Last updated: 24 August 2026
Carboxylic acids appear throughout Organic Chemistry because the –COOH group connects several important reaction pathways. Ethanoic acid, CH₃COOH, is the acid responsible for the characteristic acidity of vinegar, while benzoic acid contains the same functional group attached directly to a benzene ring. Although the rest of their molecular structures are different, both compounds show the characteristic chemistry of the carboxyl group.
The important idea is not simply to memorise a list of reactions. Once you understand why the carboxylate ion is stable and how the carbon atom of the –COOH group can be produced or transformed, the reactions begin to form a connected network. Before developing that network, try the diagnostic quiz below.
Try these five questions before reading the chapter. Focus on the carboxyl group, oxidation, acidity and characteristic reactions.
- 5 diagnostic questions
- Structures and reaction reasoning where useful
- Instant explanation for every answer option
1. The carboxyl group
A carboxylic acid contains the functional group –COOH. The carbon atom in this group is bonded to one oxygen atom by a C=O double bond and to another oxygen atom as part of an O–H bond.
Two important examples are ethanoic acid, CH₃COOH, and benzoic acid, C₆H₅COOH. Ethanoic acid contains the carboxyl group attached to a methyl group, whereas benzoic acid contains the carboxyl group attached directly to a benzene ring.
Structural principle: The characteristic chemistry of a carboxylic acid comes from the –COOH group. Changing the carbon group attached to –COOH can alter properties such as solubility and acid strength, but the fundamental reactions of the carboxyl group remain recognisable.
Physical behaviour of ethanoic acid and benzoic acid
Small carboxylic acids such as ethanoic acid interact strongly with water through hydrogen bonding and are very soluble in water. Benzoic acid, however, contains a large non-polar benzene ring and is only sparingly soluble in cold water.
Benzoic acid becomes considerably more soluble in hot water. This temperature-dependent solubility is useful when benzoic acid is purified by recrystallisation.
Ethanoic acid: A small molecule in which the polar –COOH group makes up a large proportion of the molecule, giving high water solubility.
Benzoic acid: Contains a hydrophobic benzene ring as well as –COOH, so its solubility in cold water is much lower.
2. Preparation of carboxylic acids by oxidation
Carboxylic acids can be prepared by oxidising primary alcohols or aldehydes. A primary alcohol is oxidised first to an aldehyde and then further to a carboxylic acid.
Suitable oxidising agents include acidified potassium dichromate(VI), K₂Cr₂O₇ / H⁺, or acidified potassium manganate(VII), KMnO₄ / H⁺. To obtain the carboxylic acid from a primary alcohol, the mixture is heated under reflux.
Why is reflux required?
If the aim is complete oxidation to the carboxylic acid, the intermediate aldehyde should remain in contact with the oxidising mixture. Heating under reflux allows the reaction mixture to boil while vapours condense and return to the flask.
By contrast, if an aldehyde were being prepared from a primary alcohol, the aldehyde could be distilled away as it forms to reduce further oxidation.
Example: Ethanol is oxidised to ethanoic acid according to the simplified oxidation equation CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. With acidified K₂Cr₂O₇, the characteristic observation is orange → green. With acidified KMnO₄, the purple colour is discharged.
3. Preparation of carboxylic acids from nitriles
Nitriles, RCN, can be converted into carboxylic acids by hydrolysis. During hydrolysis, the carbon atom of the –C≡N group becomes the carbon atom of the –COOH group.
Hydrolysis may be carried out using dilute acid and heat, or using dilute aqueous alkali and heat.
Acidic hydrolysis
Under acidic conditions, the organic product is obtained directly as the carboxylic acid. The nitrogen-containing product is present in protonated form, for example as NH₄⁺.
Alkaline hydrolysis
Under alkaline conditions, the initial organic product is a carboxylate salt, RCOO⁻, rather than the free carboxylic acid.
The mixture must therefore be acidified after hydrolysis if the required product is the carboxylic acid:
Important carbon-counting point: Hydrolysis of a nitrile does not itself add a carbon atom. The carbon atom of the –CN group was already present and simply becomes the carbon atom of –COOH. The nitrile route is useful for increasing chain length because forming RCN from a halogenoalkane using CN⁻ introduces that extra carbon earlier in the synthesis.
4. Why carboxylic acids are acidic
Carboxylic acids are weak acids. In water, a carboxylic acid can donate the hydrogen ion attached to oxygen:
The key question is therefore: why is RCOO⁻ sufficiently stable for this proton loss to occur?
Delocalisation in the carboxylate ion
When H⁺ is removed from –COOH, a carboxylate ion is formed. The negative charge is not localised permanently on only one oxygen atom. Instead, electron density is delocalised over the two oxygen atoms.
Delocalisation lowers the energy of the ion. In other words, the negative charge is stabilised rather than being concentrated on one oxygen atom. A more stable conjugate base means that proton loss from the acid is more favourable.
Carboxylate ion, RCOO⁻: The negative charge is delocalised over two oxygen atoms, giving substantial stabilisation.
Alkoxide ion, RO⁻: The negative charge is largely localised on a single oxygen atom, so the ion receives less stabilisation.
A useful way to think about acid strength is to look at the species left behind after H⁺ is removed. If the conjugate base is particularly stable, proton loss becomes more favourable. For carboxylic acids, delocalisation of the carboxylate ion is central to this stability.
5. Effect of electron-withdrawing groups on acidity
Not all carboxylic acids have the same acid strength. Substituents attached to the carbon chain can change the stability of the carboxylate ion.
Chlorine is more electronegative than carbon and therefore exerts a negative inductive effect, –I. It withdraws electron density through σ bonds.
Acid-strength principle: An electron-withdrawing group stabilises the negatively charged carboxylate ion. The more stable the conjugate base, the stronger the corresponding carboxylic acid.
Chlorine-substituted ethanoic acids
Replacing hydrogen atoms on the methyl group of ethanoic acid with chlorine atoms increases acidity. As more chlorine atoms are introduced, the electron-withdrawing effect becomes greater and the corresponding carboxylate ion becomes increasingly stabilised.
The order shown is an order of decreasing acid strength. Trichloroethanoic acid is therefore the strongest of these four acids.
More electron-withdrawing substituents: Greater stabilisation of RCOO⁻ and therefore a stronger acid.
Electron-withdrawing group closer to –COOH: A stronger inductive influence because the inductive effect decreases with increasing distance through the carbon chain.
6. Reactions showing the acidic behaviour of carboxylic acids
Carboxylic acids undergo the familiar reactions of acids. The acidic hydrogen of –COOH can be replaced by a metal ion to form a carboxylate salt.
Reaction with reactive metals
Hydrogen gas is evolved and a carboxylate salt is formed.
Reaction with alkalis
This is an acid–base neutralisation reaction.
Reaction with carbonates
The production of carbon dioxide gives effervescence. This provides a useful chemical test for distinguishing a carboxylic acid from many other neutral organic compounds such as alcohols and phenols.
| Reagent | Products | Characteristic observation |
|---|---|---|
| Na | Carboxylate salt + H₂ | Effervescence of hydrogen gas |
| NaOH(aq) | Carboxylate salt + H₂O | No distinctive visible observation |
| Na₂CO₃(aq) | Carboxylate salt + H₂O + CO₂ | Effervescence of carbon dioxide |
7. Esterification of carboxylic acids
A carboxylic acid reacts reversibly with an alcohol to form an ester and water. This is a condensation reaction because two organic molecules combine with elimination of a small molecule, H₂O.
The reaction is carried out by heating the carboxylic acid with the alcohol in the presence of concentrated H₂SO₄, which acts as an acid catalyst.
Example: ethanoic acid and ethanol
The ester formed is ethyl ethanoate. In naming an ester, the alkyl name derived from the alcohol comes first, followed by the carboxylate-derived name from the acid.
Exam detail: For esterification, specify an alcohol and carboxylic acid together with concentrated H₂SO₄ and heat. Simply writing “H₂SO₄” without indicating concentrated acid may lose precision.
8. Conversion of carboxylic acids into acyl chlorides
The –OH portion of the carboxyl group can be replaced by chlorine to form an acyl chloride, RCOCl. A standard reagent for this conversion is phosphorus pentachloride, PCl₅.
Formation of hydrogen chloride gives misty white fumes in moist air.
9. Reduction of carboxylic acids
Carboxylic acids can be reduced to primary alcohols using lithium aluminium hydride, LiAlH₄, in dry ether.
The carbon atom of the carboxyl group is retained and becomes the carbon atom bearing the –OH group in the primary alcohol.
Why dry ether? LiAlH₄ reacts vigorously with water and other protic substances. It is therefore used in an anhydrous ether solvent rather than in aqueous solution.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Using distillation when oxidising a primary alcohol completely to a carboxylic acid | Distillation removes the aldehyde intermediate and can prevent complete oxidation. | State acidified K₂Cr₂O₇ or KMnO₄ and heat under reflux. |
| Saying nitrile hydrolysis “adds one carbon atom” | Hydrolysis retains the nitrile carbon; it does not introduce a new carbon atom during the hydrolysis step itself. | Remember that the carbon was introduced when CN⁻ was incorporated into the molecule earlier. |
| Explaining acidity only by saying that the O–H bond is polar | This does not explain why a carboxylic acid is appreciably more acidic than an alcohol. | Discuss the delocalisation and stability of RCOO⁻. |
| Forgetting acidification after alkaline hydrolysis of a nitrile | Alkaline hydrolysis produces RCOO⁻ rather than RCOOH. | Hydrolyse with aqueous alkali and heat, then acidify to obtain the carboxylic acid. |
| Saying chlorine makes an acid stronger simply because chlorine is “reactive” | Chemical reactivity of elemental chlorine is not the relevant reason. | Explain that Cl withdraws electron density by the –I effect, stabilising the carboxylate ion. |
| Using dilute H₂SO₄ as the catalyst for esterification | The standard esterification conditions require concentrated sulfuric acid. | State concentrated H₂SO₄ and heat. |
What you should be able to do
Recognise the carboxyl group
Identify –COOH in displayed, condensed and skeletal structures, including ethanoic acid and benzoic acid.
Prepare carboxylic acids
Give suitable reagents and conditions for oxidation of primary alcohols or aldehydes and for hydrolysis of nitriles.
Explain carboxylic acid acidity
Relate acid strength to the delocalisation and stability of the carboxylate conjugate base.
Compare substituted carboxylic acids
Use electron-withdrawing inductive effects to explain changes in carboxylic acid strength.
Predict characteristic reactions
Write equations for reactions with metals, alkalis and carbonates and identify the expected observations.
Carry out functional-group transformations
Convert carboxylic acids into esters, acyl chlorides and primary alcohols using the correct reagents and conditions.
These ten questions test whether you can apply the chapter to preparation, acid strength, reactions and functional-group transformations.
- 10 mastery questions
- Structures and reaction reasoning where useful
- Instant explanation for every answer option
Frequently asked questions
Common questions about carboxylic acids
Carboxylic acid questions often become difficult when several familiar ideas—oxidation, acid strength and organic synthesis—are tested together. These answers connect the most common points.
Why must a primary alcohol be heated under reflux to prepare a carboxylic acid?
Oxidation of a primary alcohol occurs through an aldehyde intermediate. Heating under reflux keeps volatile organic material in the reaction vessel so that the aldehyde remains in contact with the oxidising agent and can be oxidised further to the carboxylic acid. Distillation is instead useful when the aldehyde itself is the required product.
Does hydrolysis of a nitrile really increase the carbon chain by one carbon?
Not during the hydrolysis step itself. The carbon atom of –CN is already part of the nitrile and becomes the carbon atom of –COOH. The apparent one-carbon chain extension occurs earlier when CN⁻ is introduced into a carbon skeleton, for example by reacting a halogenoalkane with CN⁻.
Why is a carboxylic acid more acidic than an alcohol?
After a carboxylic acid loses H⁺, it forms a carboxylate ion in which the negative charge is delocalised over two oxygen atoms. This stabilises the conjugate base. An alcohol forms an alkoxide ion in which the negative charge is much more localised on one oxygen atom, so proton loss is less favourable.
Why does chlorine make a substituted carboxylic acid stronger?
Chlorine is strongly electronegative and withdraws electron density through σ bonds by the negative inductive effect. This helps stabilise the negatively charged carboxylate ion. Increasing the number of nearby chlorine substituents therefore generally increases acidity.
What is a good chemical test for a carboxylic acid?
Add aqueous sodium carbonate. A carboxylic acid reacts to form a carboxylate salt, water and carbon dioxide, so effervescence of CO₂ is observed. This is more useful as a visible test than reaction with NaOH, because neutralisation with NaOH gives no distinctive gas or precipitate.
Carboxylic acid chemistry becomes much easier when you keep returning to the structure of –COOH. Oxidation and nitrile hydrolysis explain how the group can be made, delocalisation explains why its proton can be lost, and its reactions show how the group can be converted into salts, esters, acyl chlorides and primary alcohols. That makes the carboxyl group an important junction in the wider Organic Chemistry reaction map.