Carboxylic Acids and Derivatives (A Level)

0. What Examiners Want

This guide covers Topic 33 of Cambridge International AS & A Level Chemistry 9701 (2025–27) and follows the learning outcomes below, word for word. The right-hand column shows where each one is taught. It builds on Topic 18 (carboxylic acids and esters at AS), Topic 30 (side-chain oxidation of arenes), Topic 31 (halogenoarenes) and Topic 32 (phenol and the first acyl chloride reaction). The acyl chloride reaction with amines is met again in Topic 34.

1. Carboxylic Acids and Benzoic Acid

A carboxylic acid contains the group –COOH. At AS you met three routes to a carboxylic acid: oxidation of a primary alcohol or an aldehyde (acidified K₂Cr₂O₇, reflux), hydrolysis of a nitrile (dilute HCl, reflux), and the reactions of the acid shown in the figure. At A Level there is one more route, and it makes benzoic acid.

Amber: how a carboxylic acid is made. Green: what it does. The acyl chloride is the reactive derivative that this topic is about.

Benzoic acid from methylbenzene. An alkylbenzene is oxidised by hot alkaline KMnO₄; the whole side chain (as long as it has a C–H on the carbon next to the ring) is oxidised to –COOH. In alkali the product is the carboxylate salt, sodium benzoate. Dilute acid is then added to release the free acid, which is a white solid.

Step 1: KMnO₄/NaOH(aq), reflux (purple → brown MnO₂). Step 2: add dilute acid to give benzoic acid.

Worked example. State the reagents and conditions for the conversion of methylbenzene into benzoic acid, and describe what you see in the first step. [4]

Hot (heated under reflux) alkaline KMnO₄ [1], then dilute acid [1]. The purple solution turns colourless / forms a brown precipitate of MnO₂ [1]; adding acid gives a white precipitate of benzoic acid [1].

Like every carboxylic acid, benzoic acid reacts with bases (NaOH), reactive metals (Mg, Na) and carbonates (effervescence of CO₂) to form carboxylate salts, for example sodium ethanoate, CH₃COO⁻Na⁺.

2. Relative Acidity

All carboxylic acids are weak acids: their molecules are mostly undissociated in water. The dissociation constant K_a measures the strength: the larger the K_a (the smaller the pK_a), the stronger the acid. For ethanoic acid K_a = 1.7 × 10⁻⁵ mol dm⁻³. The order you must explain is:

The smaller the pK_a, the stronger the acid. Each added chlorine makes ethanoic acid stronger; phenol and ethanol are far weaker.

Carboxylic acids are stronger than phenols and alcohols. The reason is the stability of the anion (and the weaker O–H bond, because the C=O group pulls electrons away).

• In the carboxylate ion, the negative charge is delocalised over two oxygen atoms through the –COO⁻ group. Both C–O bonds are the same length. The ion is very stable.
• In the phenoxide ion, the charge is delocalised into the ring (Topic 32), but the ring has to share it with carbon atoms, which are far less able to hold it. It is less stable than carboxylate.
• In an alkoxide ion the charge stays on one oxygen, and the electron-donating alkyl group makes it worse.

Carboxylate > phenoxide > alkoxide in stability, so carboxylic acid > phenol > alcohol in acidity.

Chlorine-substituted acids. An electron-withdrawing atom on the carbon next to the –COOH group makes the acid stronger. Chlorine is electronegative, so it withdraws electrons through the σ bonds (a negative inductive effect, −I). This weakens the O–H bond and spreads out the charge on the carboxylate ion, so the ion is more stable and less likely to take H⁺ back. An electron-donating alkyl group does the opposite.

More Cl atoms, closer to the –COOH group, give a stronger acid. The effect falls quickly with distance.

The effect depends on how many chlorine atoms there are and how close they are to the –COOH group. 2-Chlorobutanoic acid is stronger than butanoic acid; 2,2-dichlorobutanoic acid is stronger still; 4-chlorobutanoic acid is only slightly stronger than butanoic acid because the Cl is far away.

Worked example. Place CH₃CH₂COOH, CH₃CHClCOOH and CH₃CCl₂COOH in order of acid strength, strongest first, and explain. [3]

CH₃CCl₂COOH > CH₃CHClCOOH > CH₃CH₂COOH [1]. Chlorine is electron-withdrawing [1], so it spreads the negative charge on the carboxylate ion and weakens the O–H bond; two Cl atoms do this more than one, and the ethyl group is electron-donating [1].

Worked example. Explain why ethanoic acid is a stronger acid than ethanol. [3]

In the ethanoate ion the negative charge is delocalised over two oxygen atoms through the –COO⁻ group [1], so the ion is stabilised [1]. In ethoxide the charge is localised on one oxygen and the alkyl group is electron-donating, so the ion is less stable [1].

3. Two Acids That Can Be Oxidised

Carboxylic acids are the end of oxidation: a primary alcohol is oxidised to an aldehyde, then to the acid, and the acid is not oxidised further by the same reagents. There are two exceptions.

Methanoic acid, HCOOH. The molecule has an H atom on the carbonyl carbon, so it has an aldehyde-like C–H and is a reducing agent. It is oxidised to carbon dioxide and water: HCOOH + [O] → CO₂ + H₂O.

The same reagents that test for an aldehyde work on methanoic acid; no other simple carboxylic acid gives a positive result.

Ethanedioic acid, HOOC–COOH. It is oxidised by warm acidified KMnO₄ to carbon dioxide. The purple permanganate is decolourised. This is the standard reaction for standardising KMnO₄.

The reaction is slow at first and speeds up as Mn²⁺ forms (autocatalysis). Warm the acid to about 60–70 °C.

The reaction is slow at room temperature, so the ethanedioic acid solution is warmed before the titration. The Mn²⁺ ions that form act as a catalyst, so the reaction is slow at first and then fast (autocatalysis). The end-point is the first permanent pale pink.

Worked example. 25.0 cm³ of 0.0400 mol dm⁻³ ethanedioic acid, acidified, requires 13.4 cm³ of KMnO₄(aq) for complete oxidation. Calculate the concentration of the KMnO₄. [3]

Amount of H₂C₂O₄ = 0.0250 × 0.0400 = 1.00 × 10⁻³ mol [1]. Ratio 2MnO₄⁻ : 5H₂C₂O₄, so amount of MnO₄⁻ = 1.00 × 10⁻³ × 2/5 = 4.00 × 10⁻⁴ mol [1]. Concentration = 4.00 × 10⁻⁴ / 0.0134 = 0.0299 mol dm⁻³ [1].

4. Making Acyl Chlorides

An acyl chloride has the group –COCl: it is a carboxylic acid with the –OH replaced by –Cl. The general formula is RCOCl. Ethanoyl chloride is CH₃COCl and benzoyl chloride is C₆H₅COCl. Carboxylic acids are fairly unreactive, so they are first converted into acyl chlorides, which are far more reactive. Three reagents are used.

Each reagent replaces the –OH of the acid with –Cl. SOCl₂ gives only gaseous by-products.

Steamy white fumes of HCl are seen. The reaction with SOCl₂ is the cleanest because SO₂ and HCl are both gases and escape, leaving the acyl chloride as the only liquid. With PCl₅ the liquid POCl₃ has to be separated by distillation.

Worked example. Write an equation for the reaction of propanoic acid with SOCl₂, and state one observation. [2]

CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl [1]. Steamy white fumes (HCl) / a gas is given off [1].

5. Reactions of Acyl Chlorides

The carbonyl carbon in an acyl chloride is bonded to two very electronegative atoms, O and Cl. It carries a large δ+ charge and is attacked quickly by nucleophiles, which donate a lone pair. When a nucleophile reacts, the C–Cl bond breaks and HCl is released (white fumes). The general reaction is:

All five reactions take place at room temperature and are fast.

With ammonia and amines, the HCl made is a gas that immediately reacts with unreacted NH₃ or amine, which is a base. So the equations use two moles of ammonia or amine:

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃⁺Cl⁻

For this reason you see much less white HCl fume with ammonia and amines than with water or alcohols.

Worked example. Write an equation for the reaction of propanoyl chloride with propylamine and name the organic product. [2]

CH₃CH₂COCl + CH₃CH₂CH₂NH₂ → CH₃CH₂CONHCH₂CH₂CH₃ + HCl [1] (or with 2 mol amine and the ammonium salt). The product is N-propylpropanamide [1].

The acyl chloride reactions are much faster than the corresponding reactions of the carboxylic acid, and they go to completion. They are the best routes to esters and amides in the laboratory and in industry.

6. The Addition–Elimination Mechanism

All five reactions follow the same two-stage pattern, called nucleophilic addition–elimination (sometimes called a condensation reaction, because a small molecule, HCl, is lost).

1. Addition. A lone pair on the nucleophile (O or N) attacks the δ+ carbonyl carbon. The C=O π bond breaks and the electrons move onto oxygen, giving a tetrahedral intermediate with O⁻.
2. Elimination. The lone pair on O⁻ re-forms the C=O bond and the C–Cl bond breaks, so Cl⁻ leaves. The positively charged atom (O⁺ or N⁺) loses H⁺, and H⁺ + Cl⁻ form HCl.

Hydrolysis (water as the nucleophile).

Water adds to the carbonyl carbon; HCl is then eliminated, leaving ethanoic acid.

Reaction with a primary amine (nitrogen as the nucleophile). The mechanism is the same. The amine has a lone pair on N. After Cl⁻ leaves, a second molecule of amine removes H⁺ from N⁺, giving the substituted amide and an alkylammonium chloride.

Methylamine attacks the δ+ carbon; the elimination stage gives N-methylethanamide. A second amine takes the H⁺.

Worked example. Describe, with curly arrows, the mechanism for the reaction of ethanoyl chloride with ammonia. [4]

Dipoles C^δ+=O^δ− and curly arrow from the N lone pair of NH₃ to the carbonyl carbon [1]; curly arrow from the C=O π bond to O [1]; intermediate with O⁻, Cl, CH₃ and N⁺H₃ on the central carbon [1]; curly arrow from O⁻ back to form C=O and curly arrow from C–Cl to Cl, giving CH₃CONH₂ (after loss of H⁺) and Cl⁻ [1].

7. Esters from Acyl Chlorides

An acyl chloride reacts with an alcohol or a phenol to give an ester and HCl. The name of the ester has the alkyl or aryl group from the alcohol or phenol first, and the part from the acid second.

Ethyl ethanoate. CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl. Room temperature, no catalyst, vigorous, complete (Topic 32). Compared with making the ester from ethanoic acid (concentrated H₂SO₄, reflux, reversible), the acyl chloride route is faster, needs no catalyst and gives a higher yield.

Phenyl benzoate. An acyl chloride is the only practical route to the ester of a phenol, because phenol does not form an ester directly with a carboxylic acid. Benzoyl chloride reacts with phenol to give phenyl benzoate.

Phenol is a weak nucleophile. In NaOH(aq) it forms the phenoxide ion, which attacks the δ+ carbon faster.

Worked example. Name the two reactants and the product when phenyl benzoate is made, and state the conditions. [3]

Benzoyl chloride and phenol [1]; product phenyl benzoate [1]; room temperature (with NaOH(aq) to make the phenoxide ion if needed) [1].

8. Ease of Hydrolysis

Acyl chlorides, chloroalkanes and chloroarenes all contain a C–Cl bond, but their reactivity towards water is very different. The order of ease of hydrolysis is:

The more positive the carbon and the weaker the bond, the faster the hydrolysis.

• Acyl chloride (e.g. CH₃COCl): the carbon is bonded to O and Cl, so it has a large δ+ charge. Even a neutral water molecule attacks it. The reaction is vigorous with cold water and produces HCl fumes.
• Chloroalkane (e.g. CH₃CH₂Cl): the carbon is bonded to one Cl only, so the δ+ charge is smaller. Water is too weak; the stronger nucleophile OH⁻ (NaOH(aq)) and heating under reflux are needed.
• Chloroarene (e.g. C₆H₅Cl): a lone pair on Cl overlaps with the ring π system, so the C–Cl bond has partial double-bond character and is stronger, and the electron-rich ring repels nucleophiles (Topic 31). It does not hydrolyse under these conditions.

A simple test shows the order. Add AgNO₃(aq) to each: the acyl chloride gives an immediate white precipitate of AgCl (it hydrolyses at once and releases Cl⁻); the chloroalkane gives no precipitate until it has been warmed with alkali; the chloroarene gives none at all.

Worked example. Explain the relative ease of hydrolysis of chlorobenzene, butanoyl chloride and 1-chlorobutane. [4]

Butanoyl chloride is hydrolysed most easily [1]: the carbon is bonded to both O and Cl, so it has a large δ+ charge and is attacked by water [1]. 1-Chlorobutane is hydrolysed more slowly [1]: its carbon is bonded to Cl only, so the δ+ is smaller. Chlorobenzene is not hydrolysed [1]: a Cl lone pair is delocalised into the ring, so the C–Cl bond has partial double-bond character and is stronger, and the ring repels the nucleophile.

9. Quick Sheet and Checklist

• [ ] I can state the reagents and conditions for making benzoic acid from methylbenzene.
• [ ] I can explain why carboxylic acids are stronger than phenols and alcohols.
• [ ] I can explain the effect of chlorine atoms on the strength of a carboxylic acid.
• [ ] I can write equations and give observations for the oxidation of methanoic and ethanedioic acids, and use the 2 : 5 ratio.
• [ ] I can give the three reagents that make an acyl chloride and write each equation.
• [ ] I can write the products of an acyl chloride with water, an alcohol, phenol, ammonia, and primary and secondary amines.
• [ ] I can draw the addition–elimination mechanism with curly arrows.
• [ ] I can explain why acyl chlorides hydrolyse more easily than chloroalkanes and chloroarenes.