Prepare an amine using ammonia substitution
Use ethanolic ammonia under pressure and heat to replace halogen with an amino group.
Section 15 · Halogen compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which particle supplies nitrogen in amine substitution?
What you will learn
- Identify NH₃ as a nucleophile
- State ethanol, pressure and heat
- Account for ammonium halide formation
Ammonia attacks the carbon bearing X
NH₃ has a lone pair and acts as a nucleophile. When it reacts with a halogenoalkane, it replaces the halogen to form a primary amine.
The syllabus conditions are NH₃ in ethanol, heated under pressure. For bromoethane, C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br. One ammonia supplies the nitrogen in the amine; another accepts the proton released.
Use excess ammonia to favor the primary amine
Amine products can themselves react with more halogenoalkane, forming more substituted amines. Excess NH₃ makes further alkylation less likely and favors the primary amine product.
State heated under pressure when describing the route. Do not present room-temperature aqueous ammonia as equivalent syllabus conditions.
Pause and explain
Which conditions are specified for reacting a halogenoalkane with ammonia?
Worked example
Write a balanced equation for preparing ethylamine from bromoethane and ammonia.
Show the worked solution
- NH₃ replaces Br to form C₂H₅NH₂.
- A second NH₃ accepts H⁺ and combines with Br⁻.
- Balance the ammonia and ammonium bromide.
Answer C₂H₅Br + 2NH₃ → C₂H₅NH₂ + NH₄Br; ethanolic NH₃, heat under pressure.
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.
Write the equation for 1-bromopropane reacting with ammonia to form propylamine.
Give me a hint
Use two NH₃ and show the ammonium bromide coproduct.
Compare my reasoning
- The three-carbon skeleton is retained.
- Replace Br with NH₂ to form propan-1-amine.
- Use a second NH₃ to form NH₄Br from HBr-equivalent products.
CH₃CH₂CH₂Br + 2NH₃ → CH₃CH₂CH₂NH₂ + NH₄Br; ethanol, heat under pressure.
Look for these in your work
- Skeleton preserved
- Primary amine identified
- Equation and conditions complete
Explain why excess ammonia is used in this substitution.
Give me a hint
Consider what can happen to the amine after it forms.
Compare my reasoning
- The primary amine still has a lone pair and N–H bonds.
- It can react further with halogenoalkane to form more substituted amines.
- Excess NH₃ favors reaction with ammonia and limits further alkylation.
Excess ammonia favors primary amine formation and reduces further alkylation of the amine product.
Look for these in your work
- Further reaction recognized
- Excess role stated
- Outcome linked
A learner writes RBr + NH₃ → RNH₂ + HBr and stops. Identify one missing equation detail.
Give me a hint
Check whether HBr remains as a strong acid in excess ammonia.
Compare my reasoning
- Ammonia can accept the proton from HBr.
- The ionic products form ammonium bromide.
- A balanced molecular equation uses a second NH₃: RBr + 2NH₃ → RNH₂ + NH₄Br.
Include a second NH₃ and write NH₄Br: RBr + 2NH₃ → RNH₂ + NH₄Br.
Look for these in your work
- Proton acceptor identified
- Salt product corrected
- Equation balanced
Common mistakes
- Using only one NH₃ in the balanced molecular equation
- Forgetting NH₄Br as a coproduct
- Omitting pressure from the stated conditions
What can you explain now?
Which particle supplies nitrogen in amine substitution?
Compare with the explanation
NH₃
Ammonia donates its lone pair and becomes part of the amine.
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Make it stick
Why are two ammonia molecules shown in the overall substitution equation?
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.