Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

Propyne \(\frac{\mathrm{HgSO}_{4}}{\mathrm{Hg}_{7} \mathrm{SO}_{4}} \rightarrow \mathrm{A} .\) Compound \(\mathrm{A}\) is (a) 2-propanal (b) propane (c) propene (d) 2 -propanone

Step-by-Step Solution

Answer

(d) 2-propanone

Step 1: Understanding the Reaction

The reaction involves propyne undergoing hydration with the presence of mercury(II) sulfate as a catalyst. This is an example of a keto-enol tautomerism reaction that typically converts an alkyne to a ketone.

Step 2: Reaction Mechanism Analysis

In this reaction mechanism, propyne ( ext{CH}_3-C riplenew-C-H) reacts with mercury(II) sulfate and water. The alkyne is initially converted into an enol, which is an alkene alcohol, through hydration. Because of Markovnikov's rule, water adds to the more substituted carbon.

Step 3: Tautomerization Step

The enol form is not stable and rearranges to the more stable keto form through the keto-enol tautomerism process. The double bond moves, and the hydroxyl group converts to a carbonyl group (C=O).

Step 4: Identifying the Product

For propyne, the resulting compound after rearrangement is 2-propanone, also known as acetone, which corresponds to the molecular formula ext{CH}_3- ext{C}(=O)- ext{CH}_3.

Key Concepts

Propyne

Propyne, also known as methylacetylene, is an alkyne with the chemical formula \( ext{CH}_3- ext{C}\equiv ext{C}- ext{H}\). It is the simplest alkyne containing a triple bond between carbon atoms, with three carbon atoms total in its structure. Alkynes like propyne are known for their characteristic sharp aroma and high reactivity due to their triple bonds.

Key characteristics of propyne include:

• A highly unsaturated hydrocarbons
• The sp hybridization of carbon atoms involved in the triple bond
• A linear geometry around the triple bonded carbons, making them reactive

These properties make propyne, and other alkynes, ideal candidates for addition reactions, such as hydration, which in simple terms means adding water across a triple bond.

Mercury(II) sulfate catalysis

Catalysis plays a crucial role in speeding up chemical reactions. In the context of turning propyne into 2-propanone, mercury(II) sulfate ( HgSO4) is used as a catalyst. A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.

In this reaction:

• Mercury(II) sulfate facilitates the addition of water to propyne while ensuring that the reaction proceeds smoothly and at an acceptable rate.
• It helps to stabilize the transition state during the formation of the intermediate enol.

The use of HgSO4 in this reaction is essential for converting the enol form to the more stable keto form, ensuring a successful transformation of propyne to 2-propanone.

Hydration reaction

The hydration reaction involves the addition of water ( H_2O ) across the triple bond in propyne. The presence of a catalyst, like mercury(II) sulfate, plays a pivotal role in facilitating this reaction. The triple bond present in propyne is very reactive, allowing the water to add across it, yielding an intermediate compound called an enol.

Specific steps in hydration of propyne include:

• The alkyne ( ext{C}  eq  ext{C} ) initially reacts with water in the presence of the catalyst.
• The addition of water results in an alkene alcohol known as the enol form.

Although enol is produced, it is not the final product. Enols are typically not stable, leading directly into the next stage known as tautomerization.

Markovnikov's rule

Markovnikov's rule is an essential guideline in organic chemistry for predicting the results of addition reactions. According to Markovnikov's rule, when an addition reaction proceeds, the hydrogen atom from water tends to attach to the carbon in the carbon-carbon multiple bond that already has the greater number of hydrogen atoms (less substituted carbon), with the other component (like a hydroxyl group) attaching to the more substituted carbon.

Applying Markovnikov's rule to the hydration of propyne:

• The OH (from water) will add to the more substituted carbon in the propyne molecule.
• This directional addition leads to the formation of the more stable enol intermediate, assisting the conversion to the final ketone product.

In conclusion, Markovnikov's rule helps us understand why the resulting molecule takes on the specific structure of 2-propanone, after undergoing tautomerization from its enol form.