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A-level Chemistry Year 13 · Topic 3

Isomerism, carbonyls and acid derivatives

Isomerism, carbonyls and acid derivatives takes organic chemistry into three dimensions and introduces some very reactive families of compounds.

We will look at optical isomers and why they matter for medicines, then the reactions of aldehydes and ketones, including nucleophilic addition.

Finally we study carboxylic acids, esters, acyl chlorides and acid anhydrides, and how they are used to make useful products.

In this topic

  1. 3.1Optical isomerismFree below · 23 questions
  2. 3.2Aldehydes and ketonesOpen in Full Marks · 21 questions
  3. 3.3Carboxylic acids and derivativesOpen in Full Marks · 41 questions

3.1 · Free notes

Optical isomerism

Diagram: Optical isomerism

Key points

In more detail

Practice

Optical isomerism: 10 questions

Pick an answer to check it, or open the answer for the explanation.

1. What is a chiral centre?

  1. Any carbon atom
  2. A carbon atom bonded to four different groups
  3. A carbon with a double bond
  4. A nitrogen atom
Show answer

B: A carbon atom bonded to four different groups. This lack of symmetry creates two possible mirror-image arrangements.

2. What is true of enantiomers' physical properties, such as boiling point?

  1. They are completely different
  2. They are identical, except for the direction they rotate plane-polarised light
  3. One always has a higher boiling point
  4. They have different molecular formulae
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B: They are identical, except for the direction they rotate plane-polarised light. Only their interaction with polarised light and with other chiral molecules differs.

3. What is a racemic mixture?

  1. A single pure enantiomer
  2. An equal mixture of both enantiomers, with no net effect on polarised light
  3. A mixture of different compounds
  4. A solid mixture only
Show answer

B: An equal mixture of both enantiomers, with no net effect on polarised light. The rotational effects of the two enantiomers cancel out.

4. Why is optical isomerism significant in drug design?

  1. It has no significance
  2. Enzymes and receptors are chiral, so only one enantiomer may be biologically active or safe
  3. Both enantiomers always work identically
  4. Only achiral drugs work
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B: Enzymes and receptors are chiral, so only one enantiomer may be biologically active or safe. One enantiomer of a drug can be effective while the other is inactive or even harmful.

5. What happens to plane-polarised light passed through a racemic mixture?

  1. It rotates strongly clockwise
  2. It rotates strongly anticlockwise
  3. There is no net rotation, as the effects cancel
  4. It is absorbed completely
Show answer

C: There is no net rotation, as the effects cancel. Equal amounts of each enantiomer produce equal and opposite rotations.

6. Which technique can distinguish between two enantiomers?

  1. Infrared spectroscopy
  2. Measuring the rotation of plane-polarised light
  3. Mass spectrometry
  4. Melting point
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B: Measuring the rotation of plane-polarised light. Their other physical properties are identical.

7. Which compound contains a chiral centre?

  1. Butan-2-ol
  2. Ethanol
  3. 2-methylpropan-2-ol
  4. Propan-2-ol
Show answer

A: Butan-2-ol. Butan-2-ol's C2 carries H, OH, CH₃ and C₂H₅, all different. Propan-2-ol and 2-methylpropan-2-ol have two identical methyl groups on the relevant carbon.

8. How many chiral centres are in 2,3-dibromopentane, CH₃CHBrCHBrCH₂CH₃?

  1. 3
  2. 2
  3. 1
  4. 0
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B: 2. C2 bonds to H, Br, CH₃ and CHBrC₂H₅; C3 bonds to H, Br, C₂H₅ and CHBrCH₃. Each carbon has four different groups, so there are two chiral centres.

9. Why does adding HCN to propanal give a racemic mixture of the hydroxynitrile?

  1. H⁺ attacks the carbon before CN⁻
  2. The planar C=O is attacked equally from either face
  3. The product has no chiral centre
  4. The reaction is reversible
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B: The planar C=O is attacked equally from either face. The carbonyl group is planar, so CN⁻ attacks from above or below with equal probability. This gives equal amounts of both enantiomers.

10. Pure enantiomer A rotates light by +8.0°. A mixture is 75% A and 25% of its mirror image B. What is the rotation?

  1. +6.0°
  2. +8.0°
  3. +4.0°
  4. 0°
Show answer

C: +4.0°. The 25% of B cancels 25% of A, leaving a 50% excess of A: 0.50 × 8.0° = +4.0°. The mixture is not racemic, so it still rotates the light.

Keep going in Full Marks

Full Marks has 23 questions on Optical isomerism (13 more than here), set out in practice levels. You also get notes and 3D models for all 3 subtopics of Isomerism, carbonyls and acid derivatives, plus games, timed exams and past papers.

Practise 3.1 in Full MarksOpen Isomerism, carbonyls and acid derivatives

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