Choose the order of reactions in a synthesis route
Work backwards from the target, then check forwards that every intermediate and condition is valid.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 36. Routes use reactions listed in the syllabus and state the relevant reagents and conditions. Check every intermediate and possible by-product; no route implies a practical procedure. Written practice is self-checked
Before you begin
- Use the functional-group reactions from Topics 13–19 and 29–35, including their reagents, conditions, products and limitations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the best first question when planning a synthesis backwards?
What you will learn
- Plan a route from the target functional group
- Select a workable order of syllabus reactions
- Verify intermediates and carbon counts
Work backwards from the target
Ask which syllabus reaction forms the target group. A carboxylic acid can be made by oxidation of a primary alcohol or aldehyde, or by hydrolysis of a nitrile. A nitrile can be made by substitution of a halogenoalkane with KCN in ethanol and heat.
Once a likely last step is identified, work backwards to a starting material and write the route forwards. Each arrow must begin with a structure that the previous step actually produces.
Audit route order and atom inventory
Check that reagents do not assume a functional group before it has been formed. Confirm the carbon count after each step: cyanide substitution adds one carbon, and nitrile hydrolysis retains that carbon as the carboxyl carbon.
Use only syllabus transformations and the stated conditions. If a molecule has several groups, check whether the route order targets the intended group and whether the syllabus supports the required selectivity.
Pause and explain
What happens to the carbon count from bromoethane to propanoic acid through a nitrile?
Worked example
Plan a route from bromoethane to propanoic acid.
Show the worked solution
- Substitute Br with CN using KCN in ethanol and heat.
- This forms propanenitrile and adds one carbon.
- Hydrolyse the nitrile with dilute acid to the acid.
Answer CH₃CH₂Br → CH₃CH₂CN → CH₃CH₂COOH. Use KCN in ethanol and heat, then dilute-acid hydrolysis (or alkaline hydrolysis 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.
Give me a hint
Use cyanide substitution, then hydrolyse the nitrile.
Compare my reasoning
- KCN in ethanol and heat converts the halogenoalkane to a nitrile.
- The CN carbon extends the chain to four carbons.
- Hydrolyse with dilute acid to form the acid.
CH₃CH₂CH₂Br → CH₃CH₂CH₂CN → CH₃CH₂CH₂COOH (butanoic acid).
Look for these in your work
- Correct reagents
- One-carbon extension
- Correct acid
A proposed route hydrolyses bromoethane directly to propanoic acid in one step. Identify the missing chemistry.
Give me a hint
Find the step that adds the acid's third carbon.
Compare my reasoning
- Bromoethane has two carbons.
- Propanoic acid has three.
- Cyanide substitution must first add a carbon to form a nitrile.
The route needs KCN in ethanol and heat to make propanenitrile, followed by nitrile hydrolysis to propanoic acid.
Look for these in your work
- Carbon mismatch detected
- Intermediate supplied
- Reagents and order stated
Plan a route from ethanal to 2-hydroxypropanoic acid using a hydroxynitrile intermediate.
Give me a hint
Add HCN to the carbonyl, then hydrolyse the nitrile.
Compare my reasoning
- Ethanal reacts with HCN using KCN as catalyst and heat.
- The product is 2-hydroxypropanenitrile.
- Hydrolysis converts –CN to –COOH.
CH₃CHO → CH₃CH(OH)CN → CH₃CH(OH)COOH, using HCN/KCN/heat followed by dilute-acid hydrolysis.
Look for these in your work
- Hydroxynitrile intermediate
- Added carbon tracked
- Acid product correct
Common mistakes
- Leaving a missing intermediate
- Using a reagent before the necessary group exists
- Forgetting the chain extension in cyanide substitution
- Writing the steps backwards without checking them forwards
What can you explain now?
What is the best first question when planning a synthesis backwards?
Compare with the explanation
Which syllabus reaction forms the target group?
The target suggests a possible final transformation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do you check that every intermediate in a backwards-designed route can be made in the forward direction?
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.