Compare the basicity of ammonia, ethylamine and phenylamine
Rank ethylamine above ammonia and phenylamine in aqueous basicity by comparing lone-pair availability on nitrogen.
Section 34 · Nitrogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 34. Use the stated reagents, conditions and product scope. Written practice is self-checked This online lesson cannot demonstrate supervised laboratory skills.
Before you begin
- Recall nucleophilic substitution, reduction, acyl-chloride addition–elimination, acid–base equilibria and covalent bonding.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the strongest base in water among ethylamine, ammonia and phenylamine?
What you will learn
- Order aqueous ammonia, ethylamine and phenylamine by basicity
- Explain the effect of an ethyl group
- Explain why phenylamine is less basic than ammonia
An amine base accepts a proton
A base accepts H⁺ using a lone pair. In water, ammonia and amines establish equilibria that form an ammonium ion and OH⁻.
The usual order is ethylamine > ammonia > phenylamine. Ethylamine is stronger than ammonia because the ethyl group pushes electron density toward nitrogen by the positive inductive effect, making its lone pair more available to accept H⁺.
Phenylamine's lone pair is delocalised
In phenylamine, the nitrogen lone pair overlaps with the aromatic π system and is delocalised into the ring. It is less available to bind H⁺ than the lone pair in ammonia.
This makes phenylamine weaker as a base than ammonia. Do not confuse the electron-withdrawing inductive effect of an alkyl group with lone-pair delocalisation in an aryl amine.
Pause and explain
Why is phenylamine weaker as a base than ammonia?
Worked example
Rank ethylamine, ammonia and phenylamine from strongest to weakest base.
Show the worked solution
- Ethyl donates electron density toward N.
- Ammonia has a localised lone pair.
- Phenylamine delocalises its lone pair into the ring.
Answer Ethylamine > ammonia > phenylamine.
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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 basicity order of C₂H₅NH₂, NH₃ and C₆H₅NH₂, then give one reason for each comparison.
Give me a hint
Compare electron donation and lone-pair delocalisation.
Compare my reasoning
- Ethyl increases electron density at N.
- Ammonia has a localised lone pair.
- Phenylamine delocalises its pair into the ring.
C₂H₅NH₂ > NH₃ > C₆H₅NH₂; ethyl donation strengthens, while ring delocalisation weakens basicity.
Look for these in your work
- Order correct
- Inductive effect
- Delocalisation
Explain why ethylamine forms a stronger base than ammonia.
Give me a hint
Use the ethyl group's effect through σ bonds.
Compare my reasoning
- The ethyl group is electron-releasing by induction.
- It raises electron density at nitrogen.
- Nitrogen's lone pair is more ready to accept H⁺.
The ethyl group's positive inductive effect makes the nitrogen lone pair more available, so ethylamine is more basic.
Look for these in your work
- Electron donation
- N electron density
- Proton acceptance
A learner says phenylamine is stronger than ammonia because the aromatic ring donates its π electrons to nitrogen. Correct the direction and consequence.
Give me a hint
The N lone pair delocalises into the ring.
Compare my reasoning
- The nitrogen lone pair is shared with the aromatic π system.
- This reduces lone-pair availability at nitrogen.
- Phenylamine is therefore less basic than ammonia.
The lone pair delocalises from nitrogen into the ring, making it less available to accept H⁺; phenylamine is weaker than ammonia.
Look for these in your work
- Correct direction
- Lone-pair availability
- Order
Common mistakes
- Putting phenylamine first because the phenyl ring is large
- Claiming an ethyl group withdraws electron density
- Saying phenylamine has no nitrogen lone pair
- Confusing acidity order with basicity order
What can you explain now?
Which is the strongest base in water among ethylamine, ammonia and phenylamine?
Compare with the explanation
Ethylamine
The ethyl group increases electron density at nitrogen.
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Make it stick
How do alkyl donation and aryl lone-pair delocalisation set the amine basicity order?
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.