Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{CH} \longrightarrow \mathrm{H}_{3} \mathrm{C}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_{3}\) the reagent used would include (a) \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{Na}\) in liquid \(\mathrm{NH}_{3}\), followed by \(\mathrm{CH}_{3} \mathrm{Cl}\) (c) \(1 \mathrm{~mol}\) of \(\mathrm{H}_{2} / \mathrm{Pt}\), followed by \(\mathrm{H}_{2} \mathrm{SO}_{4} / \mathrm{H}_{2} \mathrm{O}\) (d) \(2 \mathrm{~mol}\) of \(\mathrm{H}_{2} / \mathrm{Pt}\), followed by aq. \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{Hg}^{2+} / \mathrm{H}_2\mathrm{SO}_4 \) is the correct reagent.

Step 1: Analyze the Reaction Type

The reaction involves the conversion of an alkyne, \( \mathrm{H}_3\mathrm{C}-\mathrm{C} \equiv \mathrm{CH} \), to a secondary alcohol, \( \mathrm{H}_3\mathrm{C}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_3 \). This is indicative of a hydration reaction, often achieved using specific catalysts.

Step 2: Identify the Appropriate Reagent for Hydration of Alkynes

In hydration of alkynes, Mercury(II) catalyst (\( \mathrm{Hg}^{2+} \)) in an acidic medium, such as sulfuric acid \( \mathrm{H}_2\mathrm{SO}_4 \), is commonly used. This is known as the oxymercuration-demercuration process, which adds water across the triple bond to form an alcohol.

Step 3: Evaluate the Reagents Given

Evaluate each option to determine which fits the criteria for converting an alkyne to a secondary alcohol:- **Option (a):** \( \mathrm{Hg}^{2+} / \mathrm{H}_2\mathrm{SO}_4 \) is widely used for converting alkynes to ketones or alcohols via hydration.- **Option (b):** \( \mathrm{Na} \) in liquid ammonia followed by \( \mathrm{CH}_3\mathrm{Cl} \) is used for different reactions, such as formation of alkyl lithium or alkyne to alkenes, not suitable for converting to alcohols.- **Option (c):** A hydrogenation step followed by \( \mathrm{H}_2\mathrm{SO}_4 / \mathrm{H}_2\mathrm{O} \) can also hydrate alkynes but doesn't specify the secondary alcohol formation.- **Option (d):** Excess hydrogen with \( \mathrm{NaOH} \) is more suitable for complete reduction rather than controlled hydration.

Step 4: Select the Correct Answer

Considering the reaction types and requirements, option **(a)** \( \mathrm{Hg}^{2+} / \mathrm{H}_2\mathrm{SO}_4 \) is the correct reagent for the conversion of \( \mathrm{H}_3\mathrm{C}-\mathrm{C} \equiv \mathrm{CH} \) to \( \mathrm{H}_3\mathrm{C}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_3 \) via hydration to a secondary alcohol.

Key Concepts

Oxymercuration-Demercuration

Oxymercuration-Demercuration is a two-step chemical process used to hydrate alkenes and alkynes. The initial step involves the formation of a mercurinium ion intermediate by the reaction of the alkyne with a mercuric salt, such as mercuric acetate (\( \mathrm{Hg(OAc)}_2) \).) This is performed in an aqueous acidic medium, typically sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)). This step places a mercury atom and a hydroxyl group across the alkyne, resulting in a vinyl alcohol (enol) intermediate.

The second step is demercuration, where the mercury moiety is replaced by a hydrogen, often using sodium borohydride (\( \mathrm{NaBH}_4 \)). The transformation progresses from an enol to a ketone or aldehyde due to the more stable keto form through tautomerization. In the hydration of a terminal alkyne, the result is a methyl ketone, unlike the internal alkyne which leads to a simple ketone.

This reaction is preferred for selective hydration because it avoids rearrangements and produces only a single ketone product, making it a straightforward choice for converting alkynes to their corresponding alcohols or ketones.

Conversion of Alkynes to Alcohols

The conversion of alkynes to alcohols is a crucial transformation in organic synthetic chemistry. This process typically involves the addition of water across the alkyne's triple bond. In the presence of specific catalysts, an alkyne can undergo hydration to form an alcohol.

For terminal alkynes, Oxymercuration-Demercuration helps achieve hydration that yields a methyl ketone. Here, water is directly added across the triple bond. The process ensures a well-controlled and predictable outcome, often favoring the Markovnikov addition rule - wherein the hydroxyl group attaches to the more substituted carbon atom.

Such transformations are pivotal in synthesizing alcohols from inexpensive precursors. They also facilitate the creation of various functional groups by further chemical treatment. Selecting the appropriate reagent and conditions, as seen with \( \mathrm{Hg}^{2+}/\mathrm{H}_2\mathrm{SO}_4 \), ensures efficiency and specificity in the conversion, pivotal for practical applications in laboratory and industrial chemistry.

Reagents in Organic Chemistry

Reagents play a crucial role in organic chemistry, acting as the driving force for transformations. In the context of alkyne hydration, choosing the right reagent guarantees success in achieving the desired product. These reagents are typically acidic and catalyze reactions effectively.

Several reagents are known for such reactions:

• Mercuric Sulfate (\( \mathrm{HgSO}_4\)): Commonly used in conjunction with sulfuric acid, it forms an efficient catalyst for oxymercuration.
• Sulfuric Acid (\( \mathrm{H}_2\mathrm{SO}_4\)): Provides the acidic environment necessary for the oxymercuration step. It aids in stabilizing the mercurinium ion intermediate.
• Sodium Borohydride (\( \mathrm{NaBH}_4\)): Used for the demercuration step, replacing mercury in the intermediate with hydrogen and facilitating the final alcohol or ketone formation.

These reagents are selected based on their compatibility with the desired reaction pathway and their ability to provide selective hydration without side reactions. Proper understanding and utilization of these reagents are critical for carrying out efficient and successful organic syntheses.