Reactions of alkenes and alcohols (Higher)

Alkenes are reactive because of their C=C double bond — one of the bonds (the π-bond) breaks easily, so atoms can add across it.

Addition reactions of alkenes

In an addition reaction, two reactants combine to give one product.

1. With hydrogen (hydrogenation) — Ni catalyst, ~150 °C

C₂H₄ + H₂ → C₂H₆ (ethene → ethane)

Used industrially to convert vegetable oils to margarine (saturating C=C bonds).

2. With water (steam) — H₃PO₄ catalyst, 300 °C, 60 atm

C₂H₄ + H₂O → C₂H₅OH (ethene → ethanol)

This is one industrial method to make ethanol.

3. With halogens — at room temperature

C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)

This is the basis of the bromine water test (C7.4).

4. With hydrogen halides

C₂H₄ + HBr → C₂H₅Br

Alcohols — the homologous series

Alcohols contain the –OH (hydroxyl) functional group and have general formula CₙH₂ₙ₊₁OH.

Name	Formula
Methanol	CH₃OH
Ethanol	C₂H₅OH
Propanol	C₃H₇OH
Butanol	C₄H₉OH

Production of ethanol

Method 1: Hydration of ethene (above)

• Fast, cheap.
• Uses non-renewable crude oil source.
• 100% atom economy.

Method 2: Fermentation

• Yeast converts sugars (glucose) to ethanol + CO₂: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
• Conditions: ~30 °C, anaerobic, slightly acidic.
• Renewable, but slow and produces a dilute product (max ~14% ethanol — yeast dies above this).

Reactions of alcohols

Combustion

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (releases energy — ethanol is a fuel).

With sodium

2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂ (bubbles of H₂ given off — similar to Na + water).

Oxidation to carboxylic acid

With an oxidising agent (e.g. acidified KMnO₄, or air with microbial oxidation): C₂H₅OH → CH₃COOH (ethanol → ethanoic acid). Wine left exposed becomes vinegar.

Links

Builds on C7.4 (alkenes). Sets up C7.6 (carboxylic acids), C7.7 (polymerisation HT).