Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

The product(s) obtained via oxymercuration \(\left(\mathrm{HgSO}_{4}\right.\) \(+\mathrm{H}_{2} \mathrm{SO}_{4}\) ) of but-1-yne would give (a) CC(C)=O (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{CHO}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}+\mathrm{HCHO}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{HCOOH}\)

Step-by-Step Solution

Answer

The product of the oxymercuration of but-1-yne is option (a): acetone, \(\text{CC(C)=O}\).

Step 1: Understanding Oxymercuration Reaction

Oxymercuration is a reaction used to hydrate alkynes and alkenes. When applied to an alkyne like but-1-yne, it typically results in the formation of a keto product due to the addition of water across the triple bond followed by tautomerization from enol form to ketone.

Step 2: Analyzing But-1-yne Structure

But-1-yne is a four-carbon alkyne with a triple bond between the first and second carbon: \[ ext{CH}_3- ext{C} ext{#} ext{CH}- ext{CH}_2 ext{H}\]Upon reaction, the triple bond will open to form a double bond with an alcohol group initially.

Step 3: Initial Addition of Water

During the oxymercuration, the water molecule adds to the carbon atoms involved in the triple bond. The vinyl mercury intermediate forms an enol structure: \[ ext{CH}_3- ext{C}(OH)= ext{CH}- ext{CH}_3\]

Step 4: Tautomerization to Keto Form

The enol rearranges by migrating the hydrogen and forming a keto group: \[ ext{CH}_3- ext{CO}- ext{CH}_2- ext{CH}_3\]This results in the ketone, butan-2-one (methyl ethyl ketone), as the final product.

Step 5: Determine the Correct Product from Options

From the options provided, analyze which one matches the product from the reaction. The correct match is option (a), acetone, CC(C)=O representing 2-butanone.

Key Concepts

But-1-yne

But-1-yne is a type of alkyne, a hydrocarbon that features a carbon-carbon triple bond. Its chemical formula is \( \text{C}_4\text{H}_6 \), and its structure consists of four carbon atoms arranged linearly, with the triple bond situated between the first and second carbon atom: \( \text{CH}_3-\text{C} \equiv \text{CH}-\text{CH}_3 \). This positioning gives but-1-yne its unique properties, making it an excellent participant in chemical reactions such as oxymercuration.

Key characteristics of but-1-yne include:

• Possessing a terminal alkyne due to the triple bond at the end of the carbon chain.
• Being relatively reactive, particularly at the triple bond.

When but-1-yne undergoes certain reactions, like oxymercuration, the triple bond is engaged and transformed, leading to the formation of various useful products, such as enols and subsequently ketones.

Tautomerization

Tautomerization is a chemical process where tautomers—two compounds with the same formula but different connectivity—interconvert. In the context of oxymercuration reactions, this typically involves the shift from an enol form to a keto form. This rearrangement is crucial because it stabilizes the molecule by transforming a less stable structure into a more stable one.

Tautomerization typically involves:

• Hydrogen atom migration from one atom to another.
• A shift of electrons along a chain of atoms, leading to the formation of a different double bond configuration.

In our exercise, an initial hydration of but-1-yne creates an enol, which through tautomerization becomes a ketone. This transformation is spontaneous, as ketones are energetically more stable than enols. This behavior explains why in many cases, the enol form is just a transient intermediate rather than an observable product.

Keto-Enol Tautomerism

Keto-enol tautomerism is a specific form of tautomerization where a keto and an enol form interconvert. It involves two main forms: a ketone or aldehyde form (keto) and an alcohol with a double bond (enol). This switching of forms is a classic example of chemical equilibrium and plays a significant role in organic chemistry reactions.

During an oxymercuration reaction, the keto-enol tautomerism is crucial. Initially, an alcohol group attaches to the alkyne triple bond to form the enol structure:

\[ \text{CH}_3-\text{C}(\text{OH})=\text{CH}-\text{CH}_3 \]

Over time, this enol rearranges to the more stable ketone configuration:

\[ \text{CH}_3-\text{CO}-\text{CH}_2-\text{CH}_3 \]The keto form is predominantly stable, making it the favored form under typical conditions. Understanding keto-enol tautomerism is key to grasping why certain products result from reactions, and is foundational in predicting the outcomes of reactions involving alkynes like but-1-yne.