Problem 170 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the product of the following reaction. \(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)-\mathrm{CH}=\mathrm{CH}_{2} \frac{\mathrm{H}^{+}}{\mathrm{H}_{2} \mathrm{O}} \rightarrow\) Product (a) CC(C)C(C)O (b) \(\mathrm{CH}_{3}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

The product of the reaction is option (a), CC(C)C(C)O.

Step 1: Understand the Reaction Type

The given reaction involves an alkene undergoing an acid-catalyzed hydration process. In this reaction, an alkene reacts with a dilute acid (often represented by \( ext{H}^{+}/ ext{H}_2 ext{O}\)) to form an alcohol.

Step 2: Analyze the Starting Material

The starting alkene, \( ext{CH}_3- ext{CH}( ext{CH}_3)- ext{CH}= ext{CH}_2\), is a 3-methyl-1-butene. The double bond is between the third and fourth carbon atoms, with a methyl group on the second carbon.

Step 3: Predict the Carbocation Intermediate

The double bond in the alkene will first protonate, leading to the formation of a carbocation. Protonation often occurs such that the more stable carbocation is formed. For this alkene, protonating the terminal carbon (fourth carbon) will yield a secondary carbocation on the third carbon after a hydride shift to the primary position.

Step 4: Markovnikov's Rule Application

Apply Markovnikov's rule, which states that the proton (H) from \( ext{H}^{+}\) adds to the carbon with more hydrogens (the less substituted carbon). This results in the carbocation on the third carbon, which can also undergo rearrangement to form a more stable tertiary carbocation.

Step 5: Add Nucleophile

Finally, water(\( ext{H}_2 ext{O}\)) acts as a nucleophile and attacks the more stable tertiary carbocation, leading to the formation of an alcohol after deprotonation.

Step 6: Identify the Product

The product of this reaction is \( ext{CH}_3- ext{CH}( ext{CH}_3)- ext{C(OH)( ext{CH}_3)}- ext{CH}_3\) . This matches option (a), which is the tertiary alcohol formed following Markovnikov's rule and potential carbocation rearrangement.

Key Concepts

Alkene Hydration

Alkene hydration is a chemical reaction where an alkene reacts with water to form an alcohol. It's a key reaction in organic chemistry, often catalyzed by an acid. When you see a reaction notation such as \( \text{H}^{+}/\text{H}_2\text{O} \), it indicates an acid-catalyzed process. The addition of water across the double bond transforms the alkene into an alcohol. This transformation allows for the creation of versatile compounds that can be used in further reactions or as end products in industrial chemistry.
The hydration of alkenes is an important reaction to understand because it outlines how alkenes can be converted into more functional derivatives. It's also crucial in understanding how different factors affect the outcome of such transformations, which include stability of intermediate carbocations.

Markovnikov's Rule

Markovnikov's rule is a guideline used to predict the outcome of addition reactions involving asymmetrical alkenes. It states that during the addition of a protic acid to an alkene, the hydrogen atom attaches to the carbon with more hydrogen atoms bonded to it, resulting in the more substituted (and usually more stable) carbocation.
This rule plays a critical role in determining the major product of alkene reactions. Understanding Markovnikov's rule helps predict where a new functional group, like an alcohol, will form. Following this rule ensures you can correctly identify the product of a reaction not only in simple cases but also when more complex rearrangements might occur.

Carbocation Rearrangement

Carbocation rearrangement is a process that often happens during reactions involving carbocation intermediates. In the case of alkene hydration, the double bond initially forms a carbocation at one of the carbons involved in the double bond. However, if a more stable carbocation can form by rearranging the existing one, the molecule will usually undergo this rearrangement.
A common type of carbocation rearrangement is the hydride shift, where a hydrogen atom moves from an adjacent carbon to the carbon with the positive charge. This shift produces a more stable carbocation, often resulting in a tertiary structure instead of a secondary one. This stability facilitates the subsequent nucleophilic attack and can significantly influence the final structure of the reaction product.

Acid-Catalyzed Hydration

Acid-catalyzed hydration is a method commonly used to facilitate the hydration of alkenes. In this process, an acid like sulfuric acid uses its proton to promote the addition of water to the alkene. The reaction starts by protonating the alkene to create a carbocation, which is then susceptible to nucleophilic attack by water.
The acid acts as a catalyst, meaning it facilitates the reaction without being consumed in the process. This type of reaction allows for efficient conversion of alkenes into alcohols using conditions that are typically mild and suitable for various alkene structures. Mastering this reaction type is essential for progressing in organic chemistry, as it involves several key concepts such as carbocation formation and rearrangement, along with nucleophilic attack.