Chemistry • Year 12 • Module 7 • Lesson 15
Esters: Structure, Naming & Esterification
Build HSC Band 5–6 extended-response technique on ester yield maximisation, industrial hydrolysis, and source critique of common ester misconceptions.
1. Data + scenario — maximising ester yield in Australian food manufacturing (Band 5–6)
8 marks Band 5–6
Scenario. A chemical engineer at a FSANZ-approved Australian flavour company is tasked with maximising the yield of butyl ethanoate (pineapple flavour ester) for use in the confectionery industry. She reacts ethanoic acid with butan-1-ol using concentrated H₂SO₄ as catalyst under reflux. Under equimolar conditions (1:1 molar ratio), she obtains a 57% yield after 1 hour. Her production manager asks her to reach at least 80% yield without changing the total reaction time.
The table below summarises five proposed process modifications the engineer is considering.
| Modification | Description | Feasibility in industrial setting |
|---|---|---|
| A | Use 3× molar excess of butan-1-ol (3:1 ratio) | High — alcohol is inexpensive |
| B | Add anhydrous molecular sieves to absorb water | Moderate — sieves must be filtered off later |
| C | Increase catalyst concentration (more H₂SO₄) | High — cheap, easy to scale |
| D | Continuously distil off ester as it forms (ester BP 126°C) | Moderate — requires continuous distillation column |
| E | Replace conc. H₂SO₄ with solid acid catalyst (ion-exchange resin), allowing continuous operation | Moderate — higher upfront cost, continuous process |
Feasibility ratings and process parameters are illustrative.
Q1. Evaluate the five proposed modifications and recommend the best combination of two modifications for the engineer to adopt first, with full justification. In your response you must:
- Write the balanced esterification equation for butyl ethanoate formation and identify the equilibrium position issue.
- For each of Modifications A, B, C, and D: state whether it would increase yield, and explain why using Le Chatelier’s Principle (or explain why it would not increase yield if applicable).
- Justify your recommended combination using both chemical reasoning and industrial feasibility.
- Explain one limitation of your recommended combination.
2. Source critique — evaluate a claim about ester hydrolysis (Band 5–6)
7 marks Band 5–6
“To make soap from a vegetable oil (triglyceride), the oil can be hydrolysed using either dilute sulfuric acid or sodium hydroxide solution — both reactions work equally well to produce soap (a fatty acid salt). Using acid is actually preferable in industrial settings because it is cheaper and the reaction is reversible, which gives the chemist more control over the products. The reversibility is a feature, not a bug: if too much soap is made, the reaction can simply be run backwards.”
Source: fictional student revision guide, “Organic Chemistry Made Easy” (2025), pp. 84–85.
Q2. Evaluate the claim above. Identify all scientific flaws, explain the correct chemistry for each, and describe how each flaw could be detected or demonstrated experimentally.
Q1 — Sample Band 6 response (8 marks), annotated
Balanced equation: CH₃COOH + C₄H₉OH ⇌ CH₃COOC₄H₉ + H₂O (conc. H₂SO₄, heat under reflux). The ⇌ indicates a reversible equilibrium with Keq ≈ 1–4; under equimolar conditions the yield is only ~57%, well below 100%, because all four species coexist at equilibrium. [1 — balanced equation with correct arrow; 1 — equilibrium issue identified]
Modification A (3:1 excess butan-1-ol): Increases the concentration of a reactant. By Le Chatelier’s Principle, the system shifts right to oppose the increased [alcohol], producing more ester and water until a new equilibrium is reached. Yield increases significantly (model data suggest ~78% at 3:1). Industrially feasible — butan-1-ol is inexpensive and can be recovered by distillation. [1 — Le Chatelier applied correctly]
Modification B (molecular sieves): Absorbs water (a product) as it forms, reducing [H₂O]. By Le Chatelier’s Principle, the system shifts right to replace the removed product, driving further ester formation. Moderately feasible — sieves must be filtered off, adding a step, but they work without changing reagent ratios. [1 — product removal reasoning]
Modification C (more H₂SO₄): H₂SO₄ is a catalyst. Catalysts lower activation energy and increase reaction rate, but they do not change the position of equilibrium. Adding more catalyst will not increase the equilibrium yield; it will only make the reaction reach equilibrium faster. Modification C does not address the yield problem. [1 — correctly distinguishes catalyst effect from equilibrium shift]
Modification D (continuous distillation of ester): Removes the ester product (BP 126°C) as it forms, reducing [ester]. By Le Chatelier’s Principle, the system shifts right to replace the removed ester, continuously pushing the reaction toward products. In theory this gives very high yield, but requires a continuous distillation column and is energy-intensive. [1 — product removal Le Chatelier for ester]
Recommended combination: A + B. Combining a 3:1 molar excess of butan-1-ol (shifts equilibrium right via reactant increase) with molecular sieves (shifts equilibrium right via product water removal) applies Le Chatelier’s Principle from two directions simultaneously — both a reactant increase and a product removal. Together they should reliably push yield well above 80%. Both are industrially feasible: butan-1-ol is cheap and recyclable; sieves are a standard lab/industrial reagent. Industrially, A alone may suffice given the cost data. [1 — recommended combination with dual Le Chatelier justification]
Limitation: Even with A + B, 100% yield is not achievable because the water produced before the sieves absorb it can hydrolyse some ester before removal. Additionally, the excess butan-1-ol must be separated from the product, adding cost and complexity to the downstream isolation steps. [1 — identified limitation with chemical reasoning]
Marking criteria (8 marks):
- 1 mark — Balanced esterification equation for butyl ethanoate with correct reversible arrow (⇌) and conditions.
- 1 mark — Identifies that the equilibrium issue (Keq ≈ 1–4, all four species present) is why yield is below 100% under equimolar conditions.
- 1 mark — Correctly applies Le Chatelier’s Principle to Modification A (excess alcohol shifts equilibrium right).
- 1 mark — Correctly applies Le Chatelier’s Principle to Modification B (removing water shifts equilibrium right).
- 1 mark — Correctly identifies that Modification C (more catalyst) does NOT increase equilibrium yield — only increases rate of reaching equilibrium.
- 1 mark — Correctly applies Le Chatelier’s Principle to Modification D (removing ester shifts right).
- 1 mark — Recommends a valid combination of two modifications (any two of A, B, D) with a dual Le Chatelier justification and reference to industrial feasibility.
- 1 mark — States one specific and chemically grounded limitation of the recommended combination (not just “yield is less than 100%” — must explain why).
Q2 — Sample Band 6 response (7 marks), annotated
The claim contains four identifiable scientific flaws.
Flaw 1: “Acid hydrolysis produces soap (a fatty acid salt).” This is incorrect. Acid hydrolysis of a triglyceride with dilute H₂SO₄ produces the free carboxylic acid (fatty acid) and glycerol, not the carboxylate salt. Soap is a sodium (or potassium) carboxylate salt, RCOO⁻Na⁺, which requires a base (NaOH) to produce. Acid hydrolysis simply regenerates the acid and alcohol components of the ester — no salt is formed. [1 — flaw identified; 1 — correct chemistry stated]
Experimental detection: Treat the product of acid hydrolysis with pH indicator — the product would be acidic (pH < 7, from the free fatty acid). Soap (sodium carboxylate) would give a basic solution (pH > 7) and would form a lather with water; the fatty acid from acid hydrolysis would not. [1 — experimental detection]
Flaw 2: “Saponification is reversible, giving more control” and “the reaction can be run backwards.” Saponification is irreversible (single arrow →, not ⇌). The carboxylate anion (RCOO⁻) produced under basic conditions cannot act as an acylating agent — it is too stabilised by resonance to re-esterify with the alcohol. The reaction goes to completion regardless of conditions. Reversibility is not a feature of saponification; the claim reverses the fact. [1 — flaw identified; 1 — correct chemistry explained]
Experimental detection: Add glycerol and sodium stearate (soap) to water and heat under reflux — no ester would reform, confirming irreversibility. In contrast, glycerol + stearic acid + H₂SO₄ under reflux would partially re-esterify (confirming acid hydrolysis is reversible). [1 — experimental detection]
Flaw 3: “Both reactions work equally well to produce soap.” Only saponification (NaOH) produces soap (a carboxylate salt suitable for use as a surfactant). Acid hydrolysis produces the free fatty acid, which is not soap. The claim that both reactions “work equally well” for soap production is therefore entirely false — only base hydrolysis works. [1 — this flaw correctly identified and distinguished from Flaw 1]
Summary marking criteria (7 marks):
- 1 mark — Identifies Flaw 1: acid hydrolysis produces the free fatty acid, not a carboxylate salt; soap is not produced.
- 1 mark — Correctly explains that soap requires base hydrolysis (saponification) to form RCOO⁻Na⁺; acid hydrolysis regenerates RCOOH.
- 1 mark — Experimental detection for Flaw 1: pH test and/or lather test to distinguish fatty acid from carboxylate salt.
- 1 mark — Identifies Flaw 2: saponification is irreversible (single arrow), not reversible.
- 1 mark — Correctly explains why saponification is irreversible: the carboxylate anion cannot re-esterify under basic conditions.
- 1 mark — Experimental detection for Flaw 2: attempt to re-esterify soap + glycerol fails; acid hydrolysis equilibrium can be shown to be reversible under same conditions.
- 1 mark — Identifies the overall logical flaw (Flaw 3): the two reactions do not both produce soap; only saponification does, making the entire premise of the claim invalid.