Question

Evidence for carbocation intermediates in the acid-catalyzed dehydration of alcohols comes from the observation that rearrangements sometimes occur. Propose a mechanism to account for the formation of 2,3 -dimethylbut-2-ene from 3,3 -dimethylbutan-2-ol. (Review Section 7.10.)

Short answer

3,3-dimethylbutan-2-ol is protonated, loses water to form a carbocation, then deprotonates to yield 2,3-dimethylbut-2-ene.

Step-by-step solution

Protonation of the Alcohol

The mechanism begins with the protonation of the hydroxyl group of 3,3-dimethylbutan-2-ol by an acid, such as sulfuric acid. This results in the formation of an oxonium ion, making the hydroxyl group a better leaving group.

Formation of the Carbocation

Once the hydroxyl group is protonated, water (a good leaving group) departs, and a carbocation intermediate is formed at the 2-position of the original alcohol. This carbocation is relatively stable due to resonance and hyperconjugation.

Carbocation Rearrangement

The 2°C carbocation can undergo hydride or alkyl shift to form a more stable carbocation. However, in this scenario, the tert-butyl group remains intact without any major shifts due to its already stable nature.

Deprotonation to Form Alkene

A base, such as the bisulfate ion from sulfuric acid, removes a proton from one of the adjacent carbon atoms to the carbocation. This elimination step forms a double bond between the 2nd and 3rd carbon, resulting in 2,3-dimethylbut-2-ene.

Key concepts

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a crucial reaction often used in organic chemistry to convert alcohols into alkenes. In this reaction, an acid such as sulfuric acid serves as a catalyst to help remove or "dehydrate" water from alcohol molecules, leading to the formation of a double bond. The process begins with protonation, where the acid donates a proton to the alcohol's hydroxyl group (OH group). This protonated hydroxyl group now becomes an excellent leaving group, transforming into water—a stable molecule eager to leave the compound.

• Protonated hydroxyl turns into water, a great leaving group.
• This step is critical for the eventual breakage of the C-O bond.

After the leaving group departs, the remaining structure often forms a carbocation—a positively charged carbon atom, susceptible to further reactions.
Understanding these initial steps of acid-catalyzed dehydration is vital for grasping how alcohols transform into alkenes.

Mechanism of Alcohol Dehydration

The mechanism of alcohol dehydration is a sequence of transformations that turn an alcohol into an alkene. Let's delve into the detailed steps of this mechanism:

1. **Protonation of the Alcohol**: The reaction begins with the protonation of the hydroxyl group by the acid's proton. This step makes the alcohol ready to lose its water component.
2. **Formation of the Carbocation**: Once the OH group leaves, a carbocation forms. This intermediate serves as a reactive hotspot that can undergo various changes to improve stability.

• Protonation leads to a better leaving group (water).
• Carbocations can rearrange to reach more stable forms by shifting adjacent hydrogen atoms or alkyl groups.

3. **Carbocation Rearrangement**: Sometimes, the structure rearranges itself to find a more stable carbocation, although in stable systems like tert-butyl groups, this shift might be unnecessary.
These steps set the stage for the creation of alkenes, marking the end of the dehydration sequence.

Alkene Formation

The last phase of converting an alcohol to an alkene involves the creation of a double bond through a process called elimination. Initially, a base—often part of the acid used for catalysis, such as the bisulfate ion—captures a neighboring hydrogen atom. As this happens, two electrons from the C-H bond move to form a C=C double bond.

• A hydrogen is removed from a carbon next to the carbocation.
• The electrons from this hydrogen create a double bond.

This crucial step locks the molecular structure into an alkene, a more stabilized form of the molecule after the dehydration process.
By understanding the formation of alkenes, you gain insight into how simple alcohols can be transformed into more complex and reactive organic compounds. These reactions open the door to further chemical modifications, making alkene formation a cornerstone in synthetic chemistry.