AQA A Level chemistry - AS Unit 2: Section 3.2.10: Alcohols

3.2.10 Alcohols - Elimination

Students should:

know that alkenes can be formed from alcohols by acid catalysed elimination reactions (mechanism not required)
appreciate that this method provides a possible route to polymers without using monomers derived from oil

Elimination reactions

Dehydration to form alkenes or alkoxyalkanes

The products formed depend on the conditions used: Alkenes are formed in the presence of H₂SO₄ (or H₃PO₄ better, as it doesn't produce as many by-products) and the correct temperature (hot for primary, warm for secondary and cool for tertiary) alcohols lose a water molecule.

Example: Dehydration of ethanol

CH₂H-CH₂OH H₂SO₄ (and heat, 170ºC) CH₂=CH₂ + H₂O

The temperature for primary alcohols must be kept above 170ºC to prevent formation of ethers.

The alcohol group is eliminated along with hydrogen atom from the carbon adjacent to the carbon holding the alcohol group. If there is no hydrogen on this carbon then elimination is not possible.

elimination of water from alcohols

One of the advantages of this method of preparing alkenes is that the starting materials, the alcohols, can be produced by biological means such as fermentation. This means that plastic manufacture does not have to depend on the petrochemicals industry.

Fermentation alcohols dehydration alkenes polymerisation polymers

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