Alcohols | Chemistry

📖 In-Depth Theory

Structure and Properties of Alcohols

ALCOHOLS contain the functional group –OH (hydroxyl group).

GENERAL FORMULA: CₙH₂ₙ₊₁OH

First four members:

Methanol: CH₃OH

Ethanol: C₂H₅OH (CH₃CH₂OH)

Propanol: C₃H₇OH

Butanol: C₄H₉OH

PROPERTIES:

Soluble in water — the –OH group forms hydrogen bonds with water.

Higher boiling points than alkanes of similar mass — due to hydrogen bonding between molecules.

Lower members (methanol, ethanol) are liquids at room temperature.

IMPORTANT NOTE: Ethanol is the alcohol in alcoholic drinks.

Methanol is poisonous — even small amounts can cause blindness or death.

Reactions of Alcohols

1. REACTION WITH SODIUM:

Alcohols react with sodium metal to produce hydrogen gas and a metal alkoxide.

E.g. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

Less vigorous than sodium + water — alcohol –OH is less ionised than water.

2. COMBUSTION — burning in air:

Alcohols burn in air to produce CO₂ and H₂O (complete combustion).

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Clean-burning — used as fuels (ethanol as biofuel).

3. DISSOLVING IN WATER:

Lower alcohols are miscible with water (mix completely).

OH group allows hydrogen bonding with water.

4. OXIDATION — reaction with an oxidising agent:

Alcohols can be oxidised to carboxylic acids.

Ethanol → ethanoic acid (using potassium dichromate as oxidising agent).

This is how wine turns to vinegar — ethanol oxidised to ethanoic acid by bacteria.

Production of Ethanol

Ethanol is produced by two methods:

METHOD 1 — FERMENTATION:

Glucose (from sugar cane or starch crops) + yeast enzymes → ethanol + carbon dioxide.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Conditions: yeast, ~30°C, anaerobic (no oxygen), neutral pH.

Ethanol concentration limited to ~15% — above this, yeast is killed.

Further purification by fractional distillation.

Renewable source — uses plant sugar (carbon neutral in principle).

METHOD 2 — HYDRATION OF ETHENE:

CH₂=CH₂ + H₂O → C₂H₅OH

Conditions: H₃PO₄ catalyst, ~300°C, high pressure.

Source: ethene from cracking crude oil — non-renewable.

Pure ethanol produced directly — no distillation needed.

Continuous process — faster than fermentation.

COMPARISON:

Fermentation: renewable, low temperature, batch process, dilute product, uses cheaper raw materials.

Hydration: non-renewable (fossil fuels), continuous, pure product, higher energy costs.

⚠️ Common Mistake

Ethanol (not methanol) is the alcohol in drinks. Methanol is highly toxic. Fermentation uses GLUCOSE and YEAST — not alkenes. Hydration uses ethene and steam with a phosphoric acid catalyst. The two production methods use different raw materials and conditions.

📐 Key Equations

Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Hydration: C₂H₄ + H₂O → C₂H₅OH

Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

📌 Key Note

Alcohols: –OH group, CₙH₂ₙ₊₁OH. Reactions: + Na (hydrogen gas), combustion (CO₂ + H₂O), dissolve in water, oxidation → carboxylic acid. Ethanol production: fermentation (C₆H₁₂O₆, yeast, 30°C, renewable) or hydration of ethene (H₃PO₄, 300°C, non-renewable, pure product).

🎯 Matching Activity — Alcohol Reactions

Match each reaction of ethanol to the product formed. — drag the symbols on the right to match the component names on the left.

Combustion in air

Drop here

Oxidation (oxidising agent)

Drop here

Fermentation of glucose

Drop here

Hydration of ethene

Drop here

Ethanol — H₃PO₄ catalyst, ~300°C, high pressure

Ethanoic acid — how wine turns to vinegar

CO₂ and water — complete combustion like other organic molecules

Ethanol + CO₂ — yeast, ~30°C, anaerobic conditions

⭐ Higher Tier Only

Explain the oxidation of primary alcohols to aldehydes then carboxylic acids (using oxidising agents). Write equations for alcohol reactions. Evaluate ethanol as a biofuel: consider carbon neutrality claims, land use, energy balance and competition with food production.

🔬 Triple Science Only

Alcohols (4.7.2.3) is chemistry-only — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. What are the conditions needed for fermentation of glucose to produce ethanol?

2. What is the advantage of fermentation over hydration for ethanol production?

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🧪 Alcohols

Structure and Properties of Alcohols

Reactions of Alcohols

Production of Ethanol

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