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) 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

The reagent used is (a) \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\).

Step 1: Identify Reaction Type

The given reaction converts an alkyne (H3C-C≡CH) to a ketone (H3C-CH(OH)-CH3). This is a hydration reaction of an alkyne, where an alkyne is converted to a ketone via an enol intermediate.

Step 2: Determine Required Reagents

In the hydration of alkynes, to form ketones, oxymercuration-demercuration is often used. This involves using mercury(II) ions in acidic aqueous conditions, which aligns with option (a), \(\mathrm{Hg}^{2+} / \mathrm{H}_{2}\mathrm{SO}_{4}\).

Step 3: Assess All Reagent Options

(a) \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\) facilitates hydration of alkynes to ketones.(b) Na in liquid \(\mathrm{NH}_{3}\) is used for reduction, followed by \(\mathrm{CH}_{3}\mathrm{Cl}\), which is not suitable for hydration.(c) \(1 \mathrm{~mol}\) of \(\mathrm{H}_{2} / \mathrm{Pt}\) can reduce the alkyne to an alkene, but the addition of \(\mathrm{H}_{2} \mathrm{SO}_{4} / \mathrm{H}_{2} \mathrm{O}\) is not a standard method for ketone formation from alkynes.(d) \(2 \mathrm{~mol}\) of \(\mathrm{H}_{2} / \mathrm{Pt}\), would fully hydrogenate the alkyne to an alkane, which does not provide a pathway to a ketone.

Step 4: Select the Correct Reagent

The most suitable reagents for the conversion of an alkyne to a ketone by hydration are provided in option (a), \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\).

Key Concepts

Oxymercuration-Demercuration

Oxymercuration-Demercuration is a chemical reaction that hydrates organic molecules. It transforms alkynes into more complex structures like ketones.
This reaction begins with oxymercuration, where a mercury(II) acetate compound adds to an alkyne in water. Here, \ \(\text{Hg}^{2+}\) acts as a catalyst and facilitates the addition of water to the alkyne. This step forms an enol, an intermediate stage in the transformation.
Next is the demercuration stage, which involves removing mercury from the compound and replacing it with a hydrogen atom. This step usually requires a reducing agent like sodium borohydride \ \((\text{NaBH}_4)\). As a result, the enol tautomerizes, forming a more stable ketone. This concludes the hydration of the alkyne.

• First, mercury ion addition creates an organomercury intermediate.
• Water molecule attaches, forming an enol.
• Sodium borohydride removes mercury, yielding the final ketone structure.

This process is practical due to its regioselectivity and ability to add water across double and triple carbon bonds without rearrangements.

Alkyne to Ketone Conversion

Converting alkynes into ketones involves breaking a triple bond and introducing a water molecule. This transformation is important in organic chemistry for synthesizing valuable compounds.
The reaction begins with an alkyne, characterized by its carbon-carbon triple bond. For hydration to occur, the alkyne must react in the presence of catalysts like mercury(II) sulfate in acidic water.
As water and mercury ions are introduced, a critical intermediate known as an enol forms. This structure, however, is less stable and quickly rearranges in a process called tautomerization. This adjustment results in a keto tautomer forming a double bond with oxygen.

• Use a catalyst like \ \(\text{Hg}^{2+}\).
• Introduce water under acidic conditions.
• Facilitate the required tautomerization to stabilize the compound as a ketone.

This entire conversion illustrates a typical method for alkyne functionalization.

Reaction Mechanisms in Organic Chemistry

Reaction mechanisms are the step-by-step breakdown of how chemical reactions occur, offering insight into reactant transformation to product. Understanding these processes is essential in predicting and controlling chemical behavior.
In the method of hydration of alkynes to ketones, reaction mechanisms provide a clear sequence of events. They begin with the initial interaction between the alkyne and mercury ions, followed by the evolution of intermediates like enols.
Mechanisms also explain the influence of catalysts, like mercury ions, that accelerate reaction rates by stabilizing transition states or intermediates. They identify how reactant orientation and electronic configuration shifts facilitate specific product formations.

• Identify changes in reactant structures.
• Understand catalysts' roles in altering reaction speeds.
• Recognize intermediate formation.

Mastery of reaction mechanisms is crucial for advancing in chemistry and enables tailored chemical process design.