Organic synthesis

AS Level Chemistry · Organic synthesis. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Organic synthesis means making a target molecule from a starting molecule in one or more steps. Each step changes one functional group into another, using a reaction you already know. To plan a route, compare the two molecules: which group has changed, and has the carbon chain become longer?

For a molecule with several functional groups, treat each group separately. Each one reacts as it would in a simple molecule, so a reagent may react with more than one group.

For every step you should be able to give the reagent, the conditions and the type of reaction, such as electrophilic addition, nucleophilic substitution, elimination, oxidation, reduction or hydrolysis. Also think about any by-products.

Rules to remember

Common mistake

Students think only about the group they want to change. Check every group in the molecule: for example, acidified K2Cr2O7 under reflux oxidises an –OH group and a –CHO group in the same molecule.

Worked example

Suggest a three-step route from propan-1-ol to butanoic acid.

  1. The chain grows from three carbons to four, so a nitrile step is needed, and that needs a halogenoalkane first.
  2. Step 1: HBr(g) → 1-bromopropane (nucleophilic substitution).
  3. Step 2: KCN in ethanol, heat → butanenitrile, CH3CH2CH2CN (nucleophilic substitution).
  4. Step 3: dilute H2SO4, heat under reflux → butanoic acid (hydrolysis).

Answer: Propan-1-ol → 1-bromopropane → butanenitrile → butanoic acid.

Practice questions

Try each one, then open the answer.

1. In the route propene → 2-bromopropane → propan-2-ol → propanone, what types of reaction occur, in order?

  1. A
    electrophilic addition; nucleophilic substitution; oxidation
  2. B
    free-radical substitution; nucleophilic substitution; reduction
  3. C
    electrophilic addition; elimination; oxidation
  4. D
    nucleophilic addition; nucleophilic substitution; oxidation
Show answer

Answer: A. HBr adds across the C=C (electrophilic addition), OH− replaces Br (nucleophilic substitution), and the secondary alcohol is oxidised to a ketone.

2. Which two-step route converts ethene into ethanoic acid?

  1. A
    HBr(g); then KCN in ethanol, heat
  2. B
    steam with H3PO4 catalyst; then excess acidified K2Cr2O7, heat under reflux
  3. C
    H2 with Ni catalyst; then acidified KMnO4, heat
  4. D
    cold dilute acidified KMnO4; then NaBH4
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Answer: B. Steam adds to ethene to give ethanol, and refluxing with excess oxidising agent oxidises this primary alcohol fully to ethanoic acid. HBr then KCN gives propanenitrile, and ethane is not oxidised by KMnO4.

3. Compound Z decolourises bromine water, gives a red precipitate with Fehling's solution and does not react with sodium carbonate. Which functional groups does Z contain?

  1. A
    C=C and carboxylic acid
  2. B
    C=C and aldehyde
  3. C
    C=C and ketone
  4. D
    aldehyde and carboxylic acid
Show answer

Answer: B. Decolourising bromine shows a C=C bond; the red Cu2O precipitate with Fehling's shows an aldehyde (a ketone would not react); no fizzing with carbonate rules out a carboxylic acid.

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