Acid-Catalyzed Dehydration of Alcohols: Reaction & Mechanism

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

This lesson will focus on the detailed mechanism behind how organic alcohols undergo a dehydration reaction in the presence of an acid to produce alkenes.

Molecules Without Water

When you hear the term dehydration, what's the first thing that comes to your mind? Maybe you think of someone who needs water. Perhaps you jump to thinking about sports drinks such as Gatorade that help to replenish and re-hydrate someone. Whatever you tend to think of, when the term dehydration is mentioned, we all immediately think of some sort of need for water. In its simplest form, the word dehydrated means ''without water.''

What most people don't realize is that certain organic compounds (chemical compounds that contain carbon and hydrogen) can actually become dehydrated, simply meaning that they can lose water just like we do when we sweat or don't drink enough fluids. What we are going to be looking into in this lesson is how an organic alcohol undergoes a dehydration reaction by using cyclopentanol as our model substrate (substrate is a general term for compound). We will be breaking down how the reaction works by looking at each step of the reaction mechanism. Why is this important, you ask? Well, dehydration reactions help us to take certain chemicals and transform them into something else that could be of use. This is kind of like how a chef takes individual ingredients and crafts them into a culinary work of art that's delicious to eat. Let's get started!

General Dehydration Reaction Structure

As a general pattern that's nice to be aware of, a dehydration reaction is when an organic compound undergoes the loss of a water molecule to form an alkene (an organic compound with a carbon-carbon double bond) as the product. Before we get into the details of the reaction mechanism, notice the general form of the reaction in which cyclopentanol undergoes loss of a water molecule under acidic conditions to form cyclohexene.

Dehydration of cyclopentanol under acidic conditions to give cyclopentene

The Mechanism of the Dehydration Reaction

Step 1

In the first step of the reaction mechanism, the oxygen atom of the hydroxyl group uses a pair of electrons to pick up a hydrogen ion floating in our acidic solution (remember that an acid solution is simply a source of hydrogen ions).


A couple of items are worth mentioning at this point in the process:

1. Note how the mechanism arrow starts at the oxygen and points to the hydrogen ion. When drawing our mechanism, arrows always must be drawn from where electrons are (on the oxygen in the form of lone pairs) to where electrons are going (to the hydrogen ion).

2. Notice the formal charge on the oxygen after it bonds to the hydrogen ion. Anytime an oxygen has three bonds and one lone pair, it will always carry a formal +1 charge.

Step 2

At this point in the mechanism, do you notice something special about the intermediate that we just formed??? There's a water molecule attached to the cyclopentane ring! In the second step of the mechanism, the water molecule falls off and generates a carbocation (carbon with a positive charge) intermediate.

Step 2 of the dehydration mechanism in which a water molecule leaves

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