Question

In this chapter, we studied the mechanism of the acid-catalyzed hydration of an alkene. The reverse of this reaction is the acid-catalyzed dehydration of an alcohol.

Short answer

Answer: The four main steps in the acid-catalyzed dehydration of an alcohol are: 1) Protonation of the alcohol, 2) Formation of a carbocation, 3) Rearrangement (if necessary), and 4) Elimination of a proton to form an alkene.

Step-by-step solution

Protonation of the alcohol

In this step, the alcohol reacts with a strong acid, like sulfuric acid (H2SO4). The oxygen atom of the alcohol (-OH) grabs a proton (H+) from the acid, forming a water molecule (H2O) and the conjugate base of the acid. The alcohol is transformed into a protonated alcohol with an OH2+ group.

Formation of a carbocation

Next, the protonated alcohol undergoes a heterolytic cleavage, where a water molecule is released, leaving behind a positively charged carbon atom (carbocation). The carbocation is electron-deficient, making it highly reactive and susceptible to rearrangement or reaction with a nucleophile.

Rearrangement (if necessary)

If possible, the carbocation may undergo a rearrangement to form a more stable carbocation through a hydride or alkyl shift. In this process, the neighboring hydrogen or alkyl group moves to the carbocation with its bonding electron pair, filling the positive charge, and creating a new, more stable, carbocation in a neighboring carbon atom. If the carbocation is already the most stable possible (for example, a tertiary carbocation), this step doesn't happen, so we can move to the next step.

Elimination of a proton

The carbocation reacts with the conjugate base of the acid (A-) produced in step 1 to remove a proton from an adjacent carbon, releasing an electron pair to form a double bond between the carbocation carbon and the adjacent carbon, which leads to the elimination of the positive charge. This step generates an alkene and regenerates the acid catalyst. In conclusion, the acid-catalyzed dehydration of an alcohol follows these steps: protonation, formation of a carbocation, possible rearrangement, and elimination of a proton to form an alkene.