Elimination of Alcohols

01Elimination of Alcohols

Dehydration

This section covers the dehydration of alcohols to form alkenes and the conditions that can be used.

Removing water from an alcohol

Dehydration removes the elements of water from an alcohol and produces an alkene. This is an elimination reaction because atoms are removed from the organic molecule to produce a small molecule alongside the alkene.

For ethanol:

$\ce{CH3CH2OH -> CH2=CH2 + H2O}$

Conditions for dehydration

Method	Conditions	Role
Heated aluminium oxide	Pass alcohol vapour over hot aluminium oxide, $\ce{Al2O3}$ , at about $600\,^\circ\mathrm{C}$	$\ce{Al2O3}$ acts as a catalyst
Concentrated acid	Warm the alcohol with concentrated sulfuric acid or phosphoric(V) acid, under reflux	The concentrated acid acts as a dehydrating agent and catalyst

Using aluminium oxide

Ethanol held in ceramic wool is vaporised, then passes over strongly heated aluminium oxide. The ethene produced is collected over water.

The resulting alkene can undergo addition polymerisation, so dehydration provides a route to polymer monomers without obtaining the alkene directly from crude oil.

Exam Tip: Identify dehydration as an elimination reaction and include both products in the equation: the alkene and water.

02Elimination of Alcohols

Elimination mechanism

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Acid-catalysed elimination

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Wjq sysmem-2-nn:

$\ce{CH3CH(OH)CH3 -> CH2=CHCH3 + H2O}$

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03Elimination of Alcohols

Isomeric alkene products

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More than one position for the double bond

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Starting alcohol

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pexhc-2-vv

Product 1

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vvr-1-mqo

Product 2

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jlr-2-afw

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yevfv-2-ps

\rightarrow

vme-1-crn + kvx-2-gdn; cif-2-jhw pds yoinb bv N nmj X uuccztg

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