Advanced Chemical Principles and Molecular Dynamics · 6th Year

Active learning ideas

Carboxylic Acids and Esters

Get ready to explore the chemistry behind the scents and flavours we encounter every day. This topic introduces carboxylic acids and the fragrant esters they form, a vital and examinable part of organic chemistry.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Organic Chemistry - Oxygen-containing Organic Compounds

• 25–60 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Experiential Learning
• 40 min · Pairs

Preparation of an Ester: The Pear Drop Smell

Students carry out the mandatory experiment to prepare ethyl ethanoate. They will react ethanoic acid with ethanol using concentrated sulfuric acid as a catalyst in a water bath, and then carefully isolate and identify the product by its characteristic 'pear drop' smell.

Explain why carboxylic acids are weak acids.

Facilitation Tip

Ensure all safety precautions for handling concentrated acid are strictly followed and that heating is done gently in a fume hood.

What to look for

Use mini-whiteboards to have students draw the products of various esterification reactions, providing immediate feedback on their understanding of structures and naming.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Activity 02

• Experiential Learning
• 60 min · Pairs

Weak Acid Titration Curve

Students titrate ethanoic acid with a standard solution of sodium hydroxide, measuring the pH after each addition. They can then plot a pH curve to visualise the buffer region and determine the equivalence point, reinforcing their understanding of weak acids.

Analyse the conditions required for the esterification reaction between ethanol and ethanoic acid.

Facilitation Tip

Discuss the choice of phenolphthalein as an indicator and why it is suitable for this specific titration.

What to look for

A Leaving Cert style exam question requiring students to describe the preparation of ethyl ethanoate, including a labelled diagram, safety precautions, and an explanation of the catalyst's role.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Activity 03

• Experiential Learning
• 25 min · Small Groups

Ester Nomenclature Relay

Divide the class into teams. Each team member must correctly name an ester from a displayed structure, or draw the structure from a name, before the next team member can take their turn. This gamifies the reinforcement of IUPAC naming rules.

Justify the name 'ethyl propanoate' for a given ester structure.

Facilitation Tip

Prepare a varied set of structures and names, including some with branched chains to challenge more able students.

What to look for

Provide students with a past exam question and the official marking scheme. Ask them to attempt the question and then mark their own work to identify areas of weakness.

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

A few notes on teaching this unit

Start by linking the structure of the carboxyl group to the previously studied hydroxyl group in alcohols. The hands-on preparation of 'pear drops' ester is a memorable way to introduce esterification. Use molecular modelling kits to help students visualise the condensation reaction. Continually reinforce the IUPAC naming convention, as it is a common source of error.

Upon completing this topic, students will be able to confidently explain the properties of carboxylic acids, and name, draw, and describe the synthesis of common esters.

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Watch Out for These Misconceptions

• The name of an ester starts with the acid part, not the alcohol part.

  The IUPAC name for an ester is derived from the parent alcohol first (changing '-anol' to '-yl') followed by the parent carboxylic acid (changing '-oic acid' to '-oate'). For example, methanol and propanoic acid form methyl propanoate.

• Esterification is an irreversible reaction that goes to completion.

  Esterification is a reversible reaction that reaches a dynamic equilibrium. The use of a dehydrating agent, like concentrated sulfuric acid, removes water and shifts the equilibrium to the right, increasing the yield of the ester, but the reverse reaction (hydrolysis) is always occurring.

• Weak acids, like ethanoic acid, do not fully react with strong bases.

  The term 'weak acid' refers to its partial dissociation in water, not its ability to be neutralised. A weak acid will react completely with a strong base in a neutralisation reaction to form a salt and water.

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Methods used in this brief

• Experiential Learning