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F. The Dehydration of Butan-2-ol

This page builds on your understanding of the acid catalysed dehydration of 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.

 

Background

To make the diagrams less cluttered, we'll use the simplified version of the mechanism showing gain and loss of H+.

Remember that the mechanism takes place in three stages:

  • The alcohol is protonated by the acid catalyst.

  • The protonated alcohol loses a water molecule to give a carbocation (carbonium ion).

  • The carbocation formed loses a hydrogen ion and forms a double bond.

So, in the case of the dehydration of propan-2-ol:

 

 

The dehydration of butan-2-ol

 

The first two stages

There is nothing new at all in these stages.

In the first stage, the alcohol is protonated by picking up a hydrogen ion from the sulphuric acid.

 

 

In the second stage, the positive ion then sheds a water molecule and produces a carbocation.

 

 

The complication arises in the next step. When the carbocation loses a hydrogen ion, where is it going to come from?

 

Where does the hydrogen get removed from?

So that a double bond can form, it will have to come from one of the carbons next door to the one with the positive charge.

 

If a hydrogen ion is lost from the CH3 group

 

 

But-1-ene is formed.

If a hydrogen ion is lost from the CH2 group

 

 

This time the product is 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.

 

 

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.

 

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)