Carboxylic Acids and Acyl Chlorides

This guide covers subtopics 33.1, Carboxylic acids, 33.2, Esters, and 33.3, Acyl chlorides, from Topic 33, Carboxylic acids and derivatives, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content, combined here because 33.2 alone is a single narrow outcome, and acyl chlorides are both a route to and a product from carboxylic acid chemistry.

Before studying this

This resource assumes carboxylic acid production and reactions from Carboxylic Acids and Esters, the addition-elimination mechanism vocabulary from A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape, and benzene’s electrophilic substitution chemistry from Arenes and Halogenoarenes for the benzoic acid production route below.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 33

33.1 Carboxylic acids — recalling the production of benzoic acid from an alkylbenzene; describing the reaction of carboxylic acids with PCl₃/heat, PCl₅ or SOCl₂ to form acyl chlorides; recognising the further oxidation of methanoic acid and ethanedioic acid; describing and explaining the relative acidities of carboxylic acids, phenols and alcohols, and of chlorine-substituted carboxylic acids.

33.2 Esters — recalling the production of esters from alcohols and acyl chlorides.

33.3 Acyl chlorides — recalling the production of acyl chlorides from carboxylic acids; describing hydrolysis, and reactions with alcohols, phenol, ammonia and primary/secondary amines; describing the addition-elimination mechanism; explaining the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and halogenoarenes.

Producing benzoic acid

Extending the side-chain oxidation reaction from Arenes and Halogenoarenes, hot alkaline KMnO₄ followed by dilute acid oxidises the side-chain of an alkylbenzene all the way to a carboxylic acid directly on the ring, regardless of how long the original side-chain was:

C₆H₅CH₃ (methylbenzene) —[hot alkaline KMnO₄, then dilute acid]→ C₆H₅COOH (benzoic acid)

Further oxidation of carboxylic acids

Most carboxylic acids resist further oxidation, but two structurally unusual ones don’t:

Methanoic acid, HCOOH, is unusual because it retains an aldehyde-like H atom directly on its carbonyl carbon (it can be thought of as sitting between an aldehyde and a normal carboxylic acid structurally). It gives a positive result with Fehling’s reagent, Tollens’ reagent, acidified KMnO₄ or acidified K₂Cr₂O₇ — oxidised further to carbon dioxide and water, just as an aldehyde would be.

Ethanedioic acid, HOOCCOOH, is oxidised by warm acidified KMnO₄ to carbon dioxide — this is, in fact, the same MnO₄⁻/C₂O₄²⁻ reaction already met in Transition Elements: Properties, Complexes and Redox Chemistry, since the ethanedioate ion, C₂O₄²⁻, is simply ethanedioic acid’s conjugate base.

Relative acidity of carboxylic acids, phenols and alcohols

Carboxylic acids are considerably more acidic than phenols, which are in turn more acidic than alcohols — the same three-way comparison from Phenol: Reactions and Acidity, now extended to include carboxylic acids at the top.

A carboxylate ion, RCOO⁻, has its negative charge delocalised equally over both oxygen atoms of the carboxylate group (both C–O bonds become equivalent, each with partial double-bond character) — a more effective, more symmetrical delocalisation than a phenoxide ion’s charge spreading unevenly around a large aromatic ring. This stronger stabilisation of the conjugate base is why carboxylic acids are more acidic than phenols, which are in turn more acidic than alcohols (no delocalisation available at all).

Chlorine-substituted carboxylic acids are more acidic still. Chlorine is strongly electronegative and withdraws electron density inductively through the carbon chain, which further stabilises the negative charge on the carboxylate ion (spreading it partly onto the electronegative chlorine’s vicinity) — the more chlorine atoms present, and the closer they are to the carboxyl group, the greater this effect, and the more acidic the compound. For example, trichloroethanoic acid (Cl₃CCOOH) is substantially more acidic than ethanoic acid (CH₃COOH).

Esters from acyl chlorides

Alcohols react with acyl chlorides at room temperature to form an ester, already introduced in Phenol: Reactions and Acidity for a simple alcohol. The same reaction works with phenol as the nucleophile too:

CH₃COCl + C₆H₅OH → CH₃COOC₆H₅ (phenyl benzoate, if from benzoyl chloride)

• HCl

giving an aryl ester — useful precisely because phenol reacts only slowly and reversibly with carboxylic acids directly, so the acyl chloride route is the practical way to make an ester from a phenol.

Acyl chlorides: production and reactions

Producing acyl chlorides. Carboxylic acids react with PCl₃ (plus heat), PCl₅, or SOCl₂ to form the corresponding acyl chloride — the same chlorinating-reagent trio met for converting an alcohol’s –OH to –Cl, now applied to a carboxylic acid’s –OH.

Reactions of acyl chlorides, all at room temperature, all releasing HCl:

Reagent	Product
Water (hydrolysis)	carboxylic acid + HCl
Alcohol	ester + HCl
Phenol	aryl ester + HCl
Ammonia	amide + HCl
Primary or secondary amine	substituted amide + HCl

All five reactions share the same underlying mechanism.

The addition-elimination mechanism

Acyl chlorides are highly reactive because chlorine is a good leaving group and the carbonyl carbon is strongly electrophilic (both the electronegative oxygen and the electronegative chlorine pull electron density away from it). The mechanism proceeds in two steps, illustrated here for hydrolysis:

Step 1 (addition) — a lone pair on the nucleophile (here, the oxygen of a water molecule) attacks the electrophilic carbonyl carbon. Simultaneously, the C=O π electrons move onto the oxygen, forming a negatively charged tetrahedral intermediate with both the original Cl and the new nucleophile attached to the same carbon.

Step 2 (elimination) — the C–O⁻ electrons re-form the C=O π bond as chloride, Cl⁻, leaves as the leaving group, regenerating a neutral carbonyl compound. A final rapid proton transfer (from the newly formed –OH₂⁺ group, if water was the nucleophile) gives the neutral carboxylic acid product and releases H⁺, which combines with the departed Cl⁻ to give HCl overall.

The same two-step pattern applies with an alcohol, phenol, ammonia or amine as the nucleophile in step 1 — only the identity of the attacking nucleophile (and therefore the final product: ester, aryl ester, or amide) changes.

Relative ease of hydrolysis: acyl chlorides vs alkyl chlorides vs halogenoarenes

Ranking these three C–Cl-containing compounds by how readily they undergo substitution/hydrolysis, from fastest to essentially unreactive:

Acyl chlorides hydrolyse fastest, even reacting with cold water within seconds — the carbonyl carbon is made strongly electrophilic by both the electronegative oxygen (C=O) and the electronegative chlorine pulling electron density away from it together, and the addition-elimination mechanism proceeds readily because Cl⁻ is a good leaving group with nothing stabilising the starting C–Cl bond against attack.

Alkyl chlorides (e.g. chloroethane) hydrolyse much more slowly, needing warming with aqueous alkali — the carbon is electrophilic only because of the one polar C–Cl bond, a weaker pull than an acyl chloride’s combined carbonyl-plus-chlorine effect, and the mechanism (SN1 or SN2, see Halogenoalkanes) has a higher activation energy in comparison.

Halogenoarenes (e.g. chlorobenzene) essentially don’t hydrolyse under these conditions at all — as explained in Arenes and Halogenoarenes, the C–Cl bond is strengthened by lone-pair delocalisation into the ring, and nucleophilic attack at that carbon would have to disrupt the ring’s aromatic stabilisation.

Common mistakes

Trying to hydrolyse an acyl chloride the same slow way as an alkyl chloride. Acyl chlorides react with cold water almost immediately — questions describing a controlled, careful addition of water to an acyl chloride are testing awareness of just how vigorous this reaction is, not suggesting it needs warming or a catalyst.

Confusing nucleophilic addition (as with HCN and a carbonyl compound) with addition-elimination (as with acyl chlorides). Addition-elimination has the extra second step — a leaving group departs and the C=O bond re-forms — giving overall substitution, not simple addition across a double bond.

Assuming all carboxylic acids resist further oxidation equally. Methanoic and ethanedioic acid are genuine, syllabus-named exceptions, each for a specific structural reason (an aldehyde-like C–H, or being directly the conjugate base of the C₂O₄²⁻/MnO₄⁻ system) — don’t generalise “carboxylic acids don’t oxidise further” without those two exceptions in mind.

Explaining chlorine-substituted acid acidity without mentioning the inductive effect specifically. The reasoning is inductive electron withdrawal stabilising the carboxylate ion — not delocalisation (chlorine isn’t part of the delocalised carboxylate system itself, it acts through the σ-bond framework).

Quick revision checklist

• Benzoic acid: alkylbenzene + hot alkaline KMnO₄, then dilute acid
• Methanoic acid and ethanedioic acid are the two carboxylic acids that oxidise further (aldehyde-like C–H; C₂O₄²⁻/MnO₄⁻ system)
• Acidity order: carboxylic acid > phenol > alcohol; more Cl substituents → more acidic still (inductive withdrawal)
• Ester from acyl chloride: alcohol or phenol + acyl chloride → ester + HCl
• Acyl chloride production: carboxylic acid + PCl₃/PCl₅/SOCl₂
• Acyl chloride + water/alcohol/phenol/ammonia/amine, all room temperature, all release HCl, all addition-elimination
• Hydrolysis ease: acyl chloride (fastest) > alkyl chloride > halogenoarene (essentially unreactive)

Related resources

• Carboxylic Acids and Esters — the AS-level production and reactions this topic extends
• Phenol: Reactions and Acidity — the acidity comparison this topic completes with carboxylic acids
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — benzoic acid production and the halogenoarene hydrolysis comparison
• Amides and Amino Acids — the amide products of acyl chloride reactions with ammonia and amines
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.