Esterification (carboxylic acid + alcohol, concentrated H_2SO_4 catalyst, reflux) is reversible:

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How do carboxylic acids and their derivatives react, and how does structure control their acidity and hydrolysis?

Describe the acidity and reactions of carboxylic acids, the formation and hydrolysis of esters, acyl chlorides and amides, and explain the relative acid strengths of carboxylic acids in terms of inductive effects

A focused answer to the H2 Chemistry learning outcome on carboxylic acids and their derivatives. Acidity and reactions of carboxylic acids, the formation and hydrolysis of esters, acyl chlorides and amides, and how electron-withdrawing groups raise acid strength through the inductive effect.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-06

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe the acidity and reactions of carboxylic acids, the formation and hydrolysis of esters, acyl chlorides and amides, and to explain the relative acid strengths of carboxylic acids using the inductive effect. The inductive-effect acidity comparison and the reactions of acyl chlorides are reliable exam questions.

The answer

Carboxylic acids and acidity

A carboxylic acid contains the $-\text{COOH}$ group and is a weak acid: it partially ionises in water to give a carboxylate ion and $\text{H}^+$ :

\text{RCOOH} \rightleftharpoons \text{RCOO}^- + \text{H}^+

It is more acidic than an alcohol or phenol because the carboxylate ion is stabilised by delocalisation of the negative charge over the two equivalent oxygen atoms.

Typical reactions of carboxylic acids

As weak acids they react with bases, carbonates and reactive metals:

with NaOH: $\text{RCOOH} + \text{NaOH} \rightarrow \text{RCOONa} + \text{H}_2\text{O}$ .
with carbonate (effervescence of $\text{CO}_2$ , a useful test): $2\text{RCOOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{RCOONa} + \text{H}_2\text{O} + \text{CO}_2$ .

The effervescence with sodium carbonate distinguishes a carboxylic acid from a phenol (which is too weak to release $\text{CO}_2$ ).

The inductive effect on acid strength

Acid strength depends on the stability of the carboxylate ion. Electron-withdrawing groups (e.g. Cl) near the $-\text{COOH}$ pull electron density away, spreading the negative charge of the anion and stabilising it, so $\text{H}^+$ is released more readily (stronger acid). Electron-donating groups (e.g. alkyl) do the opposite (weaker acid). Hence:

\text{Cl}_3\text{CCOOH} > \text{Cl}_2\text{CHCOOH} > \text{ClCH}_2\text{COOH} > \text{CH}_3\text{COOH}

Esters

Esterification (carboxylic acid + alcohol, concentrated $\text{H}_2\text{SO}_4$ catalyst, reflux) is reversible:

\text{RCOOH} + \text{R}'\text{OH} \rightleftharpoons \text{RCOOR}' + \text{H}_2\text{O}

Esters are hydrolysed back: acid hydrolysis (reflux with dilute acid) is reversible, giving the acid and alcohol; alkaline hydrolysis (reflux with NaOH, called saponification) goes to completion, giving the carboxylate salt and the alcohol.

Acyl chlorides

Acyl chlorides ( $\text{RCOCl}$ ) are very reactive because the chlorine is a good leaving group and strongly electron-withdrawing, making the carbonyl carbon very positive. They react vigorously with nucleophiles, releasing HCl:

with water: $\text{RCOCl} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{HCl}$ .
with alcohols: an ester (a fast, irreversible route to esters).
with ammonia or amines: an amide.

Amides

Amides ( $\text{RCONH}_2$ ) are hydrolysed by reflux with acid or alkali back to the carboxylic acid (or its salt) and ammonia (or an amine).

Worked example

Place ethanoic acid, methanoic acid and 2-chloropropanoic acid in order of increasing acid strength and explain.

Order of increasing acid strength: ethanoic acid < methanoic acid < 2-chloropropanoic acid.

Ethanoic acid ( $\text{CH}_3\text{COOH}$ ) has an electron-donating methyl group, which destabilises the carboxylate ion, so it is the weakest.
Methanoic acid ( $\text{HCOOH}$ ) has only an H next to the $-\text{COOH}$ (no electron-donating alkyl group), so its carboxylate is less destabilised and it is stronger than ethanoic acid.
2-chloropropanoic acid has an electron-withdrawing chlorine that stabilises the carboxylate by drawing charge away, making it the strongest of the three.

Try it: Acid strength comparator - rank carboxylic acids by predicted strength from their substituents.

Common traps

Explaining acidity by "Cl makes it more polar": State it as the inductive effect stabilising the carboxylate ion, which makes $\text{H}^+$ release easier.
Confusing acid and alkaline hydrolysis of esters: Acid hydrolysis is reversible (gives the acid); alkaline hydrolysis is complete (gives the carboxylate salt).
Saying carboxylic acids and phenols both fizz with carbonate: Only carboxylic acids release $\text{CO}_2$ with sodium carbonate; phenols are too weak. This is a distinguishing test.
Treating acyl chlorides as unreactive: They are very reactive (good leaving group, electron-withdrawing Cl), reacting vigorously with water, alcohols and amines.
Forgetting that HCl is released by acyl chlorides: Each nucleophilic substitution at an acyl chloride gives off HCl (misty fumes with water).

Examples in context

Example 1. Soap from fats. Alkaline hydrolysis (saponification) of an ester-containing fat with sodium hydroxide gives the sodium salts of long-chain carboxylic acids, which are soaps, plus glycerol. SEAB uses this to test the irreversible nature of alkaline ester hydrolysis and to connect organic chemistry to an everyday product.

Example 2. Aspirin synthesis. Salicylic acid is acetylated using an acyl chloride (or anhydride) to make aspirin, exploiting the high reactivity of the acyl group toward the phenol oxygen. This pharmaceutical example lets SEAB test the reactions of acyl derivatives and the idea of a good leaving group in a real synthesis.

Try this

Q1. Write the equation for the reaction of propanoic acid with sodium carbonate. [1 mark]

Cue. $2\text{C}_2\text{H}_5\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{C}_2\text{H}_5\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$ .

Q2. State the products of the alkaline hydrolysis of ethyl ethanoate. [2 marks]

Cue. Sodium ethanoate ( $\text{CH}_3\text{COONa}$ ) and ethanol; reaction goes to completion.

Q3. Explain why ethanoyl chloride reacts more vigorously with water than ethanoic acid does. [2 marks]

Cue. Chlorine is a good leaving group and electron-withdrawing, making the carbonyl carbon more positive and more open to nucleophilic attack by water.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Specimen (9729)4 marksExplain why chloroethanoic acid (ClCH2COOH) is a stronger acid than ethanoic acid (CH3COOH), and predict how trichloroethanoic acid (Cl3CCOOH) compares.

Show worked answer →

Acid strength depends on the stability of the carboxylate ion formed when H+ is lost.

In chloroethanoic acid the electronegative chlorine atom withdraws electron density along the bonds (the inductive effect), pulling negative charge away from the carboxylate group. This spreads (delocalises) the negative charge of the anion more, stabilising it.

A more stable carboxylate ion means H+ is released more readily, so chloroethanoic acid is a stronger acid than ethanoic acid (which has an electron-donating methyl group instead).

Trichloroethanoic acid has three chlorine atoms, so the electron-withdrawing inductive effect is even greater, the carboxylate is even more stabilised, and it is a stronger acid still.

Markers reward the stability-of-anion argument, the electron-withdrawing inductive effect of chlorine, and the prediction that more chlorines give a stronger acid.

2023 (style)4 marksDescribe the reaction of ethanoyl chloride (CH3COCl) with (a) water and (b) ethanol, giving the organic products, and state why acyl chlorides are more reactive than the corresponding carboxylic acids.

Show worked answer →

(a) With water, ethanoyl chloride is hydrolysed vigorously to ethanoic acid, with misty fumes of HCl:
CH3COCl + H2O -> CH3COOH + HCl.

(b) With ethanol, it forms an ester (ethyl ethanoate) and HCl:
CH3COCl + C2H5OH -> CH3COOC2H5 + HCl.

Acyl chlorides are more reactive than carboxylic acids because the chlorine is a very good leaving group (as Cl-) and is strongly electron-withdrawing, making the carbonyl carbon more positive and more open to nucleophilic attack.

Markers reward both products with equations and the good-leaving-group / more-positive-carbon reason for the reactivity.