Question

The acid-catalyzed hydration of 3,3-dimethyl-1-butene gives 2,3-dimethyl-2butanol as the major product. Propose a mechanism for the formation of this alcohol.

Short answer

Question: Propose a mechanism for the formation of 2,3-dimethyl-2-butanol as the major product in the acid-catalyzed hydration of 3,3-dimethyl-1-butene. Answer: The formation of 2,3-dimethyl-2-butanol in the acid-catalyzed hydration of 3,3-dimethyl-1-butene follows a three-step mechanism: 1) protonation of the alkene forming a carbocation, 2) nucleophilic attack by water on the carbocation, and 3) deprotonation of the intermediate to form the final product. The carbocation is formed on the secondary carbon due to its stability from the inductive effect of neighboring methyl groups. Water acts as a nucleophile, attacking the carbocation and forming an intermediate. Finally, water acts as a base and deprotonates the intermediate, forming 2,3-dimethyl-2-butanol.

Step-by-step solution

Write the reaction

First, let's write the given reaction: 3,3-dimethyl-1-butene + H2O -> 2,3-dimethyl-2-butanol

Identify the acid catalyst

In an acid-catalyzed hydration, the acid (typically H2SO4 or H3PO4) acts as a catalyst to help the reaction take place. The acid donates a proton (H+) to the alkene, making it positive and more susceptible to reactions with nucleophiles.

Determine the mechanism

The acid-catalyzed hydration of alkenes follows a three-step mechanism: 1. Protonation of the alkene to form a carbocation. 2. Nucleophilic attack by water on the carbocation. 3. Deprotonation of the intermediate to form the final product. Let's go through each step for the formation of 2,3-dimethyl-2-butanol.

Protonation of the alkene

In the first step, 3,3-dimethyl-1-butene reacts with the acid catalyst (H+) and becomes protonated, creating a carbocation: \[ \text{H}^{+} + \text{CH}_{2}=\text{C}(\text{CH}_{3})-\text{C}(\text{CH}_{3})_{2} \rightarrow \text{CH}_{3}\text{-C}^{+}(\text{CH}_{3})\text{-CH-CH}_{3} \] The carbocation is formed on the secondary carbon; this is the most stable possibility due to the inductive effect of the two neighboring methyl groups.

Nucleophilic attack by water

In the next step, a water molecule (H2O) reacts with the carbocation as the nucleophile, attacking the positively charged secondary carbon center and forming an intermediate: \[ \text{H}_{2}\text{O} + \text{CH}_{3}\text{-C}^{+}(\text{CH}_{3})\text{-CH-CH}_{3} \rightarrow \text{CH}_{3}\text{-C}(\text{OH})(\text{CH}_{3})\text{-CH-CH}_{3}\text{ + }\text{H}^{+} \]

Deprotonation of the intermediate

In the final step, a water molecule acts as a base and deprotonates the intermediate: \[ \text{H}_{2}\text{O} + \text{CH}_{3}\text{-C}(\text{OH})(\text{CH}_{3})\text{-CH-CH}_{3}\text{ + }\text{H}^{+} \rightarrow \text{CH}_{3}\text{-C}(\text{OH})(\text{CH}_{3})\text{-C}\text{H}_{2}-\text{C}\text{H}_{3} \] The final product is 2,3-dimethyl-2-butanol. This mechanism explains the formation of this alcohol as the major product in the acid-catalyzed hydration of 3,3-dimethyl-1-butene.

Key concepts

Carbocation Formation

The process of carbocation formation is critical in understanding the acid-catalyzed hydration mechanism. A carbocation is a species with a positively charged carbon atom, which results from the loss of a leaving group—in this case, a proton from the acid catalyst. In organic chemistry reactions, the formation of a carbocation often determines the course the reaction will take.

For example, during the hydration of 3,3-dimethyl-1-butene, the alkene is protonated, resulting in a carbocation. Stability of the carbocation is key and is influenced by the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are more stable than secondary, which are more stable than primary ones, due to the inductive effect and hyperconjugation from neighboring carbon atoms.

This explains why in our exercise, the carbocation is formed on the secondary carbon, as it's supported by the two adjacent methyl groups, making it energetically favorable. It's this intermediate that defines the next crucial steps in the reaction.

Nucleophilic Attack

Following the formation of a carbocation, the next step in the acid-catalyzed hydration mechanism is the nucleophilic attack. A nucleophile is a molecule or ion that donates an electron pair to form a chemical bond in reaction to a positively charged center. Water, in this context, acts as a nucleophile.

The partially positive charge of the carbocation serves as a beacon, attracting the nucleophile. In our exercise, the oxygen atom in water uses its lone pair of electrons to bond with the carbocation, creating a new intermediate. This step is a pivotal moment in the hydration as the alkene is transformed into an alcohol. The subsequent steps are critical in finalizing the structure of the product, and understanding this attack helps us anticipate the product's properties and behavior in further chemical contexts.

Alkene Protonation

Alkene protonation is the first step in the acid-catalyzed hydration of alkenes and initiates the mechanism of many organic chemistry reactions. An alkene is a hydrocarbon with a carbon-carbon double bond, and protonation involves adding a hydrogen ion (H+) from an acid to one of the carbon atoms in this double bond.

In our exercise, the double bond in 3,3-dimethyl-1-butene acts as a site for protonation. The acid donates a proton to the alkene, which disrupts the double bond, leading to the formation of a more reactive carbocation. The location where the proton adds to the alkene is crucial and often follows Markovnikov's rule, where the proton adds to the carbon with the greatest number of hydrogen atoms. The new bond's location directly influences the course of the reaction, notably determining where the nucleophilic attack will occur and thus shaping the resulting product.

Organic Chemistry Reactions

Understanding the acid-catalyzed hydration mechanism gives insight into broader organic chemistry reactions. Reactions in organic chemistry can be complex, involving multiple steps and species, such as reactants, intermediates, and products. Mechanisms describe the step-by-step sequence of events that lead from reactants to products, often involving the creation and stabilization of high-energy intermediates, like carbocations.

The acid-catalyzed hydration we explored exemplifies a common type of reaction involving the conversion of an alkene to an alcohol. This particular reaction is important in both academic and industrial settings, as it forms the basis for creating various alcohols with wide-ranging applications. Educating oneself on such mechanisms is crucial because they are foundational in synthesis and understanding how and why certain products are formed under different conditions.