Acid-catalyzed dehydration of 2-butanol yields 2-butene as the major product. Draw curved arrows to show the movement of electrons in this step of the reaction mechanism.
The Correct Answer and Explanation is :
iturn0image0turn0image1turn0image3turn0image7The acid-catalyzed dehydration of 2-butanol to form 2-butene proceeds through a three-step mechanism:
- Protonation of the Alcohol: The hydroxyl group (-OH) of 2-butanol is protonated by the acid catalyst, converting it into a better leaving group, water (H₂O).
- Loss of Water: The protonated alcohol undergoes a loss of water, resulting in the formation of a carbocation intermediate at the second carbon (C₂).
- Deprotonation to Form the Alkene: The carbocation formed at C₂ is deprotonated by a base (often the conjugate base of the acid catalyst), leading to the formation of 2-butene.
This mechanism is classified as an E1 (unimolecular elimination) reaction, where the rate-determining step is the formation of the carbocation intermediate.
Explanation:
- Step 1: Protonation of the Alcohol: The lone pair of electrons on the oxygen atom of the hydroxyl group attacks a proton (H⁺) from the acid catalyst. This protonation increases the electrophilicity of the oxygen, making it a better leaving group.
- Step 2: Loss of Water: The protonated alcohol is now a good leaving group. It departs as water (H₂O), leaving behind a carbocation intermediate at the second carbon (C₂).
- Step 3: Deprotonation to Form the Alkene: The carbocation at C₂ is unstable and seeks to stabilize by losing a proton (H⁺) from an adjacent carbon (C₃). This deprotonation results in the formation of a double bond between C₂ and C₃, yielding 2-butene as the major product.
The formation of the carbocation intermediate is the rate-determining step in this mechanism. The stability of the carbocation intermediate is crucial; a more stable carbocation will form more readily, leading to a faster reaction. In the case of 2-butanol, the carbocation formed at C₂ is relatively stable due to hyperconjugation and inductive effects from the adjacent methyl group.
The E1 mechanism is favored under acidic conditions, especially with secondary alcohols like 2-butanol, because the protonation of the alcohol and the subsequent loss of water are both facilitated by the acidic environment. Additionally, the formation of a stable carbocation intermediate is more favorable in this context.
In summary, the acid-catalyzed dehydration of 2-butanol to 2-butene involves the protonation of the alcohol, loss of water to form a carbocation intermediate, and deprotonation to yield the alkene product. This E1 mechanism is characterized by the formation of a carbocation intermediate and is favored under acidic conditions.
For a visual explanation of this mechanism, you might find the following video helpful:
videoAcid-Catalyzed Dehydration Mechanismturn0search3