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Acid-Catalyzed Dehydration of Alcohols

Laura Foist, Korry Barnes
  • Author
    Laura Foist

    Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

  • Instructor
    Korry Barnes

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

Learn about acid-catalyzed dehydration of alcohols. Understand the mechanism whereby alcohol dehydrates and the reaction produces another acidic structure. Updated: 02/18/2022

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Acid-Catalyzed Dehydration of Alcohols

When fruit is dehydrated, water is removed from the fruit. When organic compounds are dehydrated, water is also removed from the compound. Typically dehydration of organic compounds is done with alcohols as the starting material, and it proceeds as an acid-catalyzed dehydration. It is acid-catalyzed, because the acid is reformed at the end of the reaction, meaning that none of it is used up during the reaction. Acid-catalyzed dehydration of alcohols occurs when an acid puts an extra hydrogen atom on the alcohol. This changes the OH alcohol group into an {eq}H_2O {/eq}, or water, group attached to the organic compound. When that molecule leaves, it leaves as water - thus dehydration occurs.

The Reaction of Alcohol Dehydration

The final product with the alcohol dehydration reaction is an alkene. Dehydration of alcohols to alkenes occurs with an acid-catalyst, to hydrogenate the alcohol, forming water. The steps that follow depend on if the alcohol is a primary alcohol or a secondary/tertiary alcohol. Primary alcohols are when the OH group is attached to the terminal carbon (the last carbon in the chain). A secondary alcohol is when the carbon attached to the alcohol is attached to two other R groups and one hydrogen. A tertiary alcohol is when the carbon attached to the alcohol is attached to three other R groups and no hydrogen atoms. No matter the starting material, the product of an alcohol dehydration is an alkene.

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The Mechanism for Secondary and Tertiary Alcohol Dehydration

The dehydration of alcohol mechanism, for secondary and tertiary alcohols, follows an E1 mechanism. This mechanism, as with all dehydration of alcohol mechanisms, starts with the protonation of the OH group by the acid catalyst. Then the E1 part of the reaction occurs in two steps, with the first step (second step of the overall reaction) having the water leave, leaving behind a carbocation intermediate. In the second step (third step of the overall reaction) the conjugate base of the acid catalyst removes a hydrogen, and the alkene forms; then the acid is reformed.

Secondary and tertiary alcohols include compounds such as:

  • {eq}C_4H_9OH {/eq}, 2-butanol
  • {eq}C_6H_{13}OH {/eq}, 2-methyl-2-pentanol
  • {eq}C_5H_9OH {/eq}, cyclopentanol

Step 1

In Step 1, the alcohol is protonated, forming a water group. The alcohol can be protonated with any strong acid, which includes:

  • Hydronium ion: {eq}H_3O^+ {/eq}
  • Sulfuric acid: {eq}H_2SO_4 {/eq}
  • Nitric acid: {eq}HNO_3 {/eq}

The hydronium ion is a common acid to use, since the conjugate base is water, {eq}H_2O {/eq}. This means that both the conjugate base and the leaving group is water. Having plenty of the conjugate base, water, around helps ensure Step 3 occurs to reform the acid. If both the conjugate base of sulfuric or nitric acid and water is around, then step three may occur using the conjugate base or water; if the water is used then the original acid is not reformed.


In Step 1 of the alcohol dehydration the acid, in this case a hydronium ion, protonates the alcohol.

Step one of alcohol dehydration with secondary alcohol


Step 2

In Step 2 (which is the same as Step 1 in an E1 reaction), water leaves the organic compound, leaving behind a carbocation. This step is the rate-limiting step in the reaction, and it is the slowest step to occur. Water is a very good leaving group, and can exist easily on its own. However, carbon does not like to hold a positive charge, so it holds onto that water for as long as it can.


In Step 2, the water molecule leaves, leaving behind a carbocation.

Step two of alcohol dehydration with a secondary alcohol


Step 3

In Step 3 (or Step 2 in an E2 reaction), a hydrogen atom is removed from a neighboring carbon atom and forms an alkene. The conjugate base, in this case water, removed the hydrogen. Water isn't usually a good base and would not be able to remove a hydrogen on a regular hydrocarbon. However, the carbocation is very unstable, and makes the hydrogen much easier to remove. So even a weak base, such as water, is able to remove the hydrogen.


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Frequently Asked Questions

How does the acid catalyst facilitate alcohol dehydration?

The acid catalyst facilitates alcohol hydration by protonating the alcohol. This forms H2O, or water, which is an excellent leaving group.

What is the mechanism for the dehydration of an alcohol to an alkene?

For primary alcohols, the mechanism follows an E2 reaction. For secondary and tertiary alcohols, the mechanism follows an E1 reaction.

What is acid-catalyzed dehydration?

Acid-catalyzed dehydration is when water is removed from a molecule (dehydration). It is acid-catalyzed when an acid protonates the alcohol, but the acid is reformed by the end of the reaction.

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