Carboxylic acids

Chemistry· 8 min read

1. Structure and Physical Properties★★☆☆☆⏱ 15 min

📘 Definition

Carboxylic Acid

RCOOH (R = alkyl/aryl, H for methanoic acid)

An organic compound containing at least one carboxyl functional group (-COOH), where a carbonyl carbon is covalently bonded to a hydroxyl group.

Example:

Ethanoic acid $C H_{3} C O O H$ , benzoic acid $C_{6} H_{5} C O O H$

Carboxylic acids have two highly polar groups: the C=O and O-H bond. This allows them to form strong hydrogen bonds with water molecules and with each other. Small carboxylic acids (up to 4 carbons) are fully miscible with water, while solubility decreases as the non-polar carbon chain length increases.

Carboxylic acids have higher boiling points than comparable alkanes, aldehydes, ketones and even alcohols of similar relative molecular mass. This is because carboxylic acids form stable dimers in both pure liquid and non-polar solution, held together by two hydrogen bonds per dimer, effectively doubling the molecular mass and increasing intermolecular attraction.

📐 Worked Example

Explain why propanoic acid ( $M_{r} = 74$ ) has a boiling point of 141°C, while butan-1-ol ( $M_{r} = 74$ ) has a boiling point of 117°C, despite having the same relative molecular mass.

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1. Identify the intermolecular forces present in each compound:
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Propanoic acid's carboxyl group allows formation of a stable hydrogen-bonded dimer:
3
$CH_3CH_2COOH ightleftharpoons (CH_3CH_2COOH)_2$
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Two hydrogen bonds form between the two carboxyl groups, creating a single larger particle with stronger intermolecular forces than individual butan-1-ol molecules.
5
Butan-1-ol has only one hydroxyl group, so can only form one hydrogen bond per molecule with other alcohol molecules.
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More energy is required to overcome the stronger intermolecular forces between propanoic acid dimers, so its boiling point is higher than butan-1-ol.

Exam tip:

Always link boiling point trends directly to intermolecular forces, not just molar mass, when molecules have similar $M_{r}$ .

2. Acidity of Carboxylic Acids★★★☆☆⏱ 20 min

Carboxylic acids are weak Brønsted-Lowry acids that dissociate partially in aqueous solution to form a carboxylate ion and a hydronium ion:

RCOOH(aq) + H_2O(l) \rightleftharpoons RCOO^-(aq) + H_3O^+(aq)

📘 Definition

Carboxylate Ion

The conjugate base formed when a carboxylic acid loses a proton, stabilized by resonance delocalization of the negative charge across the two electronegative oxygen atoms of the carboxyl group.

Example:

Ethanoate ion $C H_{3} C O O^{-}$

The acidity of carboxylic acids is directly due to this resonance stabilization of the carboxylate ion. This makes carboxylic acids more acidic than phenols and alcohols, which form less stabilized conjugate bases. Electron-withdrawing groups (e.g. -Cl, -NO₂) increase acidity by further stabilizing the carboxylate ion, while electron-donating groups (e.g. -CH₃) decrease acidity.

📐 Worked Example

Arrange the following compounds in order of increasing acidity: chloroethanoic acid, ethanoic acid, ethanol, phenol.

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1. Acidity depends on the stability of the conjugate base formed after deprotonation:
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Ethanol forms an ethoxide ion with no resonance stabilization. The negative charge is fully localized on one oxygen, so ethanol is the least acidic.
3
Phenol forms a phenoxide ion with resonance delocalization of negative charge over the benzene ring, but most of the charge rests on carbon atoms, so it is more acidic than ethanol but less acidic than carboxylic acids.
4
Ethanoic acid forms a carboxylate ion with negative charge equally delocalized over two oxygen atoms, making it far more stable than phenoxide, so it is more acidic than phenol.
5
Chloroethanoic acid has an electron-withdrawing chlorine atom that pulls electron density away from the carboxylate group, further stabilizing the ion. This makes it more acidic than ethanoic acid.
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Final order of increasing acidity: ethanol < phenol < ethanoic acid < chloroethanoic acid

3. Preparation of Carboxylic Acids★★☆☆☆⏱ 15 min

Three main preparation routes are commonly tested in CIE exams:

Oxidation of primary alcohols or aldehydes: Reflux with a strong oxidizing agent (acidified $K_{2} C r_{2} O_{7}$ or acidified $K M n O_{4}$ ) to produce the carboxylic acid with the same number of carbons as the starting material.
Hydrolysis of nitriles: Heating a nitrile (formed from nucleophilic substitution of a haloalkane with cyanide) with dilute aqueous acid gives a carboxylic acid with one more carbon than the starting haloalkane.
Oxidative cleavage of alkenes: Hot concentrated acidified $K M n O_{4}$ cleaves the C=C double bond; any doubly bonded carbon with at least one hydrogen is oxidized to a carboxyl group.

📐 Worked Example

Outline the preparation of propanoic acid starting from 1-bromopropane, including all reagents and conditions.

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1. First step: Convert 1-bromopropane to propanenitrile via nucleophilic substitution:
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Reagents and conditions: Aqueous ethanolic potassium cyanide (KCN), heat under reflux:
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$CH_3CH_2CH_2Br + KCN \rightarrow CH_3CH_2CN + KBr$
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1. Second step: Hydrolyze the nitrile group to a carboxyl group:
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Reagents and conditions: Dilute hydrochloric acid, heat under reflux:
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$CH_3CH_2CN + 2H_2O + H^+ \rightarrow CH_3CH_2COOH + NH_4^+$
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This route produces propanoic acid with good yield, extending the carbon chain by one carbon from the starting 1-bromopropane.

4. Key Reactions of Carboxylic Acids★★★☆☆⏱ 20 min

Carboxylic acids undergo several key reactions that are widely used in organic synthesis:

Neutralization: React with $N a O H$ , $N a_{2} C O_{3}$ and $N a H C O_{3}$ to form soluble carboxylate salts. Reaction with $N a H C O_{3}$ produces $C O_{2}$ gas, a diagnostic test for carboxylic acids.
Reduction: Only reduced by strong reducing agents like lithium aluminium hydride ( $L i A l H_{4}$ ) in dry ether, to form primary alcohols. Mild reducing agents like $N a B H_{4}$ do not reduce carboxylic acids.
Esterification: Reversible acid-catalyzed reaction with alcohols to form esters and water.
Decarboxylation: Sodium carboxylates heated with soda lime ( $N a O H + C a O$ ) lose $C O_{2}$ to form an alkane with one less carbon than the starting acid.

📐 Worked Example

An unknown organic compound reacts with sodium metal to produce hydrogen gas, but does not react with sodium carbonate solution. Could the compound be a carboxylic acid? Explain your answer.

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1. Recall the relative acidity of carboxylic acids compared to carbonic acid:
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Carboxylic acids are stronger acids than carbonic acid ( $H_{2} C O_{3}$ ), so they react with sodium carbonate to form a carboxylate salt, water and carbon dioxide gas.
3
The observation that the unknown compound does not produce carbon dioxide with sodium carbonate rules out a carboxylic acid.
4
The unknown is almost certainly an alcohol: alcohols react with sodium metal to form an alkoxide salt and hydrogen gas, but are not acidic enough to react with sodium carbonate.

5. Common Pitfalls

Wrong move:

Claiming carboxylic acids are strong acids because they release $H^{+}$ in solution

Why:

Carboxylic acids only partially dissociate in water, so they are classified as weak acids. CIE examiners penalize calling them strong acids

Correct move:

Always describe carboxylic acids as weak Brønsted-Lowry acids unless explicitly told otherwise

Wrong move:

Stating that sodium borohydride ( $N a B H_{4}$ ) reduces carboxylic acids to primary alcohols

Why:

$N a B H_{4}$ is a mild reducing agent that only reduces aldehydes and ketones, it cannot reduce the stable carboxyl group

Correct move:

Only strong reducing agents like $L i A l H_{4}$ in dry ether reduce carboxylic acids to primary alcohols

Wrong move:

Claiming electron-donating methyl groups increase the acidity of carboxylic acids

Why:

Electron-donating groups increase electron density on the carboxylate ion, destabilizing the negative charge and decreasing acidity

Correct move:

Electron-withdrawing groups increase acidity, electron-donating groups decrease acidity

Wrong move:

Saying the negative charge in a carboxylate ion is permanently located on one oxygen atom

Why:

The negative charge is equally delocalized across both oxygen atoms via resonance, which is the main source of carboxylate stability

Correct move:

Always mention resonance delocalization across both oxygen atoms when explaining carboxylic acid acidity

Wrong move:

Stopping at alkaline hydrolysis of nitriles and claiming you get the carboxylic acid

Why:

Alkaline hydrolysis produces a carboxylate salt, not the neutral carboxylic acid

Correct move:

After alkaline hydrolysis, add excess dilute strong acid to protonate the carboxylate ion to get the free carboxylic acid

6. Quick Reference Cheatsheet

Property	Key Exam Fact
Functional group	-COOH
Boiling point trend	Higher than alcohols of similar $M_{r}$ (hydrogen-bonded dimers)
Acidity order	Carboxylic acids > phenols > alcohols
Acidity substituent effect	EWG = increase acidity, EDG = decrease acidity
Test for carboxylic acid	Reacts with $N a H C O_{3}$ to give $C O_{2}$ effervescence
Reduction	$L i A l H_{4}$ → primary alcohol, $N a B H_{4}$ = no reaction
Common preparations	Oxidation of 1° alcohol, nitrile hydrolysis, alkene cleavage

7. Frequently Asked

Why are carboxylic acids more acidic than alcohols?

Carboxylic acids form a resonance-stabilized carboxylate ion after deprotonation, delocalizing the negative charge across two electronegative oxygen atoms. Alcohols form unstabilized alkoxide ions with localized negative charge, so they are less acidic.

How do you distinguish carboxylic acids from phenols?

Add sodium hydrogencarbonate (NaHCO₃) solution. Carboxylic acids react to produce effervescence of carbon dioxide gas, while phenols are not acidic enough to react with NaHCO₃ and produce no gas.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Acidity comparison question
2023 · 4
Multi-step synthesis problem
2021 · 1
Multiple choice on preparation

Going deeper

syllabusCIE 9701 2025-2026 SyllabusOfficial syllabus reference

What's Next

Carboxylic acids are a foundational functional group in organic synthesis, forming the starting point for many other important carbonyl compounds you will study next. Mastering their acidity and reactivity is critical for answering multi-step synthesis questions, which account for a large proportion of marks in CIE A-Level paper 4. The resonance stabilization principles you learned here will also help you understand the reactivity of other carbonyl compounds and aromatic systems. Next, you will explore carboxylic acid derivatives, which share structural similarities with carboxylic acids but have very different reactivity patterns. You will also build on this knowledge of organic acid behavior when studying buffer calculations involving weak organic acids in physical chemistry.