Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

Predict the product ' \(\mathrm{B}\) ' in the sequence of reaction \(\mathrm{HC} \equiv \mathrm{CH} \frac{30 \% \mathrm{H}_{2} \mathrm{SO}_{4}}{\mathrm{HgSO}_{4}} \mathrm{~A} \stackrel{\mathrm{NaOH}}{\longrightarrow} \mathrm{B}\) (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COONa}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The product 'B' is \(\mathrm{CH}_3\mathrm{COONa}\) (sodium acetate), option (c).

Step 1: Analyze Initial Reaction Conditions

The first reaction condition involves acetylene (\(HC \equiv CH\)) being treated with 30% sulfuric acid (\( H_2SO_4\)) in the presence of mercury sulfate (HgSO_4). This is a typical hydration reaction of an alkyne, specifically acetylene.

Step 2: Identify Product of Hydration

In the hydration of acetylene with sulfuric acid and mercury sulfate, an enol is initially formed which tautomerizes to a more stable ketone. This reaction sequence results in acetylene being converted to acetaldehyde ( CH_3CHO ). Therefore, product A is acetaldehyde.

Step 3: Understand Reaction with NaOH

Next, acetaldehyde is treated with NaOH. This is a classic reaction to form an aldol, known as the aldol condensation. However, since there is no other aldehyde or ketone, a simple reaction between acetaldehyde molecules occurs. Specifically, acetaldehyde reacts with sodium hydroxide to form sodium acetate (\(CH_3COONa\)).

Step 4: Determine Final Product

Since product A, acetaldehyde, reacts with sodium hydroxide to become sodium acetate, product B is sodium acetate (\(CH_3COONa\)).

Key Concepts

Hydration of Alkynes

The hydration of alkynes is a crucial reaction in organic chemistry, used to convert triple-bonded carbon structures into carbonyl compounds. This process typically involves adding water across the alkyne's triple bond, catalyzed by strong acids and additional catalysts like mercury sulfate.
An alkyne such as acetylene (\(HC \equiv CH\)) is treated with sulfuric acid (\(H_2SO_4\)) and mercury sulfate (\(HgSO_4\)), resulting in the formation of an unstable enol intermediate.
This enol then undergoes further transformation to yield a more stable ketone or aldehyde.
This reaction is employed frequently because it allows chemists to turn relatively simple alkynes into more complex organic molecules.

• Efficient for synthesizing aldehydes and ketones.
• Requires an acid catalyst and can involve mercury ions as a promoter.

Tautomerization in Organic Chemistry

Tautomerization is a dynamic chemical equilibrium between two structural isomers, typically involving an enol and a ketone or aldehyde.
In the hydration of alkynes, the initially formed enol tautomerizes to become an aldehyde or ketone. This process involves the migration of a hydrogen atom and a change in the location of a double bond.
In the case of acetylene hydration, the reaction forms an enol which swiftly converts into acetaldehyde (\(CH_3CHO\)).

• Common in reactions leading to carbonyl compounds.
• Usually favored in ketones or aldehydes, due to their greater stability compared to enols.

Aldol Condensation

The aldol condensation is a fundamental carbon-carbon bond-forming reaction in organic chemistry.
It typically involves the reaction of aldehyde or ketone molecules in the presence of base to form β-hydroxy aldehydes or ketones, which can further dehydrate to give α,β-unsaturated carbonyl compounds.
In this exercise, acetaldehyde (\(CH_3CHO\)) in the presence of sodium hydroxide can undergo self-condensation.
This initially results in the formation of aldol compounds but can be further processed to sodium acetate (\(CH_3COONa\)) in the absence of competing carbonyl functionalities.

• Key in synthesizing complex organic molecules.
• Involves a base like \(NaOH\) to induce carbonyl carbon nucleophilic attack.

Sodium Hydroxide Reaction

Sodium hydroxide (\(NaOH\)) is a powerful base widely used in organic transformations.
In the discussed reaction, \(NaOH\) serves as a catalyst to drive the self-condensation of acetaldehyde. It can also convert alcohols to alkenes or initiate aldol reactions.
Here, however, \(NaOH\) helps form sodium acetate (\(CH_3COONa\)) from acetaldehyde.

• Strong base, often used for deprotonation reactions.
• Helps in eliminating water with heat, especially in aldol reactions.