Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Ph}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \stackrel{\mathrm{Hg}_{B}^{+2} / \mathrm{H}^{+}}{\longrightarrow}\) A. Here, \(\mathrm{A}\) is

Step-by-Step Solution

Answer

The product A is acetophenone (Ph-CO-CH₃).

Step 1: Understanding the Reaction

The problem involves reacting phenylacetylene (Ph-C≡C-CH₃) with a reagent that contains mercury (Hg²⁺) and acid (H⁺). This is a hydration reaction where the alkyne is converted into a carbonyl compound. Specifically, terminal alkynes with Hg²⁺ and H⁺ conditions typically yield ketones.

Step 2: Identifying the Mechanism

The mechanism involves the addition of Hg²⁺ to the alkyne, forming a mercurinium ion. Following this, water adds across the triple bond. Then, tautomerization occurs where an enol (formed from water addition) rearranges to form the more stable ketone product.

Step 3: Predicting the Product Formation

Considering the addition of water to phenylacetylene, the enol initially formed changes to a ketone through tautomerization. For Ph-C≡C-CH₃, the triple bond will hydrate to form either an aldehyde or ketone, typically the latter for terminal alkynes.

Step 4: Determining the Product Structure

The hydration of the triple bond in Ph-C≡C-CH₃ results in the formation of acetophenone (Ph-CO-CH₃), as the keto form is more stable than the enol.

Key Concepts

Mercury-catalyzed hydration

Mercury-catalyzed hydration is an invaluable reaction in organic chemistry, particularly for transforming alkynes into carbonyl compounds. This reaction involves the use of a mercury(II) catalyst along with an acid, usually sulfuric acid, to add water to the alkyne.

• The process kicks off with the addition of mercury(II) ion (\( \text{Hg}^{2+} \)), which helps to activate the alkyne for further reaction. This activation forms a mercurinium ion which is crucial for the next step.
• Water then reacts with this activated complex, leading to the formation of an enol, which is an alcohol variant with a double bond.
• In a terminal alkyne, like phenylacetylene, the typical outcome is a ketone due to the positioning of added elements.

By using mercury as a catalyst, the reaction can proceed smoothly at conditions conducive to the specific alkyne being transformed. It is essential to go step-by-step, especially considering the delicate nature of organomercury intermediates.

Tautomerization

Tautomerization is a beautiful transformation in organic chemistry, allowing the rearrangement of an enol to a ketone or aldehyde. When dealing with the hydration of alkynes, this step is crucial in ensuring that a stable product is formed.
When water adds to the alkyne during mercury-catalyzed hydration, you first get an enol. Enols have the formula –C=C–OH but are often not very stable.

• The transition from an enol to a ketone or aldehyde is driven by the movement of hydrogen and the repositioning of double bonds.
• This process typically favors the keto form because it is more energetically stable.
• For example, in the case of phenylacetylene, the enol formed is quickly tautomerized into the more stable acetophenone (\( \text{Ph-CO-CH}_3 \)).

Ultimately, tautomerization is a natural and spontaneous process that plays a vital role in forming the final desired carbonyl product in these reactions.

Formation of ketones from alkynes

The formation of ketones from alkynes, especially terminal alkynes, is a fascinating aspect of organic transformations. In reactions like mercury-catalyzed hydration, the steps are carefully orchestrated to yield a ketone.
This process is especially reliable when dealing with terminal alkynes.

• Initially, the hydration of the alkyne results in the formation of an enol, but as mentioned, this enol quickly undergoes tautomerization.
• The intrinsic nature of terminal alkynes usually decides the outcome, favoring the formation of ketones over aldehydes.
• In the example of phenylacetylene, what starts as a simple triple bond, after reacting and rearranging, emerges as acetophenone.

This transformation is elegant and showcases how different structures can influence the stability and formation of carbonyl-containing compounds like ketones in alkyne hydration processes.