydroxide ion attacks either face with equal probability
  3. B1 3-methylhexan-3-ol is formed as a racemic mixture, whose equal and opposite rotations cancel

• 3 marks · Calculate · OP-Q20 Edexcel

  Calculate the percentage of the product formed by the SN1 route when a single enantiomer of 2-bromobutane is hydrolysed and the product rotates the plane by +4.0°, if pure (+)-butan-2-ol rotates it by +16.0° under the same conditions and the SN2 route gives only the (+) enantiomer.

  Mark scheme

  1. M1 the excess of (+) over (−) is 4.0 ÷ 16.0 × 100 = 25%, which is the part formed by SN2 with inversion
  2. M1 the remainder of the product is racemic, which is the part formed by SN1 through the planar carbocation
  3. A1 100 − 25 = 75% of the product formed by SN1, to 2 significant figures

• 2 marks · Identify · OP-Q3

  Identify the chiral centre in 2-methylbutan-1-ol, CH₃CH₂CH(CH₃)CH₂OH, and give the four groups attached to it.

  Mark scheme

  1. B1 the chiral centre is carbon 2, the CH carbon bonded to the methyl branch
  2. B1 its four groups are H, CH₃, CH₂CH₃ and CH₂OH, which are all different

• 2 marks · Explain · OP-Q7

  Explain why 3-methylpentane, CH₃CH₂CH(CH₃)CH₂CH₃, does not show optical isomerism.

  Mark scheme

  1. B1 carbon 3 is bonded to two ethyl groups, so it does not carry four different groups
  2. B1 every other carbon carries at least two hydrogen atoms, so the molecule has no chiral centre

Practise optical isomerism one question at a time