Question

The addition of water to propyne in the presence of \(\mathrm{HgSO}_{4}-\mathrm{H}_{2} \mathrm{SO}_{4}\) gives (1) \(\mathrm{CII}_{3} \mathrm{CH}=\) CIIOII (2) (3) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (4) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

Short answer

The product is \(\text{CH}_{3}\text{COCH}_{3}\).

Step-by-step solution

Understand the Reaction Reagents

Identify the reagents in the problem. The reagents are propyne and a mix of \(\text{HgSO}_{4} \)-\(\text{H}_{2}\text{SO}_{4} \). This combination is typically used to catalyze the hydration of alkynes.

Identify the Mechanism of Action

The hydration of an alkyne in the presence of \(\text{HgSO}_{4}\) and \(\text{H}_{2}\text{SO}_{4}\) generally follows a Markovnikov addition mechanism, where the hydroxyl group (OH) adds to the more substituted carbon atom.

Apply Markovnikov's Rule

According to Markovnikov's rule, in the hydration of propyne, the hydroxyl group (OH) will attach to the central carbon atom, resulting in the formation of a ketone after tautomerization. This intermediate forms a compound where the \(\text{C=O}\) group is attached to the second carbon.

Write the Final Product

The resulting compound from propyne hydration in this context is acetone \(\text{CH}_{3}\text{COCH}_{3}\). This compound matches option (4).

Key concepts

Markovnikov addition

In organic chemistry, Markovnikov addition is a rule that predicts the outcome of adding a proton and a hydroxyl group (OH) to an unsymmetrical alkene or alkyne. According to this rule, the proton (H) will attach to the carbon atom with more hydrogen atoms previously bonded to it, while the hydroxyl group will attach to the carbon atom with fewer hydrogen atoms.
This rule is essential in predicting the major product of such reactions. For example, when propyne (an alkyne) undergoes hydration, the hydroxyl group attaches to the more substituted carbon atom, leading to a specific product configuration after tautomerization.

Catalysis with HgSO4 and H2SO4

Hydration of alkynes usually requires a catalyst to proceed efficiently. In this reaction, a mixture of mercuric sulfate (HgSO4) and sulfuric acid (H2SO4) is used as the catalyst. HgSO4 activates the alkyne, making it more susceptible to nucleophilic attack by water (H2O).
The catalytic system helps in converting the alkyne into an enol intermediate, which quickly undergoes tautomerization to form a more stable product, typically a ketone.
This catalysis method ensures a smooth and directed transformation, aligning with Markovnikov's rule. This directed addition is crucial for forming the correct product in reactions involving asymmetrical alkynes like propyne.

Tautomerization

Tautomerization is a process where an enol (a molecule with a hydroxyl group bonded to a carbon-carbon double bond) converts into a more stable keto form (a carbon-oxygen double bond). This is an equilibrium process commonly observed in the hydration of alkynes.
After Markovnikov addition of water to an alkyne, an enol intermediate forms. However, enols are usually less stable than their keto counterparts (due to the strong C=O bond). Thus, the molecule undergoes tautomerization, shifting the hydroxyl group and forming a ketone or aldehyde, depending on the original structure of the alkyne.
For example, in the hydration of propyne, the intermediate enol quickly tautomerizes to form acetone, a more stable ketone compound.

Ketone formation

Ketones are a functional group characterized by a carbon-oxygen double bond (C=O) also bonded to two other carbon atoms. Formation of ketones is a typical outcome when dealing with the hydration of asymmetrical alkynes, following Markovnikov's rule and catalysis with HgSO4 and H2SO4.
In the given exercise, the hydration of propyne (C3H4) in the presence of the described catalysts results in the formation of a ketone. Initially, an enol intermediate forms, which tautomerizes to produce the more stable ketone.
This specific reaction ultimately produces acetone (CH3COCH3), a simple and industrially significant ketone known for its solvent properties. This final product aligns with option (4) in the multiple-choice answers provided in the exercise.