Ethanol reacts with ethanoic acid to form an ester. Describe what happens during this reaction, including the conditions needed, the names of both products formed, and explain why the reaction is described as reversible.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Esterification occurs when ethanol reacts with ethanoic acid in the presence of an acid catalyst, usually concentrated sulphuric acid. The acid protonates the carbonyl oxygen of the acid, increasing the electrophilicity of the carbonyl carbon. The lone pair on the oxygen of ethanol then attacks this carbon, forming a tetrahedral intermediate. Deprotonation and loss of a water molecule give the ester.
The products are ethyl ethanoate (the ester) and water.
The reaction is reversible because the ester and water can react in the presence of acid to regenerate the original alcohol and carboxylic acid, so the equilibrium does not lie completely to the product side.
The products are ethyl ethanoate (the ester) and water.
The reaction is reversible because the ester and water can react in the presence of acid to regenerate the original alcohol and carboxylic acid, so the equilibrium does not lie completely to the product side.
Examiner tips
- Use the term "esterification" and state the acid catalyst. Mention the protonation step and the tetrahedral intermediate. Identify both products explicitly. Explain reversibility in terms of equilibrium and the ability of water to reform the reactants.
Common mistakes
- Writing "esterification" without mentioning the acid catalyst. Forgetting to name the water product. Saying the reaction goes to completion instead of explaining equilibrium.
Mark scheme (5 marks)
- The reaction between an alcohol and a carboxylic acid is called esterification
- An acid catalyst (concentrated sulfuric acid) is required
- The ester produced is ethyl ethanoate
- Water is also produced
- The reaction is reversible because the ester and water can react to re-form the alcohol and carboxylic acid / the reaction does not go to completion
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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