Alcohols

1. Manufacture of Ethanol

1.1 Method 1 – Fermentation of Aqueous Glucose

• Definition: Fermentation is a biochemical process in which yeast enzymes convert sugars (such as glucose) into ethanol and carbon dioxide under anaerobic conditions.
• Raw material: Glucose (C₆H₁₂O₆) obtained from plant materials such as sugarcane, corn, or starch-rich crops.
• Reaction equation:
  C₆H₁₂O₆ (aq) → 2C₂H₅OH (aq) + 2CO₂ (g)
  (glucose → ethanol + carbon dioxide)
• Conditions:
  • Temperature: 25–35 °C (optimum range for yeast enzymes to work without being denatured).
  • Catalyst: Enzymes in yeast.
  • Anaerobic environment: No oxygen present; prevents oxidation of ethanol to ethanoic acid.
  • Aqueous medium: Sugar dissolved in water for yeast activity.
• Process details:
  • Glucose solution mixed with yeast.
  • Kept at the correct temperature and shielded from air.
  • Fermentation continues until ethanol concentration reaches ~15%, after which yeast is killed by alcohol toxicity.
  • The mixture is distilled to separate ethanol from water and other residues.
• Advantages:
  • Uses renewable resources (plant-derived glucose).
  • Requires low energy input (room temperature and pressure).
  • Simple technology suitable for small-scale production in developing areas.
• Disadvantages:
  • Batch process → slower production rate.
  • Product contains dilute ethanol → requires distillation (energy-intensive for purification).
  • Dependent on agricultural land and crops, which may compete with food supply.

1.2 Method 2 – Catalytic Addition of Steam to Ethene

• Definition: Industrial process in which ethene reacts with steam in the presence of an acid catalyst to form ethanol.
• Raw material: Ethene (C₂H₄) obtained from the cracking of petroleum hydrocarbons.
• Reaction equation:
  C₂H₄ (g) + H₂O (g) → C₂H₅OH (g)
  (ethene + steam → ethanol)
• Conditions:
  • Temperature: ~300 °C.
  • Pressure: ~6000 kPa (≈60 atm).
  • Catalyst: Phosphoric acid (H₃PO₄) supported on silica.
• Process details:
  • Ethene and steam are mixed in the correct molar ratio.
  • Passed over heated phosphoric acid catalyst.
  • The product mixture is condensed to separate ethanol from unreacted gases.
  • Unreacted ethene is recycled to increase efficiency.
• Advantages:
  • Continuous process → rapid production rate.
  • Produces pure ethanol without further purification.
  • Requires less labour and time.
• Disadvantages:
  • Uses non-renewable resources (petroleum).
  • Requires high energy input due to high temperature and pressure.
  • Infrastructure costs are high.

1.3 Comparison Table – Fermentation vs Catalytic Hydration of Ethene

2. Combustion of Alcohols

• Definition: Alcohols burn in oxygen to release energy, carbon dioxide, and water.
• General equation:
  CₙH₂ₙ₊₁OH + (3n/2)O₂ → nCO₂ + (n+1)H₂O
• Example for ethanol:
  C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
• Characteristics:
  • Exothermic reaction (releases heat energy).
  • Produces a clean blue flame if complete combustion occurs.
  • Incomplete combustion produces carbon monoxide (CO) and soot (C).
• Energy value: Alcohols have a high calorific value and are used as fuels in domestic and industrial applications.

3. Uses of Ethanol

3.1 As a Solvent

• Dissolves both polar and non-polar substances due to the presence of a polar –OH group and a non-polar hydrocarbon chain.
• Commonly used in:
  • Perfumes.
  • Medicines.
  • Cosmetic products (e.g., deodorants, aftershaves).
  • Cleaning agents.
  • Ink and dye production.

3.2 As a Fuel

• Used as:
  • Bioethanol (produced from fermentation) blended with petrol to form gasohol (e.g., E10 = 10% ethanol, 90% petrol).
  • Alternative fuel for cars and cooking stoves.
• Advantages:
  • Renewable if produced via fermentation.
  • Burns more cleanly than petrol (less particulate matter and greenhouse gases per energy unit).
• Disadvantages:
  • Lower energy content compared to petrol.
  • Large-scale production can compete with food crop use.