Analyse reaction steps and by-products
Classify each step, justify reagents and conditions, and identify plausible by-products in an organic route.
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 reaction type describes halogenoalkane + KCN → nitrile?
What you will learn
- Name the reaction type at each step
- Match reagents and conditions to the transformation
- Identify by-products and explain how work-up changes the product
Annotate each arrow separately
For every step, name the reaction type and link it to the bond or group that changes: substitution, addition, elimination, oxidation, reduction, hydrolysis or polymerisation. Then state the reagent and the condition that directs the reaction.
Do not let the overall route hide an incorrect step. A nitrile-to-acid conversion is hydrolysis, not oxidation; halogenoalkane plus KCN is substitution; alkene hydrogenation is addition.
Include salts and work-up products when relevant
By-products depend on the reagents and on how the equation is written. For example, bromoethane plus KCN forms a nitrile and KBr. Alkaline nitrile hydrolysis gives a carboxylate in the reaction mixture; acidification then produces the free carboxylic acid.
A complete route analysis distinguishes the desired organic product, inorganic by-products and any later work-up. Do not claim that a by-product is a waste product if it is simply an intermediate or spectator ion in the stated step.
Pause and explain
What inorganic by-product can form when bromoethane reacts with KCN?
Worked example
Analyse the route CH₃CH₂Br → CH₃CH₂CN → CH₃CH₂COOH.
Show the worked solution
- The first step replaces Br by CN.
- KCN in ethanol and heat supplies the nitrile carbon.
- The second step hydrolyses –CN; state acid work-up if alkali is used.
Answer Step 1 is nucleophilic substitution with KCN in ethanol and heat; KBr is a by-product. Step 2 is nitrile hydrolysis with dilute acid, or dilute alkali followed by acidification. The nitrile carbon becomes the carboxyl carbon.
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.
Analyse the step CH₃CH₂Br + KCN → CH₃CH₂CN + KBr.
Give me a hint
Name the bond replacement, the conditions and both product roles.
Compare my reasoning
- CN⁻ replaces Br on the ethyl group.
- Use ethanolic KCN and heat.
- The organic product is propanenitrile; KBr is the salt by-product.
This is nucleophilic substitution. Ethanolic KCN and heat form propanenitrile by adding the cyanide carbon; KBr is the inorganic by-product.
Look for these in your work
- Reaction type correct
- Conditions stated
- Organic product and by-product distinguished
A route uses dilute NaOH to hydrolyse propanenitrile, then reports propanoic acid. What should the analysis add?
Give me a hint
State the form present in alkaline solution and the follow-up work-up.
Compare my reasoning
- Alkaline hydrolysis gives propanoate.
- Propanoate is not yet the free acid.
- Acidify after hydrolysis.
Add an acidification step: dilute NaOH hydrolysis forms propanoate; acidification then gives propanoic acid.
Look for these in your work
- Intermediate identified
- Work-up stated
- Final product justified
A student labels both steps of bromoethane → propanenitrile → propanoic acid as substitution. Correct the route analysis.
Give me a hint
Check which functional group changes in the second step.
Compare my reasoning
- Bromoethane → nitrile replaces Br with CN: substitution.
- Nitrile → acid changes –CN by reaction with water: hydrolysis.
- Include acidification if alkaline hydrolysis is used.
The first step is nucleophilic substitution using KCN in ethanol and heat. The second is nitrile hydrolysis with dilute acid or dilute alkali followed by acidification; it is not substitution.
Look for these in your work
- First step classified
- Second step classified
- Conditions/work-up included
Common mistakes
- Calling all steps simply ‘organic reaction’
- Omitting the work-up after alkaline hydrolysis
- Failing to identify the salt by-product
- Giving a product without connecting it to the reagent
What can you explain now?
What reaction type describes halogenoalkane + KCN → nitrile?
Compare with the explanation
Nucleophilic substitution
CN⁻ replaces the halide on the carbon chain.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
For each reaction arrow, how do you connect reaction type, conditions, product and by-product?
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.