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

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Oxidation (oxidising agent)

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Fermentation of glucose

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Hydration of ethene

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Ethanoic acid — how wine turns to vinegar

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

CO₂ and water — complete combustion like other organic molecules

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

🔬 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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