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Prepartion of Alkenes

This page looks at ways of preparing alkenes in the lab by the dehydration of alcohols.

Dehydration of alcohols using aluminium oxide as catalyst

 

Example 1: Dehydration of Ethanol to Product Ethene

This is a simple way of making gaseous alkenes like ethene. If ethanol vapor is passed over heated aluminum oxide powder, the ethanol is essentially cracked to give ethene and water vapor.

\[ CH_3CH_2OH \overset{Al_2O_3}{\longrightarrow} CH_2=CH_2 + H2O\]

To make a few test tubes of ethene, you can use this apparatus:

 

etheneprep.gif

It wouldn't be too difficult to imagine scaling this up by boiling some ethanol in a flask and passing the vapor over aluminum oxide heated in a long tube.

Dehydration of alcohols using an acid catalyst

The acid catalysts normally used are either concentrated sulfuric acid or concentrated phosphoric(V) acid, H3PO4. Concentrated sulphuric acid produces messy results. Not only is it an acid, but it is also a strong oxidizing agent. It oxidizes some of the alcohol to carbon dioxide and at the same time is reduced itself to sulfur dioxide. Both of these gases have to be removed from the alkene. It also reacts with the alcohol to produce a mass of carbon. There are other side reactions as well.

Example 1: Dehydration of Ethanol to produce Ethene

Ethanol is heated with an excess of concentrated sulfuric acid at a temperature of 170°C. The gases produced are passed through sodium hydroxide solution to remove the carbon dioxide and sulfur dioxide produced from side reactions. The ethene is collected over water.

\[ CH_3CH_2OH \overset{conc. H_2SO_4}{\longrightarrow} CH_2=CH_2 + H2O\]

The concentrated sulfuric acid is a catalyst. Write it over the arrow rather than in the equation.

Example 2: Dehydration of Cyclohexanol to produce Cyclohexe

This is a preparation commonly used at this level to illustrate the formation and purification of a liquid product. The fact that the carbon atoms happen to be joined in a ring makes no difference whatever to the chemistry of the reaction. Cyclohexanol is heated with concentrated phosphoric(V) acid and the liquid cyclohexene distils off and can be collected and purified. Phosphoric(V) acid tends to be used in place of sulphuric acid because it is safer and produces a less messy reaction.

 

h3po4cat.gif

The dehydration of more complicated alcohols

You have to be wary with more complicated alcohols in case there is the possibility of more than one alkene being formed. Butan-2-ol is a good example of this, with no less than three different alkenes being formed when it is dehydrated. Butan-2-ol is an example to illustrate the problems. It is important that you understand it so that you can work out what will happen in similar cases. When you dehydrate an alcohol, you remove the -OH group, and a hydrogen atom from the next carbon atom in the chain. With molecules like butan-2-ol, there are two possibilities when that happens.

complex1.gif

That leads to these products:

complex2.gif

The products are but-1-ene, CH2=CHCH2CH3, and but-2-ene, CH3CH=CHCH3. In fact the situation is even more complicated than it looks, because but-2-ene exhibits geometric isomerism. You get a mixture of two isomers formed - cis-but-2-ene and trans-but-2-ene.

 

cisandtrans.GIF

 

Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene. For an explanation of the two ways of naming these two compounds, follow the link in the box below. Which isomer gets formed is just a matter of chance.

The overall result

Dehydration of butan-2-ol leads to a mixture containing:

  • but-1-ene
  • cis-but-2-ene (also known as (Z)-but-2-ene)
  • trans-but-2-ene (also known as (E)-but-2-ene)

Contributors

Jim Clark (Chemguide.co.uk)