Elimination Reactions of Alcohols

Quick Notes

• Alkenes can be formed from alcohols through an acid-catalysed elimination reaction.

• This reaction removes water (H₂O) using a strong acid catalyst such as H₂SO₄ (sulfuric acid) or H₃PO₄ (phosphoric acid).

• The reaction follows an elimination mechanism.

• Alkenes produced by this method can be used to make addition polymers without needing crude oil-derived monomers.

Full Notes

Alcohols can react by elimination reactions with an acid catalyst to form alkenes.

General reaction:
C_nH_2n+1OH → C_nH_2n + H₂O

Conditions:
Catalyst: Concentrated H₂SO₄ or H₃PO₄
Heat under reflux (~170 °C)

Products:
An alkene
Water

Example: Dehydration of Ethanol to Ethene
CH₃CH₂OH → CH₂=CH₂ + H₂O

Mechanism of Elimination

1. Protonation of the Alcohol
   The hydroxyl (–OH) group accepts a proton (H⁺) from the acid catalyst. This forms a positively charged oxonium ion (–OH₂⁺).
2. Loss of Water
   Water (H₂O) leaves, forming a carbocation intermediate. The stability of the carbocation follows the order: Tertiary > Secondary > Primary.
3. Formation of the Alkene
   A hydrogen (H⁺) is removed, forming a C=C double bond (alkene).

Importance of Producing Alkenes from Alcohols

Alkenes can be polymerised to form plastics.

This provides a sustainable alternative to crude oil-derived monomers for making polymers like poly(ethene) and poly(propene).

Summary

• Acid-catalysed dehydration of alcohols forms alkenes and water.
• Typical conditions: conc. H₂SO₄ or H₃PO₄, ~170 °C, reflux.
• Mechanism: protonation of –OH → loss of water (carbocation) → elimination to C=C.
• Alkenes produced can be used to make addition polymers, reducing reliance on crude oil-derived monomers.