Carboxylic acids and derivatives

Cambridge International AS & A Level Chemistry 9701 · A Level

All three subtopics here are printed under "A Level subject content" in the 9701 syllabus and every objective carries the tier "A Level". None of it is AS, and it is a separate topic from the AS topic 18 of the same name. It is examined on Paper 4. Paper 1 is the AS multiple-choice paper and the whole 9701 bank on this site comes from it, so no practice here is tagged to this topic.

Making benzoic acid

Heat methylbenzene with hot alkaline potassium manganate(VII), then acidify with dilute acid:

C₆H₅CH₃ → C₆H₅COO⁻ → C₆H₅COOH

The alkaline conditions produce the benzoate salt, so the acidification step is not optional; without it the product is sodium benzoate and not benzoic acid.

As noted in topic 30, the whole side-chain is oxidised back to a single COOH group whatever its original length, so ethylbenzene and propylbenzene also give benzoic acid.

Making acyl chlorides

A carboxylic acid is converted to an acyl chloride by any of three reagents:

For example:

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

SOCl₂ is the best laboratory choice, because both its by-products are gases and simply escape, leaving the acyl chloride pure with no separation step. That is a favourite short question.

The OH of a carboxylic acid is replaced by Cl. Note that these same three reagents also convert an alcohol to a halogenoalkane, which is the AS reaction, and that the misting fumes of HCl produced with PCl₅ are the standard test for an OH group of either kind.

Further oxidation of two carboxylic acids

Most carboxylic acids are the end of the oxidation sequence and cannot be oxidised further. Two exceptions are named, and the reason in each case is structural.

Methanoic acid, HCOOH, is unusual because its COOH group carries a hydrogen where every other acid has an alkyl or aryl group, so it contains what is effectively an aldehyde group. It therefore behaves as an aldehyde as well as an acid, and is oxidised to carbon dioxide and water by:

• Fehling's reagent, giving a brick-red precipitate of copper(I) oxide,
• Tollens' reagent, giving a silver mirror,
• acidified KMnO₄, decolourised from purple, and
• acidified K₂Cr₂O₇, turning from orange to green.

The Fehling's and Tollens' results are the surprising ones, since those reagents are usually presented as tests that distinguish aldehydes from everything else, and methanoic acid gives a positive result with both.

Ethanedioic acid, HOOCCOOH, has its two COOH groups joined directly to each other, and warm acidified KMnO₄ oxidises it to carbon dioxide:

5HOOCCOOH + 2MnO₄⁻ + 6H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O

This is the titration met in topic 28, where it is autocatalysed by the Mn²⁺ produced.

Relative acidities

All three classes of compound in this part of the course contain an O-H group, and the acidity order is:

carboxylic acids > phenols > alcohols

with water sitting between phenol and ethanol.

The explanation is the same question asked three times: how well is the negative charge on the conjugate base stabilised?

• Carboxylic acid. The carboxylate ion has its negative charge delocalised over both oxygen atoms and the carbon between them, so the charge is spread over a small, highly electronegative group. Both C-O bonds become equal in length, intermediate between a single and a double bond, which is the direct evidence for the delocalisation. This is the most effective stabilisation of the three, so carboxylic acids are the strongest.
• Phenol. The phenoxide ion has its charge delocalised into the benzene ring. That spreads the charge over a larger volume, but the ring carbons are much less electronegative than oxygen, so the stabilisation is less effective than in a carboxylate.
• Alcohol. The alkoxide ion has its charge localised on the oxygen, and the electron-donating alkyl group pushes further electron density onto it, intensifying the charge. Least stable conjugate base, weakest acid.

The practical consequence is the pair of tests that separates them: a carboxylic acid reacts with sodium carbonate to give carbon dioxide; a phenol does not react with carbonate but does dissolve in sodium hydroxide; an alcohol does neither.

Chlorine-substituted carboxylic acids

Substituting chlorine atoms near the COOH group makes the acid stronger, and the effect is large.

Why. Chlorine is electronegative, so it exerts an electron-withdrawing inductive effect, pulling electron density along the carbon chain away from the carboxylate group. That spreads the negative charge further and stabilises the anion, so the acid dissociates more readily.

Two refinements the syllabus expects:

• More chlorine atoms means a stronger acid, because the inductive effects add. Trichloroethanoic acid is a strong acid, roughly as strong as some mineral acids.
• Chlorine closer to the COOH group means a stronger acid, because the inductive effect weakens with distance along the chain. 2-chlorobutanoic acid is a stronger acid than 4-chlorobutanoic acid.

The same reasoning run backwards explains why a longer alkyl group makes an acid slightly weaker: alkyl groups are electron donating, so they intensify the charge instead of spreading it.

Esters from acyl chlorides

An acyl chloride reacts with an alcohol or with a phenol at room temperature, with no catalyst, to give an ester and HCl:

CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl

giving ethyl ethanoate, and

C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ + HCl

giving phenyl benzoate.

Naming an ester takes practice: the part from the alcohol or phenol comes first, then the part from the acid with the ending -oate. So phenyl benzoate is made from phenol and benzoic acid, or from benzoyl chloride and phenol.

The acyl chloride route is the only practical way to make a phenyl ester, because phenol is too poor a nucleophile to esterify with a carboxylic acid under the usual reflux conditions. That is a good reason to remember which route to choose in a synthesis question.

Reactions of acyl chlorides

All five occur at room temperature, all are vigorous, and all release HCl as steamy, misty fumes.

For example:

CH₃COCl + H₂O → CH₃COOH + HCl

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl

Note that with ammonia or an amine, an excess is used, because the HCl produced reacts with the base to form its salt. That is why the equation with ammonia is often written with two moles.

The addition-elimination mechanism

Every one of those five reactions follows the same two-stage mechanism, so learning it once covers the whole section.

The carbonyl carbon is electron deficient, because it is bonded to two electronegative atoms, the oxygen and the chlorine, and both pull electron density away. It is therefore strongly attractive to a nucleophile.

Stage 1, addition. The lone pair on the nucleophile attacks the carbonyl carbon. A curly arrow runs from the lone pair to the carbon, and a second curly arrow runs from the C=O pi bond onto the oxygen. The carbon becomes tetrahedral and the oxygen carries a negative charge: this is the tetrahedral intermediate.

Stage 2, elimination. The C=O double bond re-forms, with a curly arrow from the oxygen's negative charge back to the carbon, and the chloride ion is expelled, with a curly arrow from the C-Cl bond onto the chlorine. A proton is then lost from the attacking group, and the H⁺ and Cl⁻ leave as HCl.

The overall result is substitution, but achieved by addition followed by elimination rather than in a single step, which is exactly why the mechanism has that name.

Where the nucleophile is water, the product is the carboxylic acid; where it is an alcohol or phenol, an ester; where it is ammonia or an amine, an amide. Only the identity of the nucleophile changes.

Relative ease of hydrolysis

Acyl chloride > alkyl chloride (halogenoalkane) > halogenoarene (aryl chloride), from fastest to slowest.

• Acyl chloride, for example CH₃COCl. Hydrolyses violently on contact with cold water. The carbonyl carbon is bonded to two electronegative atoms, so it is highly electron deficient and very readily attacked by a nucleophile, and the chloride is a good leaving group.
• Alkyl chloride, for example CH₃CH₂Cl. Hydrolyses slowly, and only on warming with aqueous alkali. The carbon is attached to one electronegative atom, so it is only slightly electron deficient.
• Halogenoarene, for example C₆H₅Cl. Effectively does not hydrolyse under normal conditions. As set out in topic 31, the chlorine's lone pair is delocalised into the ring, giving the C-Cl bond partial double bond character and making it stronger, and the ring's electron density repels approaching nucleophiles.

Stated as one principle: the more electron deficient the carbon and the weaker the C-Cl bond, the faster the hydrolysis.

The practical test confirms the order. Add each compound to water, then aqueous silver nitrate. The acyl chloride gives a white precipitate immediately, the halogenoalkane gives one slowly on warming, and the halogenoarene gives none at all.

Common mistakes

• Forgetting the acidification step after oxidising an alkylbenzene, so the product is the benzoate salt.
• Saying every carboxylic acid can be oxidised further. Only methanoic and ethanedioic acid are named.
• Saying methanoic acid gives a negative Tollens' test. It gives a silver mirror, because it contains an aldehyde group.
• Putting phenol as more acidic than a carboxylic acid.
• Explaining acidity by the strength of the O-H bond only, and leaving out the stabilisation of the conjugate base.
• Saying chlorine makes an acid weaker, or that its position along the chain makes no difference.
• Saying alkyl groups withdraw electron density. They donate it, which is why longer-chain acids are slightly weaker.
• Naming an ester the wrong way round, so ethyl ethanoate becomes ethanoyl ethanoate or similar.
• Trying to make a phenyl ester by refluxing phenol with a carboxylic acid and sulfuric acid.
• Drawing the addition-elimination mechanism without the negative charge on the oxygen of the tetrahedral intermediate.
• Expelling the oxygen rather than the chloride in the elimination step.
• Writing that an acyl chloride needs heating with water. It reacts vigorously in the cold.

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