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

Question

Predict the product 'B' in the sequence of reaction $$ \mathrm{HC} \equiv \mathrm{CH} \frac{30 \% \mathrm{H}_{2} \mathrm{SO}_{4}}{\mathrm{HgSO}_{4}}-\mathrm{A} \stackrel{\mathrm{NaOH}}{\mathrm{CH}} $$

Step-by-Step Solution

Answer

The product 'B' is acetaldehyde, \[ \mathrm{CH_3CHO} \].

Step 1: Identify Reactant and Reagents

The initial reactant is acetylene, \[\mathrm{HC} \equiv \mathrm{CH}\]This undergoes treatment with 30% sulfuric acid and mercuric sulfate, which typically acts as a catalyst for hydration of alkynes.

Step 2: Hydration Reaction of Alkyne

The alkyne reacts with water in the presence of sulfuric acid (\(\mathrm{H_2SO_4}\)) and mercuric sulfate (\(\mathrm{HgSO_4}\)). This reaction converts the alkyne to an enol (compound 'A'). The general reaction forms an enol which then tautomerizes to a ketone. Thus, \[\mathrm{HC} \equiv \mathrm{CH} \rightarrow \text{vinyl alcohol (enol)} \rightarrow \text{acetaldehyde (ketone)}\] The enol form is not stable and tautomerizes, giving us acetaldehyde (\(\mathrm{CH_3CHO}\)).

Step 3: Reaction with Sodium Hydroxide

The acetaldehyde formed can undergo further reactions. In the presence of sodium hydroxide, the reaction continues via the Cannizzaro reaction or simple deprotonation. However, given typical pathways, this indicates a simple neutralization and formation of a hydroxy element. The primary understanding gets us to identify the reaction simply proceeds to an ethenate formation or similar.

Key Concepts

Hydration of Alkynes

In organic chemistry, hydration of alkynes is a crucial reaction. It involves adding water (H\(_2\)O) across the triple bond of an alkyne. This process is crucial because it transforms an alkyne into a more functional form, often an enol, which can lead to further reactions. The transformation is facilitated by acidic conditions, often using sulfuric acid (H\(_2\)SO\(_4\)) combined with a catalyst.The reaction steps:

• An alkyne like acetylene (HC≡CH) undergoes hydration in the presence of water.
• The initial reaction forms a vinyl alcohol or enol.
• Enols are usually unstable and quickly convert to their keto form through tautomerization.

The hydration of alkynes is a versatile method to create reactants that can lead to many different products, depending on conditions and catalysts.

Mercuric Sulfate Catalysis

Mercuric sulfate (HgSO\(_4\)) is commonly used as a catalyst in the hydration of alkynes. Its role is pivotal in facilitating the addition of water across the triple bond, especially in the presence of sulfuric acid.Why use mercuric sulfate?

• Mercuric sulfate contributes to generating an intermediate that enables the addition of water more efficiently.
• It increases the reaction rate, allowing the alkyne to convert to an enol quickly.
• The combination of HgSO\(_4\) and H\(_2\)SO\(_4\) acts synergistically to promote the reaction in a controlled manner.

Thus, in reactions like converting acetylene to its hydrated form, mercuric sulfate plays a crucial role in ensuring successful and rapid transformation.

Tautomerization

Tautomerization is a chemical process where a compound shifts from one isomeric form to another, often between keto and enol forms. It is a critical step in the hydration of alkynes as the reaction products are stabilized in their ketone form. The process of tautomerization:

• Initially, the alkyne hydrates to form an enol—an alcohol with a hydroxyl group attached to a carbon-carbon double bond.
• However, enols are unstable and rapidly transform into a more stable keto form, known as a tautomer.
• This transformation happens through moving a hydrogen atom and forming a carbon-oxygen double bond (C=O).

In the specific example of acetylene, the enol (vinyl alcohol) converts to acetaldehyde, a stable aldehyde. Tautomerization is vital as it determines the actual product you'll obtain from the hydration of an alkyne.

Cannizzaro Reaction

The Cannizzaro reaction is an important reaction in organic chemistry, occurring with aldehydes that lack alpha-hydrogens. It involves a self-redox process where one molecule of aldehyde is reduced to an alcohol, and another is oxidized to a carboxylate. Relevant details about the Cannizzaro reaction:

• Occurs with aldehydes that cannot undergo typical aldol condensation due to the absence of alpha-hydrogens.
• The reaction requires a strong base, commonly sodium hydroxide (NaOH).
• In standard conditions, it leads to the formation of an alcohol and a carboxylic acid salt.

However, in the outlined exercise, due to the presence of NaOH after completing the hydration reaction, the primary interaction suggested isn't a full Cannizzaro reaction but could involve neutralization or deprotonation, typically leading to different subsequent reactions.