What is the first step in the mechanism of the dehydration of 2-methyl-2-pentanol? The removal of a β-hydrogen from the alcohol The loss of OH- to form a carbocation The protonation of the hydroxyl group The loss of the proton from the hydroxyl group to give an alkoxide ion
The Correct Answer and Explanation is:
Correct Answer: The protonation of the hydroxyl group
Explanation
In the dehydration of 2-methyl-2-pentanol, the first step involves the protonation of the hydroxyl group. This process is acid-catalyzed, typically using a strong acid such as sulfuric acid (H₂SO₄), which provides protons (H⁺) to the reaction medium.
Alcohols are generally poor leaving groups because hydroxide ions (OH⁻) are not stable in acidic conditions. To make the -OH group a better leaving group, it is converted into a better one—water (H₂O)—by protonation. When the hydroxyl group on 2-methyl-2-pentanol accepts a proton, it transforms into a hydronium-like intermediate (-OH₂⁺), making it a much more stable and favorable leaving group.
Once the hydroxyl group is protonated, the subsequent step in the mechanism involves the loss of water, leading to the formation of a carbocation. In the case of 2-methyl-2-pentanol, this step results in a tertiary carbocation, which is relatively stable due to the inductive and hyperconjugative effects from the surrounding alkyl groups. The stability of this intermediate is crucial because it determines the feasibility of the reaction proceeding through this pathway.
Following carbocation formation, a β-hydrogen is then removed from a carbon adjacent to the carbocation center, resulting in the formation of a double bond. This elimination step completes the E1 (unimolecular elimination) mechanism, yielding an alkene as the final product.
Thus, the dehydration of 2-methyl-2-pentanol proceeds through three general steps: protonation of the hydroxyl group, loss of water to form a carbocation, and elimination of a β-hydrogen to form the alkene. Among these, the protonation of the hydroxyl group is the essential initiating event that enables the rest of the transformation.
