Branch a halogenoalkane route through amines or nitriles
Choose ammonia or cyanide substitution according to the target and keep the conditions and carbon-count effect explicit.
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
Which reagent introduces a nitrile carbon when reacting with a halogenoalkane?
What you will learn
- Predict products from alternative nucleophiles
- Use specified solvent and conditions
- Compare whether the carbon chain changes
Ammonia forms an amine
A halogenoalkane reacts with ammonia in ethanol when heated under pressure to form an amine. The carbon skeleton of the halogenoalkane is retained as the halogen is replaced by nitrogen.
Use the syllabus description for the intended amine product and state the pressure condition. Further alkylation can occur, so do not claim the mixture must contain only a single amine product under every reagent ratio.
Cyanide forms a nitrile and extends the chain
KCN in ethanol and heat substitutes CN for the halogen to form a nitrile. The cyanide carbon becomes part of the nitrile group, so the product has one more carbon than the starting haloalkane.
These routes branch from the same starting material but serve different targets: amine formation introduces nitrogen as –NH₂, while cyanide substitution introduces a nitrile that can later be hydrolysed or reduced.
Pause and explain
What is the carbon-count change when a haloalkane becomes a nitrile using KCN?
Worked example
Compare products from 1-bromopropane using ethanolic NH₃ or ethanolic KCN.
Show the worked solution
- Ammonia substitution replaces Br with an amino group.
- Cyanide substitution replaces Br with CN.
- Count the extra carbon in the nitrile route.
Answer Ethanolic NH₃ under pressure forms a propylamine product with three carbons. Ethanolic KCN and heat form butanenitrile with four carbons.
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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.
Give the conditions to prepare a nitrile from 1-bromobutane and predict the product carbon count.
Give me a hint
Use cyanide substitution and count the CN carbon.
Compare my reasoning
- Use KCN in ethanol and heat.
- Replace Br by CN.
- Add the cyanide carbon to the four-carbon chain.
Butanenitrile with five carbon atoms overall forms using KCN in ethanol and heat.
Look for these in your work
- Reagents/conditions correct
- Substitution shown
- Carbon count correct
Write a route from bromoethane to an amine and state which atom from the reagent enters the product.
Give me a hint
Use the ammonia route, not cyanide substitution.
Compare my reasoning
- React with NH₃ in ethanol.
- Heat under pressure.
- Nitrogen forms the C–N bond while the two-carbon skeleton remains.
Bromoethane + NH₃ in ethanol under pressure gives an ethylamine product; the carbon count remains two and nitrogen is introduced from ammonia.
Look for these in your work
- Ammonia route
- Pressure condition
- Atom and chain tracked
A proposed route uses NH₃ to turn 1-bromopropane into butanenitrile. Diagnose both the wrong reagent and the carbon-count issue.
Give me a hint
Separate the amine and nitrile branches.
Compare my reasoning
- NH₃ is the amine-forming nucleophile.
- KCN is needed to introduce the nitrile carbon.
- Butanenitrile requires the cyanide carbon to extend the three-carbon chain.
Use KCN in ethanol and heat, not NH₃, to form butanenitrile; the CN carbon adds one to the propyl chain. The ammonia route forms an amine and retains three carbons.
Look for these in your work
- Reagent corrected
- Product group corrected
- Carbon extension explained
Common mistakes
- Using KCN conditions for an amine route
- Using ammonia conditions for nitrile formation
- Forgetting CN adds one carbon
- Omitting pressure for the ammonia route
What can you explain now?
Which reagent introduces a nitrile carbon when reacting with a halogenoalkane?
Compare with the explanation
KCN in ethanol
Cyanide substitution forms –CN and extends the carbon chain by one.
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Make it stick
How do ammonia and cyanide substitution differ in products, conditions and carbon count?
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