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Carboxylic acids and esters

Key idea: A carboxylic acid and an alcohol form an ester and water reversibly. Ester hydrolysis gives an alcohol plus the acid in acidic conditions, or its salt in alkaline conditions.

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  • GCE A-Level H1 Chemistry 2027
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Vinegar fizzes with baking soda because it contains a carboxylic acid, ethanoic acid. Many fruity smells, such as pear drops, come from esters, which are made from carboxylic acids. This lesson covers how carboxylic acids form salts and esters, and how esters are split up again by hydrolysis.

Check your understanding 1 Two parts of an ester

In CH₃CH₂COOCH₃, count the carbons in the acid part (including the C=O carbon) and in the group attached to the single-bonded oxygen.

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The acid part has 3 carbons and comes from propanoic acid. The alcohol part has 1 carbon and comes from methanol. Name the alcohol part first: methyl propanoate.

Carboxylic acids and esters

A carboxylic acid contains the –COOH group, as in ethanoic acid, CH₃COOH. An ester contains the –COO– group, with a carbon group joined to the single-bonded oxygen, as in ethyl ethanoate, CH₃COOCH₂CH₃. To name an ester, give the part from the alcohol first, then the part from the acid: ethyl + ethanoate. The carbonyl carbon belongs to the acid part. For example, methyl methanoate, HCOOCH₃, has a one-carbon acid part and a one-carbon alcohol part.

Salts

Carboxylic acids are weak acids. With an alkali, the acid gives its proton to the hydroxide ion, forming a salt and water:

CH₃COOH + NaOH → CH₃COONa + H₂O

With a carbonate, a salt, water and carbon dioxide form, so the mixture fizzes:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Check your understanding 2 Gas and equation ratio

Propanoic acid reacts with potassium carbonate. Choose the observation and gas, then find the number of acid molecules needed per carbonate ion in the balanced equation.

Observation
Gas
Show answer

You see effervescence as carbon dioxide forms. Each carbonate ion needs two protons, so two propanoic acid molecules react per carbonate ion.

Guided practice 1

Propanoic acid and potassium carbonate

About 4 min

Problem

Propanoic acid, CH₃CH₂COOH, is added to aqueous potassium carbonate. State what you would see, and write the equation.

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Hints

Hint 1

products

Which familiar acid reaction makes a gas from a carbonate?
Hint 2

balance

One carbonate ion reacts with two acid molecules.
Show solution step by step

  1. Products

    Method

    Name the products.

    Reason

    The acid gives its proton to the carbonate.

    Working

    Answer

    Potassium propanoate, water and carbon dioxide; effervescence is seen.

  2. Equation

    Working

    Answer

    2CH₃CH₂COOH + K₂CO₃ → 2CH₃CH₂COOK + H₂O + CO₂

Esters

A carboxylic acid reacts with an alcohol to form an ester and water, when heated with concentrated sulfuric acid as a catalyst. This is a condensation reaction: two molecules join, and a small molecule, water, is lost. The reaction is reversible.

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Check your understanding 3 Choose the ester reactants

Choose the acid and alcohol for methyl propanoate, and the catalyst for the heated acid-catalysed esterification.

Carboxylic acid
Alcohol
Acid catalyst
Show answer

Use propanoic acid and methanol, heated with concentrated sulfuric acid as a catalyst. DCC is a coupling reagent, not the acid catalyst for this method.

Guided practice 2

Making methyl propanoate

About 4 min

Problem

Name the carboxylic acid and the alcohol needed to make methyl propanoate. Give the catalyst and conditions, and write the equation.

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Hints

Hint 1

the name

Which part of an ester name comes from its alcohol?
Hint 2

conditions

Recall the catalyst for heated acid-catalysed esterification.
Show solution step by step

  1. Reactants

    Method

    Split the name of the ester.

    Reason

    The alcohol part is named first.

    Working

    Answer

    Propanoic acid and methanol.

  2. Conditions and equation

    Working

    Answer

    Heat with concentrated H₂SO₄ catalyst: CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O

Hydrolysis of esters

Hydrolysis splits a molecule using water. An ester is hydrolysed by heating with aqueous acid or aqueous alkali:

  • with aqueous acid, the products are the carboxylic acid and the alcohol. This is the reverse of esterification:

    CH₃COOCH₂CH₃ + H₂O ⇌ (H +, heat) CH₃COOH + CH₃CH₂OH

  • with aqueous alkali, the products are the alcohol and the salt of the acid, because the acid reacts with the alkali as soon as it forms:

    CH₃COOCH₂CH₃ + NaOH → (heat) CH₃COONa + CH₃CH₂OH

Spot the mistake 3

Hydrolysis in alkali

About 4 min

Learner claim

A learner says that heating methyl ethanoate, CH₃COOCH₃, with aqueous sodium hydroxide gives methanol and ethanoic acid. Correct the claim.

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Alcohol formed
Other product
Show solution step by step

  1. Split the ester

    Method

    Identify the two parts.

    Reason

    Hydrolysis breaks the ester link.

    Working

    Answer

    Methyl ethanoate splits into methanol and an ethanoate part.

  2. The alkali

    Working

    Answer

    Ethanoic acid would react with the sodium hydroxide, so the product is sodium ethanoate, not ethanoic acid: CH₃COOCH₃ + NaOH → CH₃COONa + CH₃OH

Try it yourself 4

Getting the acid back

Minimal support

Problem

Ethyl propanoate, CH₃CH₂COOCH₂CH₃, is heated with aqueous sodium hydroxide. Enough dilute hydrochloric acid is then added to make the mixture acidic. Choose the organic products after each step.

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After heating with NaOH
After adding HCl

Hints

Hint 1

step 1

What does alkaline hydrolysis give?
Hint 2

step 2

What does a strong acid do to a carboxylate ion?
Show solution step by step

  1. After heating with NaOH

    Method

    Apply alkaline hydrolysis.

    Reason

    The acid part is present as its salt in alkali.

    Working

    Answer

    Ethanol and sodium propanoate.

  2. After adding HCl

    Working

    Answer

    The propanoate ions gain a proton: CH₃CH₂COO⁻ + H⁺ → CH₃CH₂COOH. The products are ethanol and propanoic acid.

Check your understanding 4 Which conditions?

After an ester is hydrolysed, the mixture contains ethanol, and the main acid-derived species is ethanoate rather than ethanoic acid. Was the hydrolysis done in acid or in alkali? Explain.

Show answer

In alkali. The hydroxide ions remove the proton from ethanoic acid as it forms, leaving ethanoate ions. In acid, ethanoic acid would remain.

Common mistakes

  • Naming an ester in the wrong order. The alcohol part comes first: ethyl ethanoate comes from ethanol and ethanoic acid.
  • Leaving out carbon dioxide when an acid reacts with a carbonate.
  • Giving the carboxylic acid, instead of its salt, as a product of alkaline hydrolysis.
  • Mixing up the heated sulfuric-acid method for esterification with the DCC method for amide formation.
  • Forgetting that esterification is reversible.

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