A-Level organic chemistry exam revision notes on alcohols

PART 4.6B The Dehydration of alcohols to yield alkenes

Use of dehydration of alcohols to prepare and synthesis alkenes

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Thermal dehydration at high temperature or heating with conc. sulfuric acid.

(a) Ethanol can be dehydrated to ethene - an industrial thermal decomposition process

This is done by passing the alcohol vapour over heated aluminium oxide catalyst at 300^oC.

The trend for an elimination is tertiary > secondary > primary

It is an endothermic thermal decomposition reaction.

This elimination reaction is actually the reverse of the addition reaction by which ethanol is made from ethene (from cracking oil) by addition of water.

(i) ethanol ====> ethene + water

CH₃CH₂OH ====> CH₂=CH₂ + H₂O 

====> + H₂O

You can get ethers formed in these reactions, the respective yields of products can depend on reaction conditions such as temperature e.g. the formation of ethoxyethane from ethanol.

2CH₃CH₂OH  ===> CH₃CH₂OCH₂CH₃  +  H₂O

(ii) propan-1-ol  or  propan-2-ol  ===>  propene  +  water

{CH₃CH₂CH₂OH  or  CH₃CH(OH)CH₃}  ===>  CH₃CH=CH₂  +  H₂O

Note that the two isomeric alcohols give the same alkene product.

(iii) higher alcohols may give one or two elimination products

e.g. the alcohol isomers of C₄H₉OH will yield alkenes of molecular formula C₄H₈.

butan-1-ol  ===> but-1-ene  +  water (only one alcohol product possible)

CH₃CH₂CH₂CH₂OH  ===>  CH₃CH₂CH=CH₂  +  H₂O

butan-2-ol  ===> {but-1-ene or but-2-ene}  +  water

(two isomeric products possible)

CH₃CH₂CH(OH)CH₃ ===> {CH₃CH₂CH=CH₂ or CH₃CH=CHCH₃} + H₂O

The eliminated H atom can come from the carbon atom on either side of the OH carbon atom and so two alkene isomers can be formed.

Primary alcohols give only one alkene product (a ...-1-ene).

Secondary alcohols (after propan-2-ol) can give two isomeric alkene products (e.g. a ..-1-ene and a ..-2-ene), depends on the structure.

Tertiary alcohols usually give one product e.g. a ...1-ene

In the next two examples only one alkene product is possible.

2-methylpropan-1-ol  ===>  2-methylpropene  +  water

(CH₃)₂CHCH₂OH  ===>  (CH₃)₂C=CH₂  +  H₂O

2-methylpropan-2-ol  ===>  2-methylpropene  +  water

(CH₃)₃COH  ===>  (CH₃)₂C=CH₂  +  H₂O

This reaction is potentially an important source of organic chemicals e.g. plastics made by polymerising ethene, from a renewable resource since the ethanol can be made by fermentation of carbohydrates etc.

It is being used in countries that do not have oil reserves but have large areas of agricultural land producing sugar cane or sugar beet that are the raw materials for the fermentation process to manufacture ethanol.

The ethanol, so produced, becomes an important chemical feedstock for producing lots of other chemicals including polymers from ethene.

A simple laboratory demonstration with ethanol or a propanol

You soak some mineral wool in the alcohol and stuff to the end of a pyrex boiling tube.

Place some aluminium oxide half-way along the pyrex tube.

Heat the aluminium oxide quite strongly and the hot air will vaporise some of the alcohol which passes over the hot catalyst.

The alkene gas can be collected, as shown above, quite safely over water, but note the extra safety bottle to prevent cold water sucking back into the very hot pyrex boiling tube!

Mix the gaseous product with bromine water and you should see decolourisation, evidence of alkene formation.

 

(b) Dehydration using conc. sulfuric/phosphoric(V) acid in the laboratory.

(ii) If you heat ethanol with conc. sulfuric acid (at 180^oC) or concentrated phosphoric(V) acid, ethene gas is evolved.

CH₃CH₂OH  == H₂SO₄/H₃PO₄ ==>  H₂C=CH₂  +  H₂O

You can get a con-current condensation reaction that produces an ether e.g. ethanol  ===>  ethoxyethane  +  water

2CH₃CH₂OH  ===>  CH₃CH₂OCH₂CH₃  +  H₂O

(ii) Similarly you can heat cyclohexanol to yield cyclohexene

  == H₂SO₄/H₃PO₄ ==>    +  H₂O

  == H₂SO₄/H₃PO₄ ==>    +  H₂O

You can heat a mixture of cyclohexanol and conc. sulfuric acid in a flask and distil of the cyclohexene formed.

Cyclohexanol has a bpt. of 161^oC, water 100^oC and cyclohexene 83^oC.

In this preparation, using a fractionating column reduces the impurities in the cyclohexene distillate.

You can do the preparation without the fractionating column, the product is less pure, but the product should clearly decolourise bromine water, a simple test that the product is an alkene.

If you want pure cyclohexene, you need to remove acid impurities with sodium hydrogen carbonate solution in a separating funnel. (This may remove traces of cyclohexanol too?)

Separate the organic layer and dry it.

Further fractional distillation to make it very pure.