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.

Short answer

The mechanism involves protonation, water loss, carbocation rearrangement, and elimination.

Step-by-step solution

Identify the Substrate and Reaction Conditions

We are dealing with the acid-catalyzed dehydration of the alcohol 3,3-dimethylbutan-2-ol. The conditions suggest protonation and formation of a carbocation intermediate. The goal is to rearrange and ultimately form 2,3-dimethylbut-2-ene.

Protonation of the Alcohol

The first step is the protonation of the hydroxyl group of 3,3-dimethylbutan-2-ol by the acid, typically sulfuric acid. This increases the ability of the molecule to lose the leaving group (water).

Formation of the Carbocation Intermediate

Once the hydroxyl group is protonated, it becomes water, a good leaving group, and leaves the substrate, resulting in the formation of a carbocation at the 2-position of the carbon chain. This carbocation undergoes rearrangement.

Carbocation Rearrangement by Methyl Shift

The carbocation undergoes a rearrangement by a 1,2-methyl shift, moving a methyl group from the quaternary carbon (3rd position) to the carbocation's position. This stabilizes the carbocation by relocating it to a more stable tertiary carbocation.

Elimination to Form the Alkene

Finally, a proton is eliminated from a neighboring carbon, typically in a step facilitated by the conjugate base of the acid used, resulting in the formation of a double bond. This produces the more stable 2,3-dimethylbut-2-ene.

Key concepts

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a common reaction in organic chemistry that involves the removal of a water molecule from an alcohol to form an alkene. In this reaction, an acid such as sulfuric acid is used to provide a catalyst that helps in facilitating the transformation.
The process begins by protonation of the alcohol, which makes the hydroxyl group into a better leaving group, forming water. As the reaction proceeds, water is released as a by-product, hence the term "dehydration." By using an acid, the reaction is often driven towards the alkene product through the stabilizing intermediates formed along the way. This is particularly useful when such rearrangements lead to more stable products.
Dehydrations with the aid of acid catalysts are widely applied in synthetic pathways to build complex molecules from simpler alcohols.

Mechanistic Steps

Understanding the mechanistic steps in an acid-catalyzed dehydration can greatly help in predicting the outcomes of the reaction. This process is sequential and follows a structured path that begins with protonation.

• First, the alcohol is protonated by the acid, enabling the formation of water as a good leaving group.
• The leaving of the water group leads to the formation of a carbocation, which is often the key step.
• Following this, a rearrangement such as a methyl shift can occur to stabilize the carbocation.
• Finally, the loss of a proton results in the formation of an alkene.

These steps highlight the transition from a less stable starting material to a more stable final product, the alkene, harnessing the reactivity and rearrangement capabilities of the intermediates.

Alkene Formation

Alkene formation is the ultimate goal in the acid-catalyzed dehydration process. By understanding the steps leading up to it, better predictions on the structure and stability of the resulting alkene can be made.
Alkenes are formed through the removal of a proton from the carbocation. This proton removal is usually facilitated by the conjugate base of the acid, resulting in the formation of a carbon-carbon double bond.
The stability of the resulting alkene often depends on the substitution pattern; more substituted alkenes (those with more groups attached to the double-bonded carbons) tend to be more thermodynamically stable. In this reaction, forming 2,3-dimethylbut-2-ene from the alcohol involves rearrangement steps that allow for a more stable and substituted double-bond to form.

Protonation

Protonation is the initial and crucial step in acid-catalyzed dehydration. It involves adding a proton (H⁺) to the oxygen atom of the alcohol's hydroxyl group.
This step is essential as it transforms the hydroxyl group into a better leaving group, specifically water, which can depart from the molecular structure easily. The effectiveness of this step is due to the use of strong acids like sulfuric acid, which readily donate protons, enhancing the reaction.
This protonation sets the stage for subsequent steps, allowing for the necessary formation of carbocations and subsequent rearrangements that lead to stable alkene products.

Methyl Shift

A methyl shift is a type of carbocation rearrangement beneficial in stabilizing an otherwise less stable carbocation that may have formed in the previous steps of the dehydration reaction.
In a methyl shift, a methyl group (a carbon attached to three hydrogen atoms, –CH₃) moves from an adjacent carbon atom to the carbocation's carbon, thereby enhancing stability.
This rearrangement occurs to achieve a more stabilized carbocation, typically from a secondary to a tertiary carbocation, owing to the higher electron donation and hyperconjugation capabilities of tertiary structures. Understanding methyl shifts is vital as they can explain how molecules reorganize themselves to reach the final, more stable alkene configuration.

Carbocation Stability

The formation and stability of carbocations are pivotal in the acid-catalyzed dehydration process. Carbocations are positively charged carbon intermediates which possess a significant role in organic reactions.
A carbocation's stability is determined by several factors including its degree (primary, secondary, or tertiary), with tertiary carbocations being the most stable due to optimal electron donation from surrounding carbons.

• Primary Carbocation: Least stable, least substituted.
• Secondary Carbocation: Moderately stable, somewhat substituted.
• Tertiary Carbocation: Most stable, highly substituted.

Additionally, carbocation stability can also be enhanced through resonance or hyperconjugation. This stability is crucial as more stable carbocations tend to lead to more successful and predictable reaction outcomes. Recognizing the importance of carbocation stability can aid in anticipating the potential rearrangements that occur during dehydration reactions.