AS Chemistry: Carboxylic Acids and Esters — Revision Notes

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

Carboxylic acids

Preparation:

primary alcohol + K2Cr2O7/H2SO4, REFLUX   ->  carboxylic acid
aldehyde + K2Cr2O7/H2SO4, reflux          ->  carboxylic acid
nitrile + dilute HCl(aq), reflux          ->  carboxylic acid (same number of carbons)

Reflux versus distil is the single most commonly dropped mark. Refluxing returns the aldehyde to the flask so oxidation continues to the acid; distilling removes the aldehyde as it forms, stopping at that stage.

Hydrolysing a nitrile does not change the carbon count. The nitrile carbon (–C≡N) becomes the carboxyl carbon (–COOH), so ethanenitrile (2 carbons) gives ethanoic acid (2 carbons). The chain was already lengthened one step earlier, when cyanide substituted into the halogenoalkane to make the nitrile — that step adds one carbon, so the final acid has one more carbon than the original halogenoalkane, not than the nitrile.

Why they are acidic: losing H⁺ gives a carboxylate ion in which the negative charge is delocalised over both oxygens, stabilising it. Alcohols cannot do this, which is why they are not acidic.

A Level extension (Topic 33.1.5): electron-withdrawing groups increase acid strength by pulling electron density away and stabilising the anion further — chloroethanoic acid is stronger than ethanoic acid. This comparison is not an AS 18.1 outcome; it’s included here as forward context.

Reactions:

+ reactive metal   ->  salt + hydrogen
+ metal oxide      ->  salt + water
+ alkali           ->  salt + water
+ CARBONATE        ->  salt + water + CO2   (effervescence)

Worked equations: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ (reactive metal); 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (carbonate, the effervescence test).

The carbonate test is the identification. Only a carboxylic acid is acidic enough to release CO₂ from a carbonate, distinguishing it from phenols and alcohols.

Reduction: LiAlH₄ in dry ether gives a primary alcohol — CH₃COOH + 4[H] → CH₃CH₂OH + H₂O. NaBH₄ will not work; LiAlH₄ is the stronger reducing agent, needed because a carboxylic acid is harder to reduce than an aldehyde or ketone.

Esters

Esterification:

carboxylic acid + alcohol, conc. H2SO4 catalyst, reflux  <=>  ester + water

The reaction is reversible, so the yield is limited by equilibrium. Removing water or using excess alcohol shifts it right.

Naming: the alcohol part first, then the acid part. Ethanol + propanoic acid gives ethyl propanoate. Getting the order backwards is a frequent error — the part from the alcohol supplies the alkyl name.

A Level extension: acyl chlorides (Topic 33, not examined at AS) offer a faster route to esters — an acyl chloride plus an alcohol reacts rapidly, irreversibly and near-quantitatively at room temperature, releasing HCl. See the Carboxylic Acids and Acyl Chlorides study guide for the full A Level treatment.

Hydrolysis of esters

Conditions	Products	Reversible?
Dilute acid, reflux	Carboxylic acid + alcohol	Yes
Dilute alkali (NaOH), reflux	Carboxylate salt + alcohol	No

Alkaline hydrolysis goes to completion because the carboxylate salt formed cannot react back with the alcohol — the acid is removed from the equilibrium as its anion. That is why saponification uses alkali, and it is the standard “why is alkaline hydrolysis preferred” answer.

Worked example: naming hydrolysis products

Ethyl ethanoate, CH₃COOC₂H₅, is heated under reflux with excess aqueous sodium hydroxide. Name the two organic products.

Alkaline hydrolysis breaks the ester into the alcohol and the carboxylate salt, not the free acid, since NaOH is in excess: the acyl (CH₃CO–) part becomes sodium ethanoate, CH₃COONa, and the alkoxy (–OC₂H₅) part becomes ethanol, C₂H₅OH.

CH3COOC2H5 + NaOH  ->  CH3COONa + C2H5OH

Carboxylic acids can also be made by hydrolysing a nitrile with dilute acid or dilute alkali followed by acidification, or by hydrolysing an ester with dilute acid or dilute alkali and heat followed by acidification — the reverse of esterification.

Uses

Esters are volatile with pleasant, fruity smells, so they are used in flavourings and perfumes. They are also good solvents, and are used as plasticisers.

Vegetable oils and animal fats are triesters of glycerol (propane-1,2,3-triol) with long-chain fatty acids. Alkaline hydrolysis gives soap (the sodium salts of the fatty acids) plus glycerol — the saponification reaction.

Background (beyond the 9701 syllabus, not examinable at this level): biodiesel is made by reacting these triesters with methanol, producing methyl esters of the fatty acids — a transesterification reaction, since one ester (the triglyceride) is converted into a different set of esters using a different alcohol.

A common mistake: treating acid and alkaline hydrolysis as the same reaction with different reagents. One is a reversible equilibrium; the other goes to completion — this difference is often exactly what a question is testing.

Exam traps

• Refluxing when distillation is needed, or vice versa.
• Naming an ester with the acid part first.
• Saying alkaline hydrolysis is reversible.
• Using NaBH₄ on a carboxylic acid.
• Forgetting the concentrated H₂SO₄ catalyst in esterification.
• Not stating that esterification is reversible when explaining low yield.

Self-test

1. What single change converts an alcohol oxidation from giving an aldehyde to giving an acid?
2. Why is a carboxylic acid acidic but an alcohol not?
3. Name the ester from ethanol and propanoic acid.
4. Why does alkaline hydrolysis of an ester go to completion?
5. What are soap and glycerol produced from, and by what reaction?
6. Write the equation for ethyl ethanoate hydrolysed by excess NaOH, and name both organic products.

Answers: 1. Reflux instead of distilling, so the aldehyde is returned to the flask and oxidised further. 2. The carboxylate ion formed is stabilised by delocalisation of the negative charge over both oxygen atoms; an alkoxide ion has no such stabilisation. 3. Ethyl propanoate. 4. The carboxylate salt formed cannot react back with the alcohol, so the product is removed from the equilibrium and the reaction goes to completion. 5. From the alkaline hydrolysis (saponification) of triesters of glycerol with long-chain fatty acids, found in vegetable oils and animal fats. 6. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH; the products are sodium ethanoate and ethanol.