10.5:
Acid-Catalyzed Dehydration of Alcohols to Alkenes
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In a dehydration reaction, a hydroxyl group in an alcohol is eliminated along with the hydrogen from an adjacent carbon. Here, the products are an alkene and a molecule of water. Dehydration of alcohols is generally achieved by heating in the presence of an acid catalyst. While the dehydration of primary alcohols requires high temperatures and acid concentrations, secondary and tertiary alcohols can lose a water molecule under relatively mild conditions.
The acid-catalyzed dehydration of secondary and tertiary alcohols proceeds via an E1 mechanism. First, the hydroxyl group in the alcohol is protonated in a fast step to form an alkyloxonium ion. Next, a molecule of water is lost from the alkyloxonium ion in the slow, rate-determining step, leaving behind a carbocation. Finally, water, which is the conjugate base of H3O+, removes a β hydrogen from the carbocation to yield the alkene. This step regenerates the acid catalyst.
In these reactions, the stability of the carbocation intermediate determines the major products. When possible, secondary carbocations undergo rearrangement to form more stable tertiary carbocations. Additionally, when isomeric products are possible, the more-substituted alkene, or Zaitsev's 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 a water molecule is lost. Thus, a double bond is formed, yielding a terminal alkene.
However, in the acidic solution, rehydration of the double bond (according to Markovnikov's rule) followed by a 1,2-hydride shift can yield a secondary carbocation. Loss of a proton, in accordance with Zaitsev's rule, then results in a mixture of the terminal and rearranged alkenes.
Secondary or tertiary alcohols can also undergo dehydration via the E2 mechanism if the hydroxyl group is first converted to a good leaving group, such as a tosylate. Treatment of the tosylate with a strong base yields the alkene.