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Learn7405Paper 2 · Paper 2: Organic chemistryOrganic synthesisFunctional group routes
Organic synthesis · Subtopic 1 of 2

Functional Group Routes

Organic synthesis links functional groups through known reactions. Alkene → (HBr) halogenoalkane → (NaOH aq) alcohol → (Cr₂O₇²⁻/H⁺, distil) aldehyde → (reflux) carboxylic acid → (alcohol, H₂SO₄) ester. Halogenoalkane → (KCN, ethanol) nitrile → (H⁺/H₂O) acid or (LiAlH₄) amine. Carbonyl → (HCN) hydroxynitrile; (NaBH₄) alcohol. Alcohol → (conc. H₂SO₄, heat) alkene. To lengthen a chain by one carbon, use CN⁻. Plan backwards from the target, and give reagents and conditions for each step.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Recall the key aliphatic functional group conversions.
  • Plan multi-step syntheses with reagents and conditions.
  • Recognise when the carbon chain must be lengthened.
Pause and think

How could you make propanoic acid starting from ethene, which only has two carbons?

Section 2 of 14

Core idea

Start hereChemistry as route planning

Organic synthesis is like planning a journey on a metro map: each reaction is a line between two functional groups.

The big idea

Learn the map of reactions, count carbons, and work backwards from the target one step at a time.

1. The reaction map

Aliphatic functional group map

Each arrow is a reaction with fixed reagents and conditions.

Worked exampleOverall yield

Three steps have yields 80%, 70% and 90%. Find the overall yield.

Goal
Overall %.
Strategy
Multiply the fractions.
  1. 0.80×0.70=0.560.80\times0.70=0.56
    After two steps.
  2. 0.56×0.90=0.5040.56\times0.90=0.504
    Each step works on what the last one produced.
Answer50.4%.
✓ CheckLower than the worst single step ✓.
TrickCount carbons One more carbon in the target means CN⁻ or HCN is needed.

Summary

  • Learn reagents and conditions.
  • CN⁻ lengthens chains.
  • Short routes give better yields.
Key words
Retrosynthesis
Planning a synthesis backwards from the target.
Intermediate
Compound made in one step and used in the next.
HistoryCorey's retrosynthesis. E. J. Corey formalised "retrosynthetic analysis", working backwards from the target, and won the 1990 Nobel Prize in Chemistry.
Fun factParacetamol in three steps. Paracetamol is made industrially in a few steps from phenol, a classic multi-step synthesis of the kind you'll plan here.
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