MAKING ALKENES IN THE LAB
This page looks at ways of preparing alkenes in the lab by the
dehydration of alcohols.
Dehydration of alcohols using aluminium oxide as
catalyst
The dehydration of ethanol to give ethene
This is a simple way of making gaseous alkenes like ethene. If
ethanol vapour is passed over heated aluminium oxide powder,
the ethanol is essentially cracked to give ethene and water
vapour.
To make a few test tubes of ethene, you can use this apparatus:
It wouldn't be too difficult to imagine scaling this up by boiling
some ethanol in a flask and passing the vapour over aluminium
oxide heated in a long tube.
Dehydration of alcohols using an acid catalyst
The acid catalysts normally used are either concentrated
sulphuric 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 oxidising agent. It oxidises some
of the alcohol to carbon dioxide and at the same time is reduced
itself to sulphur 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, but these aren't required
by any current UK A level (or equivalent) syllabus.
The dehydration of ethanol to give ethene
Ethanol is heated with an excess of concentrated sulphuric acid
at a temperature of 170°C. The gases produced are passed
through sodium hydroxide solution to remove the carbon dioxide
and sulphur dioxide produced from side reactions.
The ethene is collected over water.
WARNING! This is potentially an extremely dangerous
preparation because of the close proximity of the very hot
concentrated sulphuric acid and the sodium hydroxide
solution. I knew of one chemistry teacher who put several
students into hospital by getting it wrong! That was many
years ago before safety was taken quite so seriously as it is
now.
The concentrated sulphuric acid is a catalyst. Write it over the
arrow rather than in the equation.
Note: You will find the mechanism for the dehydration of
alcohols in the mechanism section of this site. You will also
find a discussion of how to cope with questions about the
dehydration of more complicated alcohols if you follow a link
at the bottom of that page.
Use the BACK button (or the HISTORY file or GO menu) on
your browser if you want to return to this page.
The dehydration of cyclohexanol to give cyclohexene
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.