1. Overview
Alcohols are a homologous series of organic compounds containing the hydroxyl (-OH) functional group. This topic focuses primarily on ethanol ($C_2H_5OH$), exploring how it is manufactured industrially, its chemical reactions, and its vital role as both a solvent and a sustainable fuel source.
Key Definitions
- Functional Group: An atom or group of atoms that determine the chemical properties of a homologous series. For alcohols, this is the –OH group.
- Fermentation: A biological process where enzymes in yeast convert sugars into ethanol and carbon dioxide in the absence of oxygen.
- Hydration (Catalytic Addition): A chemical reaction where water (in the form of steam) reacts with an unsaturated molecule like ethene to produce an alcohol.
- Anaerobic: Conditions that occur in the absence of oxygen.
Core Content
A. Manufacture of Ethanol by Fermentation
Ethanol can be produced from renewable plant sugars (glucose).
- Process: Yeast is added to an aqueous solution of glucose.
- Conditions:
- Temperature: 25–35 °C (Optimum temperature for yeast enzymes).
- Atmosphere: Absence of oxygen (anaerobic). If oxygen is present, the yeast will produce $CO_2$ and $H_2O$ instead, or the ethanol will oxidize into ethanoic acid (vinegar).
- Catalyst: Yeast (contains enzymes).
Equations:
- Word: Glucose → Ethanol + Carbon Dioxide
- Symbol: $C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$
B. Manufacture of Ethanol by Catalytic Addition (Hydration)
Ethanol is produced from ethene, which is obtained from the cracking of crude oil.
- Process: Steam is reacted with ethene.
- Conditions:
- Temperature: 300 °C.
- Pressure: 6000 kPa / 60 atm.
- Catalyst: Phosphoric acid ($H_3PO_4$).
Equations:
- Word: Ethene + Steam → Ethanol
- Symbol: $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$
A structural formula diagram showing an ethene molecule ($CH_2=CH_2$) adding an $H$ and an $OH$ across the double bond to form $CH_3CH_2OH$.
C. Combustion of Ethanol
Ethanol burns cleanly in the presence of oxygen, making it an effective fuel.
- Word: Ethanol + Oxygen → Carbon Dioxide + Water
- Symbol: $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$
D. Uses of Ethanol
- As a Solvent: Used in perfumes, cosmetics, glues, and inks because it dissolves many substances that are insoluble in water.
- As a Fuel: Used in spirit burners and mixed with petrol (bioethanol) to power car engines, reducing reliance on fossil fuels.
Extended Content (Extended Curriculum Only)
Comparison of Manufacture Methods
In the exam, you may be asked to compare these two methods.
| Feature | Fermentation | Catalytic Addition (Hydration) |
|---|---|---|
| Raw Material | Renewable (Glucose from crops) | Non-renewable (Ethene from crude oil) |
| Type of Process | Batch process (Slow) | Continuous process (Fast) |
| Purity | Low (produces a dilute solution) | High (produces pure ethanol) |
| Energy Requirement | Low (Low temperatures) | High (High temp/pressure required) |
| Reaction Rate | Very Slow | Very Fast |
Key Equations
Fermentation:
- $C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$
- Note: Glucose is aqueous; $CO_2$ is a gas.
Hydration of Ethene:
- $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$
- Note: Water must be in gas state (steam).
Complete Combustion:
- $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$
- Note: Ensure the oxygen ($O_2$) is balanced (3 moles).
Common Mistakes to Avoid
- ❌ Wrong: Forgetting the specific conditions for fermentation.
- ✅ Right: Always state 25–35 °C and absence of oxygen. If the temperature is too high (>40 °C), yeast enzymes denature.
- ❌ Wrong: Writing $H_2O(l)$ for the hydration of ethene.
- ✅ Right: Use $H_2O(g)$ because the reaction uses steam at 300 °C.
- ❌ Wrong: Using the wrong catalyst for ethene hydration.
- ✅ Right: The catalyst is Phosphoric acid ($H_3PO_4$), not yeast or nickel.
Exam Tips
- Command Words:
- "State": Give a brief answer (e.g., "State the conditions for fermentation" → 30 °C, yeast, no oxygen).
- "Name": Provide the chemical name only (e.g., "Name the catalyst" → Phosphoric acid).
- Question Types: Expect questions asking you to compare the "Greenness" of the two manufacturing methods. Fermentation is often considered "greener" as it uses renewable resources, but hydration is more efficient.
- Real-world contexts: Be prepared for questions about "Biofuel." Remember that ethanol from fermentation is a "carbon-neutral" concept because the $CO_2$ released during combustion was originally absorbed by the plants during photosynthesis.
- Typical Values: Remember 300 °C and 60 atm for hydration. These numbers appear frequently in multiple-choice and structured questions.