Amines as Bases
A concise revision guide to the basicity of amines: how the nitrogen lone pair accepts a proton, the reactions of butylamine and phenylamine with water and with acids to form salts, and why primary aliphatic amines are stronger bases than ammonia while aromatic amines are weaker.
Why Amines Are Bases
Choose a step to watch the nitrogen lone pair in ammonia, ethylamine and phenylamine, and see which one accepts a proton most readily.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
Amines behave like ammonia. Check the acid-base ideas this page builds on.
- A Brønsted–Lowry base is a proton acceptor. Ammonia accepts H⁺ using the lone pair on nitrogen: NH₃ + H⁺ → NH₄⁺.
- Ammonia is a weak base: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, with the equilibrium well to the left.
- Ammonia reacts with acids to form ammonium salts: NH₃ + HCl → NH₄Cl.
- Primary, aromatic and aliphatic amines and the formula of butylamine, CH₃CH₂CH₂CH₂NH₂: Amines, Amides and Amino Acids: Structure and Naming.
GCSE Recap: Alkalis, Neutralisation and Strength
Three quick questions on the GCSE ideas this page builds on.
The Lone Pair Accepts a Proton
Every amine has a lone pair on its nitrogen. The lone pair forms a dative covalent bond to a proton, so amines are Brønsted–Lowry bases. The same lone pair makes them nucleophiles (Amines as Nucleophiles).
With water: an alkaline solution
- CH₃CH₂CH₂CH₂NH₂ + H₂O ⇌ CH₃CH₂CH₂CH₂NH₃⁺ + OH⁻: the butylammonium ion and hydroxide ions form.
- The equilibrium lies well to the left: amines are weak bases, and most molecules stay unprotonated.
- Enough OH⁻ forms for the solution to be alkaline: 0.10 mol dm⁻³ butylamine has a pH of about 11 to 12.
- Phenylamine, C₆H₅NH₂, dissolves only slightly and its solution is barely alkaline.
Remember: Write the ion in full and show the charge on nitrogen: CH₃CH₂CH₂CH₂NH₃⁺, the butylammonium ion. The hydroxide ion is what makes the solution alkaline.
Salts with Acids
With an acid the proton transfer is complete. The amine is converted into an alkylammonium salt, an ionic solid that dissolves in water.
| Reaction | Equation | Salt |
|---|---|---|
| butylamine + hydrochloric acid | CH₃CH₂CH₂CH₂NH₂ + HCl → CH₃CH₂CH₂CH₂NH₃⁺Cl⁻ | butylammonium chloride |
| butylamine + sulfuric acid | CH₃CH₂CH₂CH₂NH₂ + H₂SO₄ → CH₃CH₂CH₂CH₂NH₃⁺HSO₄⁻ | butylammonium hydrogensulfate |
| ethylamine + hydrochloric acid | CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻ | ethylammonium chloride |
| phenylamine + hydrochloric acid | C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻ | phenylammonium chloride |
- Name the salt by changing -amine to -ammonium and adding the anion: butylammonium chloride, ethylammonium nitrate.
- Phenylamine is almost insoluble in water, but phenylammonium chloride dissolves: adding acid to an oily amine gives a clear solution.
- Adding sodium hydroxide to the salt releases the amine again: CH₃CH₂CH₂CH₂NH₃⁺ + OH⁻ → CH₃CH₂CH₂CH₂NH₂ + H₂O. This is used to purify amines.
Three proton transfers with the lone pair shown: butylamine with water (an equilibrium), ethylamine with HCl and phenylamine with HCl.
What is seen in the test tube: the alkaline solution, the salt forming with acid, phenylamine dissolving as its salt and reappearing with alkali.
Naming the salts: -amine becomes -ammonium, then add the anion. The pattern is the same for every acid.
Exam focus: The salt is ionic: write it as two ions, CH₃CH₂CH₂CH₂NH₃⁺Cl⁻ or CH₃CH₂NH₃⁺Cl⁻, not as a molecule. The new N–H bond is a dative covalent bond.
Check: Alkaline Solutions and Salts
Write the equations for an amine with water and with an acid, name the salts and explain why the solution of an amine is alkaline.
Quick Check: New Salts and a Titration
Work each one out on paper, then flip the card to compare your working.
Comparing Base Strength
Base strength depends on how available the lone pair is. The order is phenylamine (weakest) < ammonia < butylamine (strongest). Two effects explain it.
Aliphatic amines are stronger bases than ammonia
- An alkyl group releases electrons towards the nitrogen: the positive inductive effect.
- The electron density on nitrogen rises, so the lone pair is more available to accept a proton.
- The positive charge on the butylammonium ion is also spread on to the alkyl group, which stabilises the ion formed.
- Secondary amines are slightly stronger again: two alkyl groups push electrons (an extension).
Aromatic amines are much weaker bases than ammonia
- In phenylamine the nitrogen lone pair overlaps the ring π system and is partly delocalised into the ring, as the OH lone pair is in phenol.
- The electron density on nitrogen falls, so the lone pair is far less available: phenylamine is about 4 × 10⁴ times weaker than ammonia.
- Protonating phenylamine would also remove the lone pair from the ring and lose that extra stability.
Ammonia, ethylamine and phenylamine side by side: the inductive push along the C–N bond, and the lone pair spread into the ring.
The ladder of base strength and the one-line reason for each rung. The numbers are pKb values: smaller means stronger. Your specification does not ask you to use them.
Exam focus: Always argue through the lone pair: alkyl group pushes electron density on to N (more available, stronger); ring delocalises the lone pair (less available, weaker).
Quick Check: Ranking Base Strength
Argue through the availability of the nitrogen lone pair in each compound.
Check: Base Strength and the Lone Pair
Explain through the lone pair which of two amines is the stronger base, read pH data and predict what an amine does with water, acids and metal ions.
Common Exam Points
- CH₃CH₂CH₂CH₂NH₂ + H₂O ⇌ CH₃CH₂CH₂CH₂NH₃⁺ + OH⁻: the equilibrium lies to the left, so the amine is a weak base and the solution is alkaline.
- CH₃CH₂CH₂CH₂NH₂ + HCl → CH₃CH₂CH₂CH₂NH₃⁺Cl⁻, butylammonium chloride: an ionic salt, soluble in water. Phenylamine gives phenylammonium chloride.
- The new N–H bond forms from the lone pair on nitrogen: amines are Brønsted–Lowry bases and proton acceptors.
- Order of strength: phenylamine < ammonia < butylamine. Alkyl groups push electrons on to N (positive inductive effect); the ring delocalises the lone pair.
- The more available the lone pair on nitrogen, the stronger the base: ethylamine is stronger than ammonia, and ammonia is much stronger than phenylamine.
- Adding NaOH to an amine salt releases the free amine: RNH₃⁺ + OH⁻ → RNH₂ + H₂O.
- Phenylamine is only slightly soluble in water but dissolves in acid as its salt.
FAQs
Use these quick answers to check why butylamine and phenylamine are bases, the salts they form with acids and the order of base strength.
Why is butylamine a base?
The nitrogen has a lone pair of electrons that can accept a proton, so amines are Brønsted–Lowry bases.
In water, CH₃CH₂CH₂CH₂NH₂ + H₂O ⇌ CH₃CH₂CH₂CH₂NH₃⁺ + OH⁻, and the equilibrium lies well to the left, so the solution is only weakly alkaline.
What forms when an amine reacts with an acid?
A salt. Butylamine and hydrochloric acid give butylammonium chloride, CH₃CH₂CH₂CH₂NH₃⁺Cl⁻, an ionic solid that dissolves in water. Phenylamine gives phenylammonium chloride in the same way, which is one way of dissolving phenylamine, because the free amine is barely soluble.
Why is phenylamine a weaker base than ammonia?
In phenylamine the lone pair on nitrogen is partly delocalised into the π system of the benzene ring, so it is less available to accept a proton.
Why is butylamine a stronger base than ammonia?
The alkyl group releases electrons towards nitrogen (a positive inductive effect), raising the electron density on N, so the lone pair is more available and the butylammonium ion is stabilised.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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