Dehydration of Cyclohexanol: Mechanism & Overview

Instructor: Korry Barnes

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

In this lesson, we will focus on the dehydration reaction that cyclohexanol undergoes under acidic conditions to produce cyclohexene, a useful building block in organic chemistry.

Thirsty Organic Compounds

Did you know, just like living organisms that depend on water for life, that some organic compounds can be dehydrated? When a human being is dehydrated, it means what? That they need water, of course! You see, the word dehydrated simply means 'without water.' So when you hear that term your mind immediately jumps to thinking about the need for water, because something or someone is without it.

When we say that an organic compound has been dehydrated, just think about it the same way you normally would, without water! When a dehydration reaction happens, a molecule of water is lost from an organic substrate. Unlike a human that can become dehydrated just by not consuming water, an organic compound typically doesn't lose water on its own. For the reaction to happen it needs some help, and usually the most common helper is an acid of some sort. Acids are relatively cheap, abundant, and readily available, which is why they find such common use in this type of transformation. Let's discuss the acid-promoted dehydration of cyclohexanol and explore in detail how this process works!

Alcohols as Dehydration Candidates

In terms of what kinds of organic compounds can undergo dehydration reactions, alcohols are among the most common. Why might that be the case? What makes alcohols unique that they lend themselves to this type of reaction? The reason lies in the fact that the oxygen atom on an alcohol has two lone pairs (non-bonding electrons) that it can use to bond to a hydrogen ion from our acid. Once that happens, we form a very special intermediate. Let's dive into the reaction mechanism to see how this works.

Dehydration Reaction Mechanism

First Step:

As we mentioned previously, it's the lone pairs of electrons on the oxygen atom of the alcohol group that play a major role in the reaction. In the first step, the OH group on cyclohexanol accepts a hydrogen ion from an acid. Note that any acid will work in terms of the reaction, but more common acids that are used include sulfuric acid and phosphoric acid.


A couple of things are worth mentioning about the first step in the mechanism:

1. Notice how the arrow goes from the oxygen to the hydrogen ion. When drawing any mechanism, arrows always need to 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. Note 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.

Second Step:

Do you notice something special about the intermediate that we just formed??? There's a water molecule attached to the cyclohexane ring! In the second step of the mechanism, the water molecule pops off and generates a carbocation (carbon with a positive charge) intermediate.

Second mechanistic step of dehydration where a water molecule leaves

Third Step:

Okay, so now we've formed a carbocation intermediate, which is a VERY reactive species. Normally we wouldn't really think of water as being that great of a base. In this case, however, since the organic carbocation is so reactive, even water is sufficient enough to come in, pull off a hydrogen, and form our final product. Once the hydrogen has been removed, the product is. . . cyclohexene!

Final mechanistic step of the dehydration to form the alkene product

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