Question

Acid-catalyzed dehydration of 3 -methyl-2-butanol gives three alkenes: 2 -methyl-2butene, 3 -methyl-1-butene, and 2 -methyl-1-butene. Propose a mechanism to account for the formation of each product.

Short answer

Question: Provide an overview of the acid-catalyzed dehydration of 3-methyl-2-butanol and propose the mechanisms for the formation of three different alkene products, namely 2-methyl-2-butene, 3-methyl-1-butene, and 2-methyl-1-butene. Answer: Acid-catalyzed dehydration of 3-methyl-2-butanol is an elimination reaction that forms three alkene products: 2-methyl-2-butene, 3-methyl-1-butene, and 2-methyl-1-butene. The reaction mechanism involves protonation of the alcohol to form an oxonium ion intermediate, followed by the formation of a secondary carbocation intermediate. To form each alkene product, a base abstracts a proton from the carbocation intermediate or its shifted counterpart. The formation of 2-methyl-2-butene occurs directly from the secondary carbocation, while 3-methyl-1-butene forms after a hydride shift. The formation of 2-methyl-1-butene also takes place directly from the secondary carbocation.

Step-by-step solution

Acid-catalyzed dehydration reaction overview

Acid-catalyzed dehydration of alcohols is an elimination reaction in which the alcohol loses a water molecule (H₂O) in the presence of an acid catalyst, forming an alkene. The general mechanism involves protonation of the alcohol, followed by the formation of a carbocation intermediate, and finally, the elimination of a proton to form the alkene product.

Protonation of 3-methyl-2-butanol

The first step in all three mechanisms will be the same. An acid catalyst (for example, H₂SO₄) donates a proton to the oxygen atom in the 3-methyl-2-butanol, forming a positively charged oxonium ion intermediate. This intermediate is represented as: CH₃-C(CH₃)-CH₂-CH₂-OH₂⁺

Formation of carbocation intermediate

In this step, the oxonium ion undergoes a loss of a water molecule (H₂O), resulting in the formation of a carbocation intermediate. There are two possible carbocation formation, primary and secondary. Choosing the most stable carbocation is important. A secondary carbocation is more stable than a primary one, so we consider the secondary carbocation: CH₃-C⁺(CH₃)-CH₂-CH₃

Formation of 2-methyl-2-butene

In this case, a base present in the solution (for example, HSO₄⁻) abstracts a proton from the carbon adjacent to the positively charged carbon: CH₃-C(json):CH(CH₃)-CH₃ This results in the formation of the double bond and the product 2-methyl-2-butene.

Formation of 3-methyl-1-butene

To form 3-methyl-1-butene, there will be a hydride shift from the carbon adjacent to the carbocation. This causes the carbocation to be relocated: CH₃-CH(CH₃)-C⁺-CH₃ Next, a base abstracts a proton from the carbon adjacent to the positively charged carbon: CH₂=C(CH₃)-CH₂-CH₃ This leads to the formation of the double bond and the product 3-methyl-1-butene.

Formation of 2-methyl-1-butene

Similarly, to form 2-methyl-1-butene, a base abstracts a proton from the carbon adjacent to the carbocation in the secondary carbocation: CH₃-CH=C(CH₃)-CH₃ This results in the formation of the double bond and the product 2-methyl-1-butene. These three mechanisms account for the formation of each product in the acid-catalyzed dehydration of 3-methyl-2-butanol.

Key concepts

Alkene Formation

Under the umbrella of organic transformations, the formation of alkenes is a fundamental reaction. Alkenes are hydrocarbons characterized by at least one double bond between carbon atoms, offering diverse chemical properties and vast industrial applications. In the context of acid-catalyzed dehydration, alkenes emerge from alcohols where the -OH group and a hydrogen atom from an adjacent carbon atom are removed.

Considering our example with 3-methyl-2-butanol, the end game is the formation of different alkenes by repositioning the double bond. The crux of this conversion lies in the stability of the intermediate forms and the acid's role in facilitating the elimination process. By abstracting a proton, the acids tip the scales to favor the creation of a double bond, bringing the alkene into existence. The subsequent stabilization of the molecule guides which of the possible alkenes is the major product.

Carbocation Intermediate

The carbocation intermediate forms the beating heart of various chemical reactions, including the acid-catalyzed dehydration of alcohols. This highly reactive species is a carbon atom bearing a positive charge, and its stability is pivotal for the overall reaction pathway. Secondary carbocations, like the one we encounter in the dehydration of 3-methyl-2-butanol, trump their primary counterparts in terms of stability owing to additional alkyl groups that provide electron-donating effects through hyperconjugation.

Moreover, the rearrangement of the carbocation, a process where the positive charge hops to a more stable location, may occur. This can result in different positional isomers of the final alkene product. For instance, a hydride shift transforms a less stable secondary carbocation into a more stable tertiary one, allowing for a fascinating array of possible products and demonstrating the profound impact of carbocation stability on reaction outcomes.

Protonation of Alcohols

In the magical molecular dance of converting an alcohol to an alkene, the first move is protonation. Picture the alcohol, in our scenario 3-methyl-2-butanol, as a dancer awaiting the right partner to initiate the steps. The dance floor? Acidic conditions. The acid—a chemical matchmaker—distributes protons to the oxygen atom in the alcohol, morphing it into a favorable state for parting with water. This is the oxonium ion, a transient yet crucial intermediary, bearing an endearing positive charge that makes it a good leaving group.

From an electron's-eye view, this attraction of electrons toward the proton paves the way for the oxygen to relinquish its hold on the hydrogen atom attached to the carbon. This is the tipping point, which sets the stage for the elimination of water and the subsequent formation of the carbocation intermediate, ultimately leading the alcohols down the path to become alkenes.

Dehydration Reaction

The dehydration reaction is a chemical ballet, where molecules, particularly alcohols, gracefully part with water molecules. It's all about loss but in a productive sense, as it cultivates new connections between carbon atoms—a double bond, the hallmark of alkenes. The acid-catalyzed dehydration bursts into action in the presence of a strong acid such as sulfuric acid or phosphoric acid. These proton providers assist in the departure of water, leaving the stage open for the alkene to form.

In the transformation of 3-methyl-2-butanol, dehydration is the overarching theme that involves the loss of the -OH group and a neighboring hydrogen. By employing an acid to add tension—protonating the alcohol—the reaction ensures that water slips away, smoothly and inevitably. This enables the carbon atoms, previously part of the alcohol, to forge a new path as part of a double-bonded and more hydrophobic alkene structure.