Check whether a proposed synthesis route is consistent
Audit each route step for a valid starting group, reagent, condition, product structure and carbon count.
Section 36 · Organic synthesis
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 a strong first check for a proposed multi-step route?
What you will learn
- Find missing or mismatched route steps
- Check atom and functional-group continuity
- Correct reagents, conditions and by-products
Check the start and end of every arrow
For each step, ask whether the starting structure contains the group required by the reagent and whether the product shows the expected bond change. A reducing agent cannot hydrolyse a nitrile to an acid; acidified oxidant under reflux does not form an aldehyde as the final product from a primary alcohol.
Follow the atoms across the arrow. Confirm carbon counts, location of substituents and whether a group such as –OH or –COOH should remain unchanged.
Check what the route leaves out
A complete route includes every intermediate, key reagent and condition, and the relevant by-product when the question requests it. For a multi-step sequence, verify that the intermediate from one step is exactly the starting material for the next.
If the syllabus does not support a claimed selectivity or condition, mark that limitation rather than assuming the proposed route works. This is a route audit, not a guarantee of practical yield or laboratory safety.
Pause and explain
A route changes a two-carbon haloalkane to a three-carbon acid with no nitrile or other carbon source shown. What is the issue?
Worked example
Audit the proposed route: bromoethane → propanoic acid using dilute acid in one step.
Show the worked solution
- Compare the carbon counts: two in bromoethane, three in propanoic acid.
- No carbon-adding step is shown.
- Insert nitrile formation and hydrolysis.
Answer The route is incomplete. First use KCN in ethanol and heat to form propanenitrile, then hydrolyse the nitrile to propanoic acid.
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.
Correct the route: propan-1-ol → propanoic acid labelled ‘distil aldehyde from the mixture’.
Give me a hint
The target is the acid, so oxidation must continue beyond the aldehyde.
Compare my reasoning
- Distillation isolates an aldehyde intermediate.
- Use reflux for the acid target.
- Keep the acidified oxidising reagent.
Use acidified K₂Cr₂O₇ or KMnO₄ under reflux to form propanoic acid; distillation would favour isolating propanal.
Look for these in your work
- Condition corrected
- Target matched
- Reagent retained
A synthesis diagram shows nitrile → primary amine using dilute HCl. What should be corrected?
Give me a hint
Separate hydrolysis from reduction.
Compare my reasoning
- Nitrile hydrolysis with dilute acid gives a carboxylic acid.
- A nitrile-to-amine route needs LiAlH₄ or H₂/Ni.
- Match the reagent to the target group.
Use LiAlH₄ or H₂/Ni to reduce the nitrile to a primary amine. Dilute acid hydrolyses the nitrile toward a carboxylic acid instead.
Look for these in your work
- Target read correctly
- Reducing reagent supplied
- Hydrolysis distinguished
A proposed aromatic route warms phenylamine with water first, then adds NaNO₂/dilute acid below 10°C to make phenol. Reorder and explain the steps.
Give me a hint
Form the diazonium salt cold before warming with water.
Compare my reasoning
- Diazotise phenylamine below 10°C using NaNO₂/dilute acid.
- Name the diazonium salt intermediate.
- Warm the salt with water to form phenol, releasing N₂.
First form the diazonium salt from phenylamine below 10°C; then warm it with water to give phenol and nitrogen. The original order reverses the required intermediate sequence.
Look for these in your work
- Cold stage first
- Intermediate included
- Warm hydrolysis and by-product correct
Common mistakes
- Accepting a route because the final product is plausible
- Ignoring a carbon-count mismatch
- Treating a reagent as if it supports several unrelated changes
- Assuming an unlisted step or selectivity
What can you explain now?
What is a strong first check for a proposed multi-step route?
Compare with the explanation
Each intermediate must match the next step's required starting structure
A route must be continuous from one valid product to the next.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do structural continuity, reagents and conditions reveal an invalid route step?
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.