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6.8.2Organic Chemistry and Analysis

Synthetic Routes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

5 min read 5 parts Lesson 26 of 33

📚 OCR A Level Chemistry A H432 Hub / Topical Revision
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Part 1 of 5Functional Group Conversions

6.8.2 Synthetic Routes

A synthetic route is a chain of reactions, so you need a toolkit of single-step conversions.

Alkene to alcohol
  • Steam with an acid catalyst adds water across C=C; HBr gives a bromoalkane instead.
Haloalkane to alcohol
  • Heating under reflux with NaOH(aq) replaces the halogen by –OH through nucleophilic substitution.
Alcohol oxidation
  • Acidified K₂Cr₂O₇ with distillation turns a primary alcohol into an aldehyde,
    • and reflux gives the carboxylic acid; secondary alcohols give ketones.
Carbonyl reduction
  • NaBH₄ converts aldehydes to primary alcohols and ketones to secondary alcohols.
Chain extension
  • KCN followed by hydrolysis or reduction adds one carbon, and Friedel–Crafts reactions add carbon to a benzene ring.
ConversionReagents and conditions
Haloalkane to nitrileKCN in ethanol, reflux
Nitrile to amineLiAlH₄ in dry ether
Nitrobenzene to phenylamineSn / conc. HCl, reflux, then NaOH
Alcohol and acid to esterConcentrated H₂SO₄ catalyst, heat
Acyl chloride to esterAlcohol, room temperature

Part 2 of 5Planning a Route

1
Identify the functional groups in the starting material and the target, and note any change in carbon number.
2
Work backwards from the target, asking which reaction could form that functional group in one step.
3
Match each step to a known reaction and write down its reagents, conditions and organic product.
4
Check that each reagent will not react with other groups, and put steps in the right order where directing effects matter.
5
Write the complete sequence with reagents above each arrow, and estimate the overall yield.

Part 3 of 5Judging and Carrying Out a Route

Judging and Carrying Out a Route
Overall yield
  • Multiply the fractional yields of every step; many steps with modest yields lead to a low overall yield.
Practical techniques
  • Reflux keeps volatile reactants in the flask, distillation separates products, and recrystallisation purifies solids.
Purity check
  • A sharp melting point or a boiling point matching the data book value indicates a pure product.
Fewer steps
  • Shorter routes usually give better yields, produce less waste and cost less, so the shortest sensible route is preferred.

Part 4 of 5Worked Example

Question

  • Propylamine is made from bromoethane in two steps: KCN gives propanenitrile with 80% yield, and reduction gives propylamine with 75% yield.
  • Calculate the overall percentage yield, and state the reagents.

  1. Step 1 uses KCN in ethanol under reflux and Step 2 uses LiAlH₄ in dry ether.
  2. Convert yields to fractions0.80 and 0.75.
  3. Overall yield = 0.80 × 0.75 = 0.60.
  4. Express as a percentage60%.

Answer Overall yield = 60%

Part 5 of 5Exam Checklist & Tips

6.8.2 – Key Points for the Exam

  • Synthetic route A sequence of reactions that converts a starting material into a target compound.
  • Overall yield The product of the fractional yields of all steps.
  • Functional group interconversion Changing one functional group into another.
  • Reflux Heating with a condenser to prevent loss of volatile substances.

Common Mistakes

  • Adding the yields of each step instead of multiplying them.
  • Omitting the conditions, such as 'in ethanol' or 'under reflux', when stating reagents.
  • Choosing a route that uses a reagent which would also change another group in the molecule.
  • Forgetting that cyanide steps add exactly one carbon to the chain.

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