A reaction pathway is a chain of single-step reactions. Each arrow changes one functional group.
This lesson is part of SPM Chemistry carbon compounds. It uses the reactions in writing characteristic organic reactions.
How do I plan a route?
Work backwards from the target, one family at a time. Then write the route forwards and add the reagent on every arrow.
- Identify the family of the target.
- Name the reaction that makes that family, and what it starts from.
- Repeat until you reach the given compound.
- Rewrite forwards with reagents and conditions.
Worked example: ethene to ethyl ethanoate
The target is an ester. Backwards: an ester comes from an acid and an alcohol. The acid comes from oxidising an alcohol, and the alcohol comes from ethene by hydration.
Forwards:
- Ethene to ethanol: steam, phosphoric acid catalyst, high temperature and pressure.
- Ethanol to ethanoic acid: acidified potassium manganate(VII), heat.
- Ethanoic acid plus more ethanol to ethyl ethanoate: concentrated sulfuric acid, heat.
Only some of the ethanol is oxidised. The rest stays as ethanol for step 3.
Worked example: a route in reverse
Propan-1-ol to propene. Backwards: an alkene can come from dehydrating an alcohol. So one step: pass the alcohol vapour over heated aluminium oxide, or heated porcelain chips, to give propene and water.
Write the equation as C₃H₇OH → C₃H₆ + H₂O.
The mistake that loses marks
A student jumps from ethene straight to ethanoic acid with “oxidation”. There is no single step on the syllabus that does this. The route must pass through ethanol.
Count arrows against families: alkene, alcohol, acid each need their own step. If an arrow skips a family, add the missing step.
Check yourself
Plan a two-step route from ethanol to poly(ethene). Name the reagents and the intermediate.
Answer
Step 1: dehydrate ethanol by passing the vapour over heated aluminium oxide to form ethene, C₂H₅OH → C₂H₄ + H₂O.
Step 2: polymerise ethene under high pressure with a catalyst. The monomers join by addition to form poly(ethene), a long chain with no double bond. The intermediate is ethene.
What to study next
Polymer steps are explained in comparing polymers and monomers. Then test routes in the practice set.
If wrong arrows keep appearing, log them in the mistake log and paper-error review. For a teacher to plan fresh routes with you, see online one-to-one Chemistry tuition.