Elimination Reactions

What are Elimination Reactions?

Elimination reactions are a type of organic reaction where atoms or groups of atoms are removed from a molecule, resulting in the formation of a double or triple bond.

These reactions are the opposite of addition reactions and are commonly seen in the formation of alkenes from alcohols or haloalkanes.

Dehydration of Alcohols

One of the most important elimination reactions is the dehydration of alcohols, where water ( $H₂O$ ) is removed from an alcohol to form an alkene.

This reaction is typically catalyzed by an acid, such as sulfuric acid ( $H₂SO₄$ ) or phosphoric acid ( $H₃PO₄$ ), and requires heating.

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Example: Dehydration of Ethanol When ethanol ( $C₂H₅OH$ ) is heated in the presence of concentrated sulfuric acid, it undergoes dehydration to form ethene ( $C₂H₄$ ) and water:

\text{C₂H₅OH} \xrightarrow{\text{H₂SO₄, heat}} \text{C₂H₄} + \text{H₂O}

This reaction involves the removal of an $–OH$ group and a hydrogen atom ( $H$ ) from adjacent carbon atoms, resulting in the formation of a double bond between the two carbon atoms.

Mechanism of Alcohol Dehydration

The dehydration of alcohol typically follows a three-step mechanism:

Protonation: The alcohol's hydroxyl group ( $–OH$ ) is protonated by the acid, making it a better-leaving group.
Loss of Water: The protonated $–OH$ group leaves, forming a carbocation (a positively charged carbon species).
Formation of a Double Bond: A hydrogen atom is removed from an adjacent carbon atom, and a double bond is formed between the two carbons, resulting in an alkene.

Factors Affecting the Dehydration Reaction

Type of Alcohol: The ease of dehydration depends on the structure of the alcohol. Tertiary alcohols dehydrate more easily than secondary or primary alcohols due to the relative stability of the carbocation intermediate.
- Tertiary alcohols > Secondary alcohols > Primary alcohols
Temperature: Higher temperatures are usually required for the dehydration of primary alcohols compared to secondary and tertiary alcohols.

Industrial Importance of Alcohol Dehydration

The dehydration of alcohols, particularly ethanol to ethene, is an important industrial process. Ethene is a key raw material used in the production of various plastics, such as polyethylene, and in the synthesis of other chemicals like ethanol and ethylene glycol.

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Key Points on Dehydration Reactions

Dehydration of alcohols is an elimination reaction resulting in the formation of alkenes.
Requires an acid catalyst ( $H₂SO₄$ or $H₃PO₄$ ) and heat.
Follows a three-step mechanism: protonation, loss of water, and formation of a double bond.
The type of alcohol and reaction conditions influence the ease of dehydration.
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Elimination reactions are fundamental in both organic synthesis and industrial chemistry, enabling the production of important alkenes from simple alcohols.

Elimination Reactions Simplified Revision Notes for Leaving Cert Chemistry