Topic 15 · page 19 of 28

Hydrolysis of esters

Heating an ester under reflux with dilute acid reverses esterification and gives an equilibrium. Heating it with aqueous sodium hydroxide goes to completion and gives the sodium salt of the acid and the alcohol.

13 min read15.15

Read first: Esterification

Two ways to break an ester 15.15

Hydrolysis means breaking a bond by reacting with water. An ester can be hydrolysed in acidic or alkaline solution, and the two are different in an important way.

Hydrolysis of ethyl ethanoate. Heating under reflux with dilute acid gives ethanoic acid and ethanol in an equilibrium, so the reaction is incomplete. Heating under reflux with aqueous sodium hydroxide gives sodium ethanoate and ethanol and goes to completion.
Acid hydrolysis is reversible. Alkaline hydrolysis goes to completion.

Acid hydrolysis 15.15

Heat the ester under reflux with dilute acid, such as dilute sulfuric or hydrochloric acid. The acid is the catalyst and the water in the solution is the reactant.

CHX3COOCHX2CHX3+HX2O⇌CHX3COOH+CHX3CHX2OH \ce{CH3COOCH2CH3 + H2O <=> CH3COOH + CH3CH2OH}

This is the reverse of esterification. It is reversible and ends at an equilibrium, so the hydrolysis is incomplete: all four substances are present. A large excess of water pushes the position towards the products.

Alkaline hydrolysis 15.15

Heat the ester under reflux with aqueous sodium hydroxide.

CHX3COOCHX2CHX3+NaOH→CHX3COONa+CHX3CHX2OH \ce{CH3COOCH2CH3 + NaOH -> CH3COONa + CH3CH2OH}

The products are the sodium salt of the carboxylic acid, here sodium ethanoate, and the alcohol. The reaction is not reversible and goes to completion, because the acid is removed as its carboxylate ion, which does not react with the alcohol.

To obtain the carboxylic acid itself, acidify the product with dilute hydrochloric acid after the reaction.

CHX3COOX−(aq)+HX+(aq)→CHX3COOH(aq) \ce{CH3COO-(aq) + H+(aq) -> CH3COOH(aq)}
Acid hydrolysis Alkaline hydrolysis
reagent dilute H₂SO₄ or HCl NaOH(aq)
conditions reflux reflux
products carboxylic acid + alcohol sodium salt of the acid + alcohol
extent reversible, equilibrium complete

To find the products of hydrolysing any ester, split it at the C–O bond next to the C=O. The acyl part becomes the acid (or its salt) and the alkyl part becomes the alcohol. Methyl propanoate gives propanoic acid and methanol.

PracticalHydrolysis of an ester

This is the further suggested practical. Wear eye protection. Sodium hydroxide solution is corrosive, and the mixture is flammable so use a hot water bath or electric heater.

  1. Put about 3 cm³ of ethyl ethanoate in a round-bottomed flask with 20 cm³ of sodium hydroxide solution and anti-bumping granules.
  2. Fit a reflux condenser and heat gently for about 20 to 30 minutes.
  3. The ester is a separate layer above the aqueous solution at the start. As the reaction goes on the layer disappears, because the products are soluble in water.
  4. Cool the flask and add dilute hydrochloric acid dropwise until the solution is acidic. The ethanoic acid is released, which you can smell.

Quantities in the practical are typical amounts. Use the volumes on your worksheet.

Check yourself

  1. Write an equation for the acid hydrolysis of ethyl propanoate and name the products.
  2. Write an equation for the alkaline hydrolysis of methyl ethanoate with sodium hydroxide.
  3. Explain why alkaline hydrolysis goes to completion but acid hydrolysis does not.
  4. Name the products of heating propyl methanoate under reflux with sodium hydroxide solution.
  5. In the practical, how can you tell that the hydrolysis is complete?

Answers cover these until you have tried the questions

  1. CHX3CHX2COOCHX2CHX3+HX2O⇌CHX3CHX2COOH+CHX3CHX2OH\ce{CH3CH2COOCH2CH3 + H2O <=> CH3CH2COOH + CH3CH2OH}. The products are propanoic acid and ethanol.
  2. CHX3COOCHX3+NaOH→CHX3COONa+CHX3OH\ce{CH3COOCH3 + NaOH -> CH3COONa + CH3OH}. The products are sodium ethanoate and methanol.
  3. Acid hydrolysis is the reverse of esterification, so it is reversible and reaches an equilibrium. In alkaline hydrolysis the carboxylic acid is converted into its carboxylate salt, which does not react with the alcohol, so the reaction is not reversible and goes to completion.
  4. Sodium methanoate, HCOONa, and propan-1-ol.
  5. The separate layer of ester has disappeared and the mixture is one clear layer, because the products are all soluble in water.