Acid-Catalyzed Dehydration of Alcohols to Alkenes

The elimination of a hydroxyl group in an alcohol along with the hydrogen from an adjacent carbon can yield an alkene. As a molecule of water is lost, this is a dehydration reaction. Alcohols are commonly dehydrated by heating in the presence of an acid catalyst.

Dehydration of primary alcohols is the most difficult and thus needs harsh conditions. Secondary alcohols require lower temperatures and acid concentrations, while tertiary alcohols can lose a water molecule under relatively mild conditions.

Acid-catalyzed dehydration of secondary and tertiary alcohols proceeds via an E1 mechanism. First, the oxygen atom of the hydroxyl accepts a proton from the acid in a fast step, thereby becoming a better leaving group.

Next, a molecule of water is lost from the oxonium ion in the slow, rate-determining step, leaving behind a carbocation. Then, the conjugate base removes a β hydrogen to yield the alkene. This step regenerates the acid catalyst.

Here, the stability of the carbocation intermediate determines the major products.

For instance, when 3,3-dimethyl-2-butanol undergoes acid-catalyzed dehydration, the secondary carbocation rearranges to a more stable tertiary carbocation.

When isomeric products are possible, the more-substituted alkene, or Zaitsev product, is favored.

Primary alcohols would yield highly unstable primary carbocations. As a result, their dehydration occurs via the E2 mechanism.

This mechanism also begins with the protonation of the alcohol. In the next step, a base removes the β hydrogen and water departs, forming the double bond to yield a terminal alkene.

However, in the acidic solution, the double bond can be rehydrated according to Markovnikov’s rule. A 1,2-hydride shift yields a secondary carbocation, which then loses a proton in accordance with Zaitsev's rule. Thus, 2-butene is the major product.

If the hydroxyl is converted to a better leaving group in secondary or tertiary alcohols, treatment with a strong base can enable dehydration via the E2 mechanism.