Intramolecular Dehydration

A very important reaction by which alcohols can be converted to alkenes is intramolecular dehydration.

An equation for such a reaction is shown here (and in Example 10-a in your workbook).

H H
| |
H-C-C-H
| |
H OH

H₂SO₄
žžžŽ
heat

H H
| |
H-C=C-H

+ H₂O

In this reaction the alcohol is heated in the presence of sulfuric acid. The -OH group on one carbon atom and the hydrogen atom on an adjacent carbon atom are split away from the molecule and are essentially replaced by a double bond. How concentrated the acid has to be and how hot the mixture has to be heated depends on the particular alcohol that is being dehydrated.

Equation for intramolecular dehydration of an alcohol. [63rxn05.JPG]

Another important thing to note, something that will become even more important later, is that since this dehydration occurs within a molecule it is called intramolecular dehydration. That name is used to distinguish this reaction from an intermolecular dehydration reaction in which the H- and the -OH come from two different molecules.

Notice that this is just the opposite of the reaction in which an alkene is converted to an alcohol (refer to Example 7-a in your workbook). The fact that this kind of a reaction can go in both directions using the same catalyst, tells you that we are dealing with an equilibrium reaction.

Equation showing the reversible conversion of alcohol to alkene. [63rxn06.JPG]

Because of this, when you start with an alkene and change it into an alcohol, you should expect to end up with a mixture containing some of the original alkene as well as some of the alcohol being made. If you start with an alcohol and change it into an alkene you would expect to have some of the original alcohol remaining in solution.

You should also realize that the ratio of alkene to alcohol in the resulting solution is going to depend upon the conditions under which the solution is kept. Conditions such as the concentration of acid, the temperature of the solution, and even the abundance of water will affect the equilibrium position and consequently the ratio of alkene to alcohol in the reaction mixture.

As an aside, let's relate this to the equilibrium constant expressions that you worked with last term. In reactions like this the concentration of water will be low enough to change considerably as the reaction takes place. Consequently, you cannot simply ignore the concentration of water as we did when we were dealing with ionic reactions.

Equation showing alcohol-alkene equilibrium and equilibrium constant expression.[63rxn07.JPG]

Those earlier ionic reactions were taking place in aqueous solution where the concentration of water was so large (about 55 moles per liter) that the formation or reaction of water had very little effect upon the position of equilibrium. That is, the changes in the amount and concentration of water were so small compared to what was there that they did not influence the equilibrium position very much. Therefore, we didn't even include it in the equilibrium constant expressions. If you were to deal with the equilibrium expression of a reaction like this one above, you would have to take water into account.

As I pointed out earlier, this kind of dehydration reaction in which you form an alkene from an alcohol is called intramolecular dehydration because all of the atoms for the water molecule came from the same alcohol molecule. The H and OH were on adjacent carbon atoms within the same molecule. This is intramolecular dehydration.

63rxn05.JPG (3515 bytes)

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E-mail instructor: Eden Francis

Clackamas Community College
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