Design a multi-step route to a target molecule
Work backwards from the target group, choose syllabus reactions, then verify each intermediate and condition.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Synthetic routes use reactions in this syllabus; reagent hazards are not practical instructions. Written practice is self-checked
Before you begin
- Use the functional-group reactions, reagents and conditions from Topics 13–20. Check connectivity and carbon count at each step.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a useful first question when planning a route backwards?
What you will learn
- Plan a multi-step route using syllabus reactions
- Choose a useful order for functional-group changes
- Check intermediates and carbon-skeleton changes
Plan backwards, then verify forwards
Start with the target functional group and ask which syllabus reaction forms it. If the target is a carboxylic acid, a nitrile can be hydrolysed; if the nitrile is not present, a halogenoalkane can first undergo cyanide substitution.
Work backwards to the starting material, then check the route forwards. At each arrow write the reagent, solvent or condition, the product group and any carbon-count change. A route is only complete when every intermediate can be made from the preceding one by a reaction in the syllabus.
Track carbon atoms and competing groups
Cyanide substitution adds one carbon because CN supplies the nitrile carbon. Nitrile hydrolysis retains that carbon when it becomes the carboxyl carbon. This makes a haloalkane-to-acid route a useful chain-extension sequence.
When a starting material contains several groups, choose conditions that target the intended group and check whether another group would react too. Do not add unlisted transformations simply to make a route shorter.
Pause and explain
What happens to the carbon count in haloalkane → nitrile → carboxylic acid?
Worked example
Design a route from bromoethane to propanoic acid.
Show the worked solution
- Bromoethane can form a nitrile with KCN in ethanol and heat.
- Cyanide substitution adds one carbon to give propanenitrile.
- Hydrolyse the nitrile and acidify to form propanoic acid.
Answer CH₃CH₂Br →(KCN, ethanol, heat) CH₃CH₂CN →(dilute acid hydrolysis) CH₃CH₂COOH. An alkaline hydrolysis route must be followed by acidification.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Plan a route from 1-bromopropane to butanoic acid, naming reagents and conditions.
Give me a hint
Use cyanide substitution for one-carbon chain extension, then hydrolyse the nitrile.
Compare my reasoning
- React with KCN in ethanol and heat.
- The product is butanenitrile.
- Hydrolyse with dilute acid, or use dilute alkali followed by acidification.
1-bromopropane → butanenitrile using KCN in ethanol and heat → butanoic acid by dilute-acid hydrolysis. If dilute alkali is used, acidify after hydrolysis.
Look for these in your work
- First reagent/conditions stated
- Correct four-carbon nitrile
- Hydrolysis and work-up stated
Design a route from ethanal to 2-hydroxypropanoic acid using syllabus reactions.
Give me a hint
First form a hydroxynitrile, then hydrolyse its nitrile group.
Compare my reasoning
- Add HCN using KCN catalyst and heat to form 2-hydroxypropanenitrile.
- Hydrolyse –CN with dilute acid or dilute alkali.
- Acidify after alkaline hydrolysis; retain –OH.
Ethanal → 2-hydroxypropanenitrile using HCN, KCN catalyst and heat → 2-hydroxypropanoic acid by nitrile hydrolysis (followed by acidification if alkali is used).
Look for these in your work
- Hydroxynitrile intermediate correct
- Reagents/conditions included
- Hydrolysis retains OH and yields acid
A proposed route from bromoethane to propanoic acid is bromoethane → ethene → ethanal → ethanoic acid. Identify why this route misses the target.
Give me a hint
Count carbons at each step and compare with the three-carbon acid required.
Compare my reasoning
- Bromoethane has two carbons.
- The listed transformations do not add a carbon.
- Propanoic acid needs a three-carbon chain.
The route retains a two-carbon skeleton and ends at ethanoic acid, not propanoic acid. Use KCN to add the nitrile carbon, then hydrolyse to propanoic acid.
Look for these in your work
- Carbon count checked
- Wrong product identified
- Correct chain-extension strategy proposed
Common mistakes
- Leaving out a reagent or condition on an arrow
- Using hydrolysis before the nitrile exists
- Forgetting cyanide substitution adds a carbon
- Claiming a route is complete when an intermediate is not formed
What can you explain now?
What is a useful first question when planning a route backwards?
Compare with the explanation
Which syllabus reaction forms the target functional group?
The target group points to a reaction that can create it.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do you plan a route backwards and verify it forwards without losing carbon atoms or skipping an intermediate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.