A Level Chemistry: Carboxylic Acids and Acyl Chlorides — Revision Notes

Condensed for the final weeks. For the full explanation, use the Carboxylic Acids and Acyl Chlorides study guide.

Carboxylic acids

Why they are acidic: the carboxylate ion formed on losing H⁺ is stabilised by delocalisation of the negative charge over both oxygen atoms. That stabilisation makes the loss of H⁺ favourable, which alcohols cannot match.

Acid strength is increased by electron-withdrawing groups, which pull electron density away and stabilise the anion further. So chloroethanoic acid is stronger than ethanoic acid, and trichloroethanoic acid stronger still. Electron-donating alkyl groups have the opposite effect.

Reactions:

+ metal          ->  salt + hydrogen
+ metal oxide    ->  salt + water
+ alkali         ->  salt + water
+ carbonate      ->  salt + water + CARBON DIOXIDE   (effervescence)

The carbonate reaction is the identifying test. Effervescence with sodium carbonate distinguishes a carboxylic acid from a phenol or alcohol, because only the acid is strong enough to release CO₂.

Reduction: LiAlH₄ in dry ether reduces a carboxylic acid to a primary alcohol. NaBH₄ is not powerful enough — a common error.

Esterification: acid + alcohol, concentrated H₂SO₄ catalyst, heat under reflux. The reaction is reversible and slow, giving a modest equilibrium yield.

Producing benzoic acid: 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 the original side-chain length — C₆H₅CH₃ → C₆H₅COOH.

Further oxidation — two exceptions. Most carboxylic acids resist further oxidation, but methanoic acid (HCOOH) retains an aldehyde-like H atom on its carbonyl carbon, giving a positive result with Fehling’s, Tollens’, acidified KMnO₄ or K₂Cr₂O₇. Ethanedioic acid (HOOCCOOH) is oxidised by warm acidified KMnO₄ to CO₂ — the same MnO₄⁻/C₂O₄²⁻ reaction seen in transition-metal redox chemistry.

Acyl chlorides

Made from the carboxylic acid using SOCl₂ (or PCl₅).

Acyl chlorides are far more reactive than carboxylic acids because the carbonyl carbon is bonded to two electronegative atoms (O and Cl) and, unlike the –OH oxygen of an acid, chlorine delocalises little lone-pair density into the C=O group, so the carbonyl carbon keeps a greater δ+ charge and is more readily attacked by nucleophiles; chloride is also a good leaving group.

All reactions produce HCl — visible as steamy white fumes, which is the practical identification.

Reagent	Product
Water	Carboxylic acid
Alcohol	Ester
Ammonia	Primary amide
Primary amine	N-substituted amide
Phenol	Ester (where direct esterification fails)

The reason acyl chlorides matter synthetically is exactly this: esterification with an acid is reversible and low-yielding, but with an acyl chloride the reaction is fast, irreversible and near-quantitative at room temperature. Phenols in particular react only slowly and reversibly with carboxylic acids directly, so an acyl chloride is used instead to make esters of phenol reliably.

The mechanism is nucleophilic addition–elimination, in two steps. Addition: a lone pair on the nucleophile attacks the electrophilic carbonyl carbon as the C=O π electrons move onto oxygen, forming a negatively charged tetrahedral intermediate. Elimination: the C–O⁻ electrons re-form the C=O π bond as Cl⁻ leaves, and a rapid proton transfer gives the neutral product plus HCl.

Relative ease of hydrolysis: acyl chlorides hydrolyse fastest (even with cold water) because the carbonyl carbon is made strongly electrophilic by both the C=O oxygen and the chlorine together; alkyl chlorides hydrolyse far more slowly, needing warm aqueous alkali, since only one polar C–Cl bond makes the carbon electrophilic; halogenoarenes essentially don’t hydrolyse at all, because lone-pair delocalisation into the ring strengthens the C–Cl bond.

Acid anhydrides

Less vigorous than acyl chlorides, cheaper, and produce a carboxylic acid rather than corrosive HCl — which is why ethanoic anhydride is used industrially to make aspirin rather than ethanoyl chloride.

Exam traps

• Explaining acidity without delocalisation of the carboxylate ion.
• Getting the electron-withdrawing effect backwards.
• Using NaBH₄ to reduce a carboxylic acid.
• Forgetting that only carboxylic acids give CO₂ with carbonates.
• Not explaining why acyl chlorides are preferred for esterification.
• Omitting the HCl produced in acyl chloride reactions.

Self-test

1. Why is a carboxylic acid more acidic than an alcohol?
2. Why is trichloroethanoic acid stronger than ethanoic acid?
3. What test distinguishes a carboxylic acid from a phenol?
4. Why are acyl chlorides more reactive than carboxylic acids?
5. Why is an acyl chloride preferred to a carboxylic acid for making an ester?
6. Why does methanoic acid give a positive Fehling’s/Tollens’ test when most carboxylic acids do not?
7. Rank acyl chlorides, alkyl chlorides and halogenoarenes by ease of hydrolysis, and explain the ranking.

Answers: 1. The carboxylate ion formed is stabilised by delocalisation of the negative charge across both oxygen atoms, making loss of H⁺ favourable. 2. The three electron-withdrawing chlorine atoms pull electron density away from the carboxylate, stabilising the anion further. 3. Adding sodium carbonate — only the carboxylic acid produces effervescence of carbon dioxide. 4. Chlorine is strongly electronegative and a good leaving group, so the carbonyl carbon is more δ+ and more readily attacked by nucleophiles. 5. Esterification with a carboxylic acid is reversible, slow and low-yielding, whereas the acyl chloride reacts rapidly, irreversibly and in near-quantitative yield at room temperature — and it works reliably with phenols, which react only slowly and reversibly with carboxylic acids directly. 6. It retains an aldehyde-like H atom directly on its carbonyl carbon, so it can be oxidised further just as an aldehyde would be. 7. Acyl chlorides fastest (electrophilic carbon activated by both C=O and Cl), then alkyl chlorides (only one polar C–Cl bond), then halogenoarenes (barely react at all, since lone-pair delocalisation into the ring strengthens the C–Cl bond).