Alcohols (5) Dehydration of alcohols

The dehydration of alcohols appears fairly commonly in school and college chemistry courses.

Of course, dehydration means the removal of the elements of water from an organic molecule like an alcohol. 

Several different reagents are or can be used but the common ones are aluminium oxide, concentrated sulphuric acid or phosphoric acid. 

With aluminium oxide, Al₂O₃, the apparatus used is given below:

The problem with this and all other diagrams I can find on the Internet and in text books is that they suggest that the amount of aluminium oxide used is far too low. 

The better method is to put the 2-3ml of alcohol in the bottom an upright test-tube, vertical at first not horizontal.  

Then you add enough mineral wool to absorb the alcohol.  

After that, fill the whole tube with aluminium oxide granules leaving just enough room for the bung and delivery tube. 

Now turn the test-tube horizontal and you are ready to dehydrate your alcohol.

Heating the granules not the alcohol on the mineral wool should generate a steady flow of alkene bubbles. 

But remember the first bubbles will be air from the tube—this stops briefly and then starts again and this gas is your alkene.

The equation is given on the diagram above.

You should be able to satisfy yourself that you can construct these dehydration equations.

You also ought to be able to see why butan-2-ol generates a mixture of two different alkenes.

Dehydration using a strong acid catalyst uses a different method. 

The liquid alcohol is heated with the acid catalyst and the more volatile alkene (it no longer contains hydrogen bonds)  is distilled over or collected as a gas over water. 

Advanced courses will also require you to build the reaction mechanism as here:

The stages are as follows

—Initial protonation of the alcohol —O—H by the acid catalyst,

—Loss of the elements of water leaving a carbocation.

—Rearrangement of the carbocation and loss of a proton  

   facilitated by the very polar acid solvent.

Test for the Alkene:

The usual test for an alkene can be used just to confirm the product is what it is supposed to be

Add bromine water to the suspected alkene and cork and shake the test tube.

The orange bromine water turns colourless and confirms the presence of an alkene C=C double bond.