Problem 23 - Chemistry in 30 days

Question

Which among the following compounds will give mixture of two ketones on hydration with \(\mathrm{HOH} / \mathrm{H}_{2} \mathrm{SO}_{4} / \mathrm{HgSO}_{4}\) : (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{C}=\mathrm{C}-\mathrm{CH}_{3}\) b) \(\mathrm{CH}_{3}-\mathrm{C}=\mathrm{C}-\mathrm{CH}_{3}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{C}=\mathrm{C}-\mathrm{C}_{2} \mathrm{H}_{5}\) d) All of these

Step-by-Step Solution

Answer

Compound (a) will give a mixture of two ketones on hydration.

Step 1: Understanding the Reaction

The reaction involves hydration of alkynes in the presence of dilute sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)) with mercuric sulfate (\(\mathrm{HgSO}_4\)) as a catalyst. This typically results in the Markovnikov addition of water, forming enols that tautomerize into ketones.

Step 2: Analyzing Compound (a)

For compound \((a)\), \(\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_3\), hydration occurs at the internal triple bond, yielding two possible keto tautomer products: 2-pentanone and 3-pentanone.

Step 3: Analyzing Compound (b)

In compound \((b)\), \(\mathrm{CH}_3-\mathrm{C}\equiv\mathrm{C}-\mathrm{CH}_3\), hydration of the internal alkyne results first in an enol, which tautomerizes into a single ketone: acetone.

Step 4: Analyzing Compound (c)

For compound \((c)\), \(\mathrm{C}_2\mathrm{H}_5-\mathrm{C}\equiv\mathrm{C}-\mathrm{C}_2\mathrm{H}_5\), hydration also yields an enol that will tautomerize into a single ketone product: 3-pentanone.

Step 5: Conclusion

Only compound \((a)\) produces a mixture of two ketones upon hydration under the given conditions due to the potential for different tautomers, unlike \((b)\) and \((c)\) that produce only one ketone each.

Key Concepts

Markovnikov Addition

When discussing the hydration of alkynes, a key principle is Markovnikov addition. This rule guides how different atoms or groups attach to a carbon-carbon multiple bond. In our scenario, an alkyne is the starting material. The presence of \(\mathrm{H}_2\mathrm{SO}_4\) with \(\mathrm{HgSO}_4\) as a catalyst helps in adding water across the triple bond. During this process, the hydrogen atom in water preferentially bonds to the carbon already bearing the greater number of hydrogen atoms. This results in an intermediate enol, a compound with a hydroxyl group bonded to a double-bonded carbon atom.
Ultimately, the hydration process leads to the formation of a more stable ketone due to the tautomerization of the initial enol product.

Tautomerization

Tautomerization is an essential mechanism in organic chemistry reactions, particularly in alkyne hydration. The enol formed through Markovnikov addition is not very stable. As a result, it undergoes a process called tautomerization. This is a form of isomerization where the enol converts into its more stable form, a ketone. The mechanism involves a series of proton transfers, where the hydroxyl hydrogen shifts to the neighboring carbon, and the double bond shifts to create a carbonyl group (C=O).

• The original OH group transforms into a carbonyl group adjacent to the double bond.
• The enol-to-ketone shift is energetic-saving, making the ketone the predominant product.

This process is crucial in determining the ultimate product distribution in alkyne hydration reactions.

Ketone Formation

In hydration reactions of alkynes, the end goal is to form a ketone. This chemical species contains a carbonyl group (C=O) bonded to two carbon-containing groups. The significance of ketone formation from alkynes cannot be understated. It is a straightforward method of converting simple molecules into more versatile and reactive species.
Upon hydration, many alkynes may initially form different enols. However, through the tautomerization process, these enols eventually convert to ketones.

• Ketones serve as vital intermediates in organic synthesis.
• The transformation often results in fewer steps with high efficiency.

The formation of ketones is especially important for the production of various chemicals and pharmaceuticals.

Organic Chemistry Reactions

Organic chemistry reactions, such as the hydration of alkynes, are central to understanding how various chemical transformations occur. These reactions involve breaking and forming bonds to convert reactant molecules into more useful products. The hydration of alkynes demonstrates several key principles:

• Reaction conditions: The use of \(\mathrm{H}_2\mathrm{SO}_4\) and \(\mathrm{HgSO}_4\) creates an environment conducive for water addition.
• Reactant structure: Alkynes have unique triple bonds that, when activated, allow for rich chemical transformations.
• Mechanistic pathways: Processes like Markovnikov addition and tautomerization illustrate the intricacies of reaction mechanisms.

Importantly, mastering these concepts equips students with tools to predict reaction outcomes and synthesize desired end products.