Label each step in a synthetic route
For every arrow, name the reaction type, reagent, condition, product change and any likely by-product.
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 should be recorded for each arrow in a route analysis?
What you will learn
- Analyse a route one step at a time
- Classify reaction types using structural changes
- Match reagents and conditions to each arrow
Classify the structural change
Compare the structures before and after each arrow. Replacing a halogen with CN is substitution; converting –CN to –COOH is hydrolysis; forming –COO– from acid and alcohol is esterification; changing a primary alcohol to an aldehyde is oxidation.
Name the smallest accurate reaction type for each step. A multi-step route may contain different reaction types, so do not label the whole route with just one word.
Read the arrow label completely
For each step, identify the reagent and conditions from the syllabus, then check that they produce the displayed intermediate. Include solvent, catalyst, temperature or distillation/reflux when they distinguish the route.
If a proposed reagent does not match the functional-group change, flag the inconsistency instead of silently accepting the product.
Pause and explain
Which reaction type converts R–Br to R–CN using KCN?
Worked example
Analyse CH₃CH₂Br → CH₃CH₂CN → CH₃CH₂COOH.
Show the worked solution
- The first step replaces Br with CN using KCN in ethanol and heat.
- The second changes the nitrile to a carboxylic acid by hydrolysis.
- Track the added carbon through both steps.
Answer Step 1: nucleophilic substitution, KCN in ethanol, heat; nitrile forms and the chain gains one carbon. Step 2: nitrile hydrolysis with dilute acid (or dilute alkali then acidification); propanoic acid forms.
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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.
Label the steps in propan-1-ol → propanal → propanoic acid, including the condition difference.
Give me a hint
The first product can be oxidised further.
Compare my reasoning
- Both steps use acidified oxidising agent.
- Distillation isolates the aldehyde.
- Reflux continues oxidation to the acid.
Step 1: oxidation with acidified K₂Cr₂O₇ or KMnO₄ and distillation gives propanal. Step 2: further oxidation under reflux gives propanoic acid.
Look for these in your work
- Reaction type named
- Distillation matched
- Reflux matched
Analyse a route step: propanone + HCN, KCN catalyst, heat → hydroxynitrile.
Give me a hint
Identify the carbonyl bond change and product class.
Compare my reasoning
- HCN adds across the C=O.
- KCN is the catalyst and heat is stated.
- The product contains OH and CN on the former carbonyl carbon.
Nucleophilic addition forms a hydroxynitrile using HCN, KCN catalyst and heat.
Look for these in your work
- Addition identified
- Reagents/conditions included
- Product group change stated
A route labels nitrile → carboxylic acid as ‘reduction with dilute HCl’. Correct the reaction type and reagent logic.
Give me a hint
Hydrolysis, not reduction, makes the carboxylic acid.
Compare my reasoning
- Use dilute acid for nitrile hydrolysis or dilute alkali followed by acidification.
- Reduction instead gives a primary amine.
- Name the target route accurately.
Nitrile-to-carboxylic-acid is hydrolysis with dilute acid, or dilute alkali then acidification; dilute HCl is not a reducing agent.
Look for these in your work
- Reaction corrected
- Reagent route valid
- Reduction contrasted
Common mistakes
- Calling every arrow ‘conversion’ without naming the reaction
- Omitting a condition required by the syllabus
- Matching reagents to the wrong arrow
- Ignoring carbon-count changes
What can you explain now?
What should be recorded for each arrow in a route analysis?
Compare with the explanation
Reaction type, reagents, conditions and product change
These show whether the step is chemically consistent.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do structural changes, reagents and conditions let you classify each arrow in a multi-step route?
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.