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Think First, Before You Read

Brazil vs. Germany: Two Factories, One Molecule

In 2023, Brazil produced approximately 35 billion litres of ethanol from sugar cane, almost entirely by fermentation. At the same time, chemical plants in Europe and Asia produced ethanol from ethene and steam over a phosphoric acid catalyst in high-pressure reactors. Both processes make identical molecules of CH₃CH₂OH. The Brazilian factory runs at 35°C with yeast in giant vats. The European plant runs at 300°C and 65 atmospheres of pressure with stainless steel vessels.

Before you read on, write down: Which method do you think produces higher-purity ethanol? Which costs more to run? Which is more sustainable? You will return to evaluate each prediction at the end of the lesson.

Key Equations, Production of Alcohols

Alkene Hydration (Industrial)

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH

H₃PO₄ catalyst · ~300°C · ~65 atm · reversible (equilibrium)

Haloalkane Substitution

R–X + NaOH(aq) → R–OH + NaX

aqueous NaOH or KOH · heat under reflux · single arrow (→)

Fermentation

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

yeast (zymase enzyme) · ~35°C · anaerobic (no oxygen)

Reflux Purpose

condenser returns vapour → flask

prevents loss of volatile reactants/products · improves yield

Today's hook: In the 1990s, BASF switched its ethanol production from traditional fermentation (using yeast and sugars) to direct hydration of ethylene over a phosphoric acid catalyst, cutting production time from days to hours. Both routes produce ethanol, but the mechanism, purity, and cost differ significantly. By the end of this lesson you will understand both pathways at the reaction level.

0/5TASKS

Learning Intentions

goals

Know

Three methods to produce alcohols: alkene hydration, haloalkane substitution, fermentation
Conditions for each: catalyst, temperature, pressure, solvent, atmosphere
Balanced equations with correct arrow notation (⇌ for hydration, → for substitution and fermentation)

Understand

Why alkene hydration uses ⇌, reversible equilibrium, Le Chatelier's principle applies
Why 300°C is a rate-yield compromise for alkene hydration
Why fermentation requires anaerobic conditions and is limited to ~15% ethanol
How aqueous vs. alcoholic NaOH determines substitution vs. elimination

Can Do

Write and balance equations for all three methods with conditions
Apply Le Chatelier's principle to explain pressure and temperature effects on hydration yield
Evaluate and compare the three methods using rate, purity, cost, and sustainability criteria

Scan these before reading

vocab

Hydration of alkenesAddition of water (steam) across a C=C double bond in the presence of acid catalyst (H₃PO₄); produces an alcohol.

Nucleophilic substitutionReaction where a halogenoalkane reacts with aqueous NaOH; the –OH replaces the halogen (SN1 or SN2).

FermentationEnzymatic conversion of glucose to ethanol and CO₂ by yeast; occurs at ~30–35°C without catalyst.

Fractional distillationUsed to purify ethanol from fermentation mixture; separates based on differences in boiling points.

Markovnikov's ruleIn addition of H–X to an alkene, H adds to the carbon with more H atoms; predicts major product of hydration.

Industrial vs biological productionIndustrial hydration gives pure product quickly; fermentation uses renewable biomass but gives dilute ethanol.

Core Concepts

Alkene Hydration, Industrial Context & Equilibrium

You met alkene hydration in L06 as a reaction type, now you examine it as an industrial process, which means thinking about equilibrium, yield, scale, and the economic context that determines whether this reaction is worth running at all.

The hydration of ethene to produce ethanol is one of the most important industrial organic reactions in the world:

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH

The reaction is reversible written with ⇌, not →. At equilibrium, both the forward reaction (hydration → ethanol) and the reverse reaction (dehydration → ethene + steam) occur simultaneously.

Conditions Chosen to Maximise Yield, Le Chatelier's Principle

HIGH PRESSURE (~65 atm): The left side has 2 moles of gas (ethene + steam); the right side has effectively 0 moles of gas (ethanol condenses as liquid). Increasing pressure shifts equilibrium to the side with fewer gas moles, the right side, increasing ethanol yield.

TEMPERATURE (~300°C): The forward reaction is exothermic. Lower temperature would favour the forward direction and increase yield. However, lower temperature also means slower rate and lower throughput. 300°C is a rate-vs-yield compromise not optimal for yield, but acceptable for rate. (Same trade-off as the Haber process for ammonia.)

H₃PO₄ CATALYST: Speeds up attainment of equilibrium without shifting the equilibrium position. More ethanol is produced faster per unit time.

RECYCLE LOOP: Single-pass conversion is only ~5%, most ethene does not react in one pass. Unreacted ethene is separated from the product stream, recycled, and passed through again. This continuous recycling achieves high overall yield despite low single-pass conversion.

Conditions, Alkene Hydration (Industrial Ethanol)

Reagent: H₂O (steam)
Catalyst: H₃PO₄ (absorbed on silica support, heterogeneous)
Temperature: ~300°C (rate-yield compromise)
Pressure: ~65 atm (high pressure, shifts equilibrium right)
Arrow: ⇌ (reversible equilibrium)

Must Do: Write ⇌ not → for the hydration equation. Using → implies complete conversion and tells the marker you don't understand that this is an equilibrium reaction. The ⇌ signals that you know conditions (high pressure, catalyst, temperature) are chosen to maximise yield within equilibrium constraints.

Common Error: "High temperature is used to increase ethanol yield." This is wrong for an exothermic reaction. High temperature shifts the exothermic equilibrium LEFT (Le Chatelier, opposes heating by favouring the endothermic reverse reaction), which DECREASES yield. 300°C is a rate compromise, not a yield-maximising choice. Distinguish rate effects from equilibrium effects.

Insight: The H₃PO₄ catalyst is not dissolved in the reaction mixture, it is absorbed onto solid silica and acts as a heterogeneous catalyst. Gaseous reactants flow over the solid catalyst surface. This continuous-flow design allows the catalyst to stay in the reactor while ethanol vapour flows out and is condensed downstream. Such reactors can operate without stopping for weeks or months.

The industrial hydration of ethene operates at ~65 atm pressure. What is the main reason high pressure is used?

Le Chatelier's Principle: the left side has 2 moles of gas (ethene + steam); the right side has 0 moles of gas (ethanol condenses). High pressure shifts equilibrium to the side with fewer gas moles, the right, increasing ethanol yield.

Substitution of Haloalkanes, Making Alcohols from Halides

Haloalkanes are the most versatile building blocks in organic synthesis, and one of their most important transformations is nucleophilic substitution with hydroxide ion, which converts a C–X bond directly into a C–OH bond without needing a double bond at all.

When a haloalkane is heated with aqueous NaOH or KOH, the halogen is replaced by a hydroxyl group. This is nucleophilic substitution OH⁻ acts as a nucleophile and replaces the halogen (which leaves as halide X⁻).

R–X + NaOH(aq) → R–OH + NaX

Worked Equations

Haloalkane	Equation	Product
bromoethane	CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr	ethanol
2-bromopropane	CH₃CHBrCH₃ + NaOH(aq) → CH₃CHOHCH₃ + NaBr	propan-2-ol
2-bromo-2-methylpropane	(CH₃)₃CBr + NaOH(aq) → (CH₃)₃COH + NaBr	2-methylpropan-2-ol

Key Features

(1) Aqueous NaOH: The hydroxide must be aqueous (dissolved in water) to give substitution. If NaOH is in ethanol (alcoholic NaOH), elimination occurs instead, producing an alkene.

(2) Reflux: Heating increases rate. Reflux prevents volatile haloalkane and alcohol vapours from escaping, condensed vapours drip back into the flask, keeping reactants in contact and improving yield.

(3) Halide leaving group reactivity: R–I (fastest) > R–Br > R–Cl (slowest). C–I bonds are weakest and most easily broken; C–Cl bonds are strongest.

Conditions, Haloalkane Substitution to Alcohol

Reagent: NaOH(aq) or KOH(aq), aqueous hydroxide (NOT alcoholic)
Catalyst: None
Conditions: Heat under reflux
Equipment: Round-bottom flask, reflux condenser, heating mantle
Arrow: → (goes essentially to completion)

Reflux apparatus for haloalkane substitution, condenser returns volatile reactants to the flask, preventing loss and improving yield

Must Do: The word AQUEOUS in "NaOH(aq)" is critical, it determines the reaction type. Aqueous NaOH → substitution → alcohol. Alcoholic NaOH (NaOH dissolved in ethanol) → elimination → alkene. This is one of the most commonly tested condition-based questions in Module 7. Always check: aqueous or alcoholic?

Common Error: Writing the product side as "R–OH + Na + X" as if NaOH splits into separate Na and X products. The correct product is NaX (sodium halide salt), Na⁺ and X⁻ combine. The equation must balance: one NaOH consumed, one NaX produced.

True or False: Using alcoholic NaOH (NaOH dissolved in ethanol) with 2-bromopropane will produce propan-2-ol by nucleophilic substitution.

FALSE. Alcoholic NaOH gives ELIMINATION, not substitution. The product would be propene (an alkene), not propan-2-ol. Substitution requires AQUEOUS NaOH.

Fermentation, Biological Production of Ethanol

Fermentation is the oldest chemical process humans have used deliberately, and it works because yeast cells contain an enzyme (zymase) that catalyses the conversion of glucose to ethanol under conditions impossible to replicate in an industrial pressure reactor.

Fermentation is the biological conversion of glucose to ethanol and carbon dioxide, catalysed by the enzyme zymase found in yeast cells:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

6C + 12H + 6O → (4C + 12H + 2O) + (2C + 4O) = 6C + 12H + 6O ✓

All Three Conditions Required

🌡️

~35°C

Optimal for enzyme activity. <15°C = too slow. >45°C = enzyme denatures.

🚫

Anaerobic

No oxygen. With O₂, yeast respires aerobically → CO₂ + H₂O, not ethanol.

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Yeast (zymase)

Enzyme catalyst. Not consumed overall, but reproduces and dies in vessel.

Maximum ethanol concentration: ~15%. When ethanol reaches ~15%, it becomes toxic to yeast cells and fermentation stops, regardless of remaining glucose. Spirits (40%) require fractional distillation of the fermented liquid.

Feedstock: Any sugar or starch, sugar cane, sugar beet, corn, wheat, barley, cassava. Starch must first be hydrolysed to glucose by amylase enzymes before zymase can act.

Conditions, Fermentation

Reagent: Glucose solution (C₆H₁₂O₆ in water)
Catalyst: Yeast (enzyme zymase, biological catalyst)
Temperature: ~35°C (enzyme optimum)
Atmosphere: Anaerobic (no oxygen / air excluded)
Equipment: Sealed fermentation vessel (vented to release CO₂)
Arrow: → (goes essentially to completion, stops at ~15% ethanol)

Fermentation equation, C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ · carbon balance: 6C glucose splits into 4C (ethanol) + 2C (CO₂)

Must Do: Three conditions are always required: (1) temperature ~35°C; (2) anaerobic (no oxygen / air excluded); (3) yeast or zymase enzyme. The most commonly omitted condition is anaerobic. Without anaerobic conditions, yeast respires aerobically and produces CO₂ + H₂O instead of ethanol, no ethanol at all.

Common Error: Writing C₆H₁₂O₆ → C₂H₅OH + CO₂ without coefficients, this is unbalanced. The balanced equation requires coefficient 2 on both products: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. An unbalanced equation loses marks even if the rest of the answer is correct.

Insight: The CO₂ produced in fermentation is not always waste. In beer and sparkling wine, CO₂ is retained to carbonate the drink. In bread-making, CO₂ bubbles are trapped by gluten proteins, causing dough to rise. The same equation, the same two products, underlies beer, wine, spirits, and artisan bread.

Safety: Fermentation continuously produces CO₂ gas. Never seal the fermentation vessel completely, the gas pressure will build up and can burst the container or blow out the bung. Fit an airlock or loosely vented bung so CO₂ can escape while air (oxygen) is kept out. Because CO₂ is denser than air and can displace oxygen, large-scale fermentation areas must be well ventilated to avoid an asphyxiation hazard in confined spaces.

Complete the sentence: Fermentation requires three conditions, temperature of approximately °C, conditions (no oxygen), and the enzyme found in yeast.

35°C · anaerobic · zymase. If oxygen is present, yeast performs aerobic respiration instead, producing only CO₂ and H₂O, no ethanol.

Comparing the Three Methods, Feedstock, Yield & Sustainability

Choosing between hydration, haloalkane substitution, and fermentation is not a chemistry question, it is an engineering, economic, and environmental question that chemistry informs but does not answer alone.

Feature	Hydration (alkene + H₂O)	Haloalkane substitution	Fermentation
Starting material	Ethene (crude oil)	Haloalkane (from alkane halogenation)	Glucose (plant biomass)
Catalyst	H₃PO₄ (heterogeneous)	None	Yeast (zymase enzyme)
Temperature	~300°C (high)	Reflux (~80–100°C)	~35°C (very low)
Pressure	~65 atm (high)	Atmospheric	Atmospheric
Reaction type	Addition (⇌ equilibrium)	Nucleophilic substitution (→)	Biological (→)
Ethanol purity	High (~95%)	Variable, must purify	Low (~15%, must distil)
Rate	Fast (continuous)	Moderate	Slow (batch, days–weeks)
Renewable feedstock?	No (fossil fuel)	No (fossil fuel)	Yes (plant biomass)
Carbon footprint	High	High	Lower (near-carbon-neutral)
Scale	Very large industrial	Lab to small industrial	Small to very large
Second organic product	None	NaX salt (waste stream)	CO₂ (useful, carbonation, bread)

HSC Comparison Key Points

Rate: hydration > substitution > fermentation. Hydration is continuous industrial; fermentation is slow batch.

Purity: hydration gives highest purity; fermentation gives lowest (15%, must distil to get concentrated product).

Sustainability: fermentation wins on renewable feedstock, plant biomass grown using solar energy; net CO₂ near zero for the growth phase. Hydration and substitution both depend on fossil fuel feedstocks.

Energy: fermentation at 35°C = low energy input. Hydration at 300°C + 65 atm = very high energy and infrastructure cost.

Must Do, HSC Comparison Questions: Structure your response as explicitly paired comparisons, not as separate descriptions. Marks go to COMPARATIVE statements: "fermentation is slower than hydration because…", not "fermentation is slow" and "hydration is fast" in isolation. Use comparison language: faster/slower, higher/lower, more/less renewable.

Common Error: Writing "fermentation is better than hydration because it is renewable." This earns one mark but ignores trade-offs: fermentation is slower, produces lower-purity ethanol requiring energy-intensive distillation, and cannot supply full global industrial demand. HSC evaluation questions require you to acknowledge both advantages AND limitations of each method.

Which one of the following is the odd one out for fermentation?

C is the odd one out. High pressure (~65 atm) is a condition for alkene HYDRATION, not fermentation. Fermentation occurs at atmospheric pressure with yeast, ~35°C, and renewable glucose feedstock.

Common Misconceptions, Alcohol Production

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"Higher temperature increases ethanol yield in alkene hydration." Wrong, the forward reaction is exothermic. Higher temperature favours the reverse (endothermic) reaction by Le Chatelier, DECREASING yield. 300°C is chosen for rate, not yield.

✗

"Fermentation is always more sustainable than hydration." Oversimplified, distillation of low-purity fermented ethanol consumes significant energy, and large-scale fermentation requires land clearing, irrigation, and may compete with food crops. Context matters.

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"NaOH always gives substitution with haloalkanes." Only aqueous NaOH gives substitution (→ alcohol). Alcoholic NaOH gives elimination (→ alkene). The solvent determines the product.

✗

"Fermentation equation: C₆H₁₂O₆ → C₂H₅OH + CO₂ (no coefficients)." Unbalanced, must be C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Missing coefficients always lose marks.

Worked Examples

Example 1 Straightforward

Writing Balanced Equations for All Three Production Methods

Problem: Write balanced equations for the production of ethanol using: (a) hydration of ethene; (b) substitution of bromoethane with NaOH; (c) fermentation of glucose. State the conditions for each.

Part (a), Alkene Hydration:
Equation: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH
Note: reversible arrow (⇌), this is an equilibrium reaction.
Conditions: H₂O (steam), H₃PO₄ catalyst, ~300°C, ~65 atm.

Part (b), Haloalkane Substitution:
Equation: CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr
Note: single arrow (→), substitution goes essentially to completion.
Conditions: NaOH(aq) [aqueous], heat under reflux.

Part (c), Fermentation:
Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Balance check: Left, 6C, 12H, 6O. Right, 4C+12H+2O + 2C+4O = 6C, 12H, 6O ✓
Conditions: yeast (zymase enzyme), ~35°C, anaerobic (no oxygen).

Answer:
(a) CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH [H₃PO₄, ~300°C, ~65 atm]
(b) CH₃CH₂Br + NaOH(aq) → CH₃CH₂OH + NaBr [aqueous NaOH, reflux]
(c) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [yeast/zymase, ~35°C, anaerobic]

Example 2 Intermediate

Explaining Equilibrium Considerations in Alkene Hydration

Problem: The industrial hydration of ethene operates at ~300°C and ~65 atm with a H₃PO₄ catalyst. Single-pass conversion is only ~5%. (a) Explain why high pressure is used. (b) Explain why 300°C is a compromise. (c) Explain how low single-pass conversion is managed without sacrificing overall yield.

(a) High pressure:
Equation: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH
Left side: 2 moles of gas (ethene + steam). Right side: 0 moles of gas (ethanol condenses as liquid).
By Le Chatelier's Principle: increasing pressure shifts equilibrium to the side with fewer moles of gas, the right side (products). ∴ High pressure increases ethanol yield.

(b) 300°C as rate-yield compromise:
The forward reaction is exothermic. By Le Chatelier, DECREASING temperature favours the exothermic direction → shifts equilibrium right → increases ethanol yield.
However, lower temperature also means slower reaction rate and lower throughput.
300°C gives a rate fast enough for continuous industrial production while still achieving an acceptable (if not maximum) equilibrium yield. This is a kinetics-vs-thermodynamics compromise, analogous to the Haber process.

(c) Managing low single-pass conversion:
After each pass through the reactor, the exit stream contains mostly unreacted ethene + small amounts of ethanol. The ethanol is condensed and separated. Unreacted ethene is recycled, pumped back to the reactor inlet and passed through again.
This continuous recycling achieves high overall yield despite only ~5% conversion per pass. Almost all ethene is eventually converted across multiple passes.

Answer:
(a) High pressure (65 atm) favours the side with fewer gas moles (right side), Le Chatelier, increasing ethanol yield.
(b) Lower temperature would increase yield (exothermic forward reaction) but would slow the rate unacceptably. 300°C is a kinetics-thermodynamics compromise.
(c) Unreacted ethene is separated from the product stream and recycled to the reactor inlet, continuous recycling achieves high overall yield despite low single-pass conversion.

Example 3 Extended Response

Evaluating Production Methods, Brazil vs. Germany (7 marks)

Problem: Brazil produces ethanol almost exclusively by fermentation of sugar cane; Germany produces industrial ethanol by catalytic hydration of ethene. (a) Write balanced equations for both processes with all conditions. (b) Explain why fermentation is preferred in Brazil. (c) Explain why hydration is preferred in Germany for industrial use. (d) Evaluate the claim: "Fermentation is always the more sustainable choice."

(a) Equations and conditions:
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ · yeast (zymase), ~35°C, anaerobic.
Hydration: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH · H₂O (steam), H₃PO₄, ~300°C, ~65 atm.

(b) Fermentation preferred in Brazil:
Brazil has abundant agricultural land for sugar cane, a renewable, photosynthetically produced glucose source. Fermentation requires no high-pressure infrastructure, operates at 35°C (low energy input), and is near-carbon-neutral (CO₂ released during fermentation is offset by CO₂ absorbed during sugar cane growth). The resulting ethanol is used as car fuel, reducing fossil fuel import dependence. Brazil's warm climate enables continuous sugar cane cultivation year-round.

(c) Hydration preferred in Germany:
Germany has sophisticated petrochemical infrastructure, ethene is available from oil refinery cracking. Catalytic hydration produces high-purity ethanol (~95%) continuously at large scale, suitable for industrial solvents, pharmaceutical manufacturing, and chemical synthesis where strict purity is required. The process has far higher throughput than fermentation. Fermentation's slow rate and low purity (requiring energy-intensive distillation) make it less competitive for high-volume industrial applications in a high-wage economy with existing large-scale infrastructure.

(d) Evaluation of the claim:
Arguments FOR (fermentation more sustainable): renewable feedstock; lower energy requirement (35°C vs 300°C + 65 atm); near-carbon-neutral when plant growth is accounted for; no fossil fuel dependence.
Arguments AGAINST (claim is oversimplified): fermentation produces only ~15% ethanol, purification to industrial grade requires fractional distillation consuming significant energy. Large-scale fermentation requires clearing land for biomass crops, potentially involving deforestation, habitat loss, and food-vs-fuel competition. Water consumption for irrigation is very high.
Conclusion: Fermentation has genuine sustainability advantages in many contexts. However, it is not universally "better", net sustainability depends on feedstock, land use, water use, energy source for distillation, and local infrastructure. The claim is too absolute.

Model Answer Summary:
(a) Equations with conditions as above.
(b) Fermentation in Brazil: renewable agricultural feedstock, low energy/infrastructure, near-carbon-neutral, reduces import dependence.
(c) Hydration in Germany: ethene from refinery cracking, high purity, high throughput, no distillation needed for industrial-grade concentration.
(d) Partially correct but oversimplified. Fermentation has genuine sustainability advantages but distillation energy cost, land use, water use, and food-vs-fuel trade-offs mean it is not automatically "more sustainable" in every context.

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Interactive Tool, Alcohols Laboratory Open fullscreen ↗

In the Alcohol Properties tool, which type of alcohol CANNOT be oxidised to a carboxylic acid?

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Place these steps in the correct order for the industrial production of ethanol by catalytic hydration of ethene:

Pass steam (H₂O) and ethene gas over a phosphoric acid catalyst on silica

Maintain temperature at 300°C and pressure at 60–70 atm

Ethene reacts with steam: CH₂=CH₂ + H₂O → CH₃CH₂OH

Separate unreacted ethene and recycle it back into the reactor

Purify the crude ethanol by fractional distillation

Practice

Apply what you've learned. Complete Activities, then answer the MC and Short Answer questions.

Activity A

Arrow Notation Audit, Correct the Equations

Each equation below contains an error in the arrow notation or equation balance. Identify and correct each one.

CH₂=CH₂ + H₂O → CH₃CH₂OH (industrial ethanol)
CH₃Br + NaOH(aq) ⇌ CH₃OH + NaBr
C₆H₁₂O₆ → C₂H₅OH + CO₂ (fermentation)

Activity B

Conditions Matching, Which Method?

For each set of conditions below, identify which method of alcohol production is being described and write the balanced equation:

Yeast, 35°C, sealed vessel, no oxygen
H₃PO₄ catalyst, 300°C, 65 atm, steam
KOH(aq), round-bottom flask, reflux condenser, heating mantle, 1-bromopropane as starting material

Check Your Understanding

Question 1. A student reacts 1-chloropropane with aqueous NaOH under reflux. Which product is formed and what is the reaction type?

Question 2. In the industrial production of ethanol by alkene hydration, single-pass conversion is approximately 5%. Which combination correctly explains how high overall yield is still achieved?

Question 3. Which set of conditions correctly describes fermentation of glucose to produce ethanol?

Question 4. Which statement correctly compares the three methods of ethanol production?

Question 5. The industrial hydration of ethene is described as operating at a rate-yield compromise temperature. Which statement best explains this?

Short Answer Questions

Question 6 4 marks

Write a balanced equation for the fermentation of glucose to produce ethanol. State all three conditions required for fermentation, and explain why each condition is necessary.

Question 7 5 marks

The industrial hydration of ethene is carried out at ~300°C, ~65 atm, with a H₃PO₄ catalyst. Using Le Chatelier's Principle, explain: (a) why high pressure is used rather than atmospheric pressure; (b) why 300°C is described as a compromise temperature and not the temperature that maximises ethanol yield.

Question 8 6 marks

Brazil produces most of its transport fuel ethanol by fermentation of sugar cane, while Germany produces most of its industrial ethanol by catalytic hydration of ethene. Evaluate which method is more sustainable for large-scale ethanol production, considering feedstock, energy, purity, and land use.

Show All Answers

Multiple Choice Answers

Q1, B: Propan-1-ol + NaCl, nucleophilic substitution. 1-chloropropane (CH₃CH₂CH₂Cl) + NaOH(aq) → CH₃CH₂CH₂OH (propan-1-ol) + NaCl. Nucleophilic substitution, OH⁻ replaces Cl on the same carbon (C1), giving propan-1-ol. Propane-2-ol (C) would require -OH to move to C2, which does not happen in simple substitution. Propene (A) forms with alcoholic NaOH (elimination), not aqueous NaOH.

Q2, C: Unreacted ethene is recycled. The low 5% single-pass conversion is compensated by continuous recycling of unreacted ethene back to the reactor inlet. Over many passes, almost all ethene is converted. A is wrong, excess ethene shifts equilibrium slightly right but cannot achieve full conversion per pass. D is wrong, 300°C is a rate compromise; higher temperature shifts the exothermic equilibrium LEFT (away from products), decreasing yield.

Q3, B: Glucose solution, yeast (zymase enzyme), ~35°C, anaerobic. Fermentation requires all three: yeast (zymase), ~35°C (enzyme optimum), and anaerobic conditions (no oxygen). D is the most dangerous distractor, adding excess oxygen causes aerobic respiration (→ CO₂ + H₂O), not increased ethanol. A describes alkene hydration conditions applied incorrectly to fermentation.

Q4, C: Fermentation lower temperature, lower purity. Fermentation operates at ~35°C (lower than hydration's ~300°C) but produces only ~15% ethanol (much lower purity than hydration's ~95%). Purification by fractional distillation is required for industrial-grade product. A is wrong (hydration gives higher purity). B is wrong, the feedstocks are the reverse: fermentation uses renewable plant biomass, hydration uses non-renewable ethene from crude oil.

Q5, D: Higher temperature increases rate but decreases yield for exothermic reaction. The forward hydration reaction is exothermic. By Le Chatelier, higher temperature shifts the equilibrium LEFT (favours endothermic reverse), decreasing ethanol yield. However, higher temperature increases reaction rate. 300°C is chosen to balance these opposing effects, fast enough rate for industrial viability, acceptable (not maximum) yield. A is incorrect, higher temperature does NOT increase equilibrium yield for an exothermic reaction.

Short Answer Sample Answers

Q6, Fermentation equation and conditions (4 marks):
Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [1 mark, must have coefficient 2 on both products]
Condition 1, Yeast (zymase enzyme): biological catalyst that facilitates the multi-step conversion of glucose to ethanol; without it the reaction is too slow to be useful [1 mark]
Condition 2, ~35°C: optimal temperature for enzyme activity; above ~45°C enzyme denatures and loses function; below ~15°C rate is too slow [1 mark]
Condition 3, Anaerobic (no oxygen/air excluded): in the presence of oxygen, yeast undergoes aerobic respiration (→ CO₂ + H₂O) rather than fermentation; oxygen must be excluded for ethanol to be produced [1 mark]

Q7, Le Chatelier pressure and temperature (5 marks):
(a) High pressure [2–3 marks]: Equation: CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. Left side: 2 moles of gas (ethene + steam). Right side: 0 moles of gas (ethanol condenses as liquid under process conditions). Le Chatelier's Principle: increasing pressure shifts equilibrium to the side with fewer moles of gas, the right side (products). ∴ High pressure increases the equilibrium yield of ethanol.
(b) Temperature compromise [2–3 marks]: Forward reaction is exothermic. By Le Chatelier, decreasing temperature favours the exothermic direction (forward), shifting equilibrium right and increasing yield. However, lower temperature also decreases reaction rate, reducing economic throughput. 300°C is a compromise, the rate is fast enough for continuous industrial production while still giving an acceptable equilibrium yield.

Q8, Sustainability evaluation (6 marks):
Arguments for fermentation being more sustainable: Uses renewable feedstock (plant biomass, sugar cane, corn); grown using solar energy and photosynthesis. Near-carbon-neutral, CO₂ released during fermentation is largely offset by CO₂ absorbed during plant growth. Operates at 35°C, very low energy input compared to 300°C + 65 atm for hydration. No dependence on declining fossil fuel reserves. [up to 3 marks]
Arguments against fermentation being universally more sustainable: Produces only ~15% ethanol, must be distilled to reach industrial-grade concentrations; distillation is energy-intensive and erodes the sustainability advantage. Large-scale fermentation requires clearing land for biomass crops, potentially involving deforestation, habitat loss, and food-vs-fuel competition. Very high water consumption for crop irrigation. Slow batch process with high labour requirements at scale. [up to 3 marks]
Conclusion: Fermentation has genuine sustainability advantages, particularly in countries with surplus agricultural land, warm climate, and renewable energy for distillation (e.g. Brazil). However, it is not universally more sustainable. The net outcome depends on land use, water availability, energy source for distillation, and scale. [1 mark for qualified conclusion acknowledging context-dependence]

Revisit, Think First

Brazil vs. Germany, Were Your Predictions Right?

Return to your predictions. Check each one:

Higher-purity ethanol: Alkene hydration (industrial, Germany) produces ~95% purity directly from the reactor. Fermentation produces only ~15%, the Brazilian ethanol requires fractional distillation to reach fuel grade. If you predicted hydration, you were right.

Higher running cost: This is context-dependent, but alkene hydration requires 300°C and 65 atm pressure, enormous energy and infrastructure costs per unit of product. Fermentation at 35°C costs far less per batch energy-wise, though distillation adds cost. If you said hydration costs more to run in terms of energy and capital infrastructure, you were broadly right.

More sustainable: Fermentation has genuine sustainability advantages, renewable feedstock, low energy, near-carbon-neutral. But it's not a simple win: distillation energy, land clearing, and water use complicate the picture. The honest answer is "it depends on context", which is exactly the kind of nuanced evaluation the HSC rewards in top-band responses.

Coming up in Lesson 11: You will see what happens when ethanol burns, comparing its enthalpy of combustion with fossil fuels and evaluating the biofuel debate using actual energy data.

I have completed Lesson 10, Production of Alcohols

Year 12 Chemistry Module 7, Organic Chemistry ⏱ ~45 min Lesson 10 of 23

Production of Alcohols, Hydration, Substitution & Fermentation

Ethanol is simultaneously the world's most consumed psychoactive substance, a major industrial solvent, a fuel additive, and a feedstock for dozens of other chemicals, and it can be made three completely different ways, each with different feedstocks, conditions, costs, and environmental footprints.

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Review

Quick-fire drill, read the prompt, recall the answer, then reveal to check.

What arrow notation is used for the industrial hydration of ethene to ethanol, and why?

Reveal

⇌ (reversible/equilibrium arrow), because the hydration reaction is reversible; ethanol can dehydrate back to ethene + steam. The reaction reaches a dynamic equilibrium, not full completion. Using → would imply complete reaction and earn no marks.

State THREE conditions required for fermentation of glucose to produce ethanol.

Reveal

(1) Yeast (enzyme zymase, biological catalyst)
(2) Temperature ~35°C (enzyme optimum; denatures above ~45°C)
(3) Anaerobic conditions (no oxygen; with O₂, yeast performs aerobic respiration → CO₂ + H₂O instead of ethanol)

Using Le Chatelier's Principle, explain why high pressure (~65 atm) is used in the industrial hydration of ethene.

Reveal

CH₂=CH₂ + H₂O ⇌ CH₃CH₂OH. Left side: 2 mol gas. Right side: 0 mol gas (ethanol is liquid). By Le Chatelier, increasing pressure shifts equilibrium to the side with fewer moles of gas, the right side (products). ∴ High pressure increases the equilibrium yield of ethanol.

Compare the purity and rate of alkene hydration vs fermentation for ethanol production.

Reveal

Purity: Hydration produces ~95% purity ethanol directly. Fermentation produces only ~15%, must be concentrated by fractional distillation for industrial use.
Rate: Hydration is fast and continuous (industrial throughput). Fermentation is slow (days to weeks, batch process).
Feedstock: Hydration uses non-renewable ethene (fossil fuel); fermentation uses renewable plant biomass (glucose).

What determines whether a haloalkane reacts with NaOH to give an alcohol (substitution) or an alkene (elimination)?

Reveal

The solvent of the NaOH solution determines the reaction pathway:
• Aqueous NaOH [NaOH(aq)] → nucleophilic substitution → alcohol (R–OH) + NaX
• Alcoholic NaOH [NaOH in ethanol] → elimination → alkene (R–CH=CH₂) + NaX + H₂O
This is one of the most tested condition-dependent choices in Module 7.