Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH} \equiv \mathrm{CH}+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH} \stackrel{\mathrm{HgSO}_{4}}{-\mathrm{Y}} \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow \mathrm{X}}\) \(\mathrm{X}\) is (a) \(\mathrm{CH}_{3} \mathrm{OH}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (d) \(\mathrm{CH}_{2}=\mathrm{CHOH}^{2}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

Step 1: Identify the starting materials

We begin by identifying the starting materials in the given chemical reaction: \(\mathrm{CH} \equiv \mathrm{CH}+\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}\). This represents acetylene (ethyne) and ethyl alcohol (ethanol) respectively.

Step 2: Understand the reagents and conditions

The reaction is conducted in the presence of \(\mathrm{HgSO}_{4}\) (mercuric sulfate) and water \(\mathrm{H}_{2}\mathrm{O}\). This reagent and condition leads to the hydration of alkynes to form carbonyl compounds, a reaction known as oxymercuration-demercuration.

Step 3: Predict the intermediate formation

Acetylene, \(\mathrm{CH} \equiv \mathrm{CH}\), upon hydration in the presence of \(\mathrm{HgSO}_{4}\) and \(\mathrm{H}_{2}\mathrm{O}\), forms an enol intermediate (ethenol), which quickly undergoes tautomerization to acetaldehyde, \(\mathrm{CH}_{3}\mathrm{CHO}\).

Step 4: Determine the product X

The compound \(\mathrm{X}\) formed after the reaction is the addition of acetaldehyde and ethanol. Since acetaldehyde (\(\mathrm{CH}_{3}\mathrm{CHO}\)) and ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\)) are the two products, \(\mathrm{X}\) refers to their combination.

Step 5: Match with options given

From the given options, (c) \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) matches the predicted product \(\mathrm{X}\).

Key Concepts

Hydration of Alkynes

Hydration of alkynes is a chemical reaction where a water molecule is added across the triple bond of an alkyne, resulting in the formation of a carbonyl compound.
This is typically achieved through a process called oxymercuration-demercuration, which involves the use of mercuric sulfate \(\mathrm{HgSO}_4\) and water \(\mathrm{H}_2\mathrm{O}\).
In the context of the original exercise, acetylene (\(\mathrm{CH} \equiv \mathrm{CH}\)) and ethanol (\(\mathrm{C}_2\mathrm{H}_3\mathrm{OH}\)) react in the presence of \(\mathrm{HgSO}_4\) and water. This leads to the addition of water across the triple bond, forming an unstable enol intermediate.

• Alkynes are saturated hydrocarbons with at least one carbon-carbon triple bond.
• During hydration, the triple bond is converted to a double bond, with an -OH group attaching to the carbon bearing more hydrogen atoms.
• The reaction requires specific catalysts like mercuric sulfate to increase reaction efficiency.

Acetylene Chemistry

Acetylene, known chemically as ethyne, is the simplest alkyne with the formula \(\mathrm{C}_2\mathrm{H}_2\). It features a carbon-carbon triple bond, giving it unique reactivity.
This high reactivity is what allows acetylene to participate in hydration reactions to form various organic products.
In the scenario of hydration reactions, acetylene plays the role of the foundational building block.

• Its simplistic structure with just two carbon atoms makes acetylene a key starting material in synthesis.
• It is often involved in reactions yielding carbonyl compounds as end products.
• Acetylene's ability to form enol intermediates further underpins its usefulness in organic chemistry.

Tautomerization

Tautomerization is a key chemical process where an enol, a compound with a hydroxyl group (-OH) bound to a carbon with a double bond, converts into its isomer known as a keto form.
It is this rearrangement of bonding within the molecule that transforms the unstable enol intermediate, formed during the hydration of alkynes, into a stable carbonyl compound.
In our original exercise, the enol formed from acetylene quickly undergoes tautomerization to yield acetaldehyde.

• This process helps stabilize the molecule by forming a carbon-oxygen double bond (carbonyl group).
• Though the process sounds complex, it's essentially a shift in the position of a proton and a rearrangement of electrons.
• The rapid nature of tautomerization makes the originally formed enol intermediate practically undetectable.

Carbonyl Compounds Formation

Carbonyl compounds are a class of organic compounds that feature a carbon-oxygen double bond, referred to as the carbonyl group. The formation of such compounds is a crucial outcome of reactions like the hydration of alkynes.
In the presence of \(\mathrm{HgSO}_4\) and water, the enol intermediate from acetylene is transformed into acetaldehyde, which is a simple carbonyl compound.
The presence of the carbonyl group imparts specific chemical properties, making carbonyl compounds highly reactive and useful in further organic synthesis.

• Acetaldehyde, formed during the hydration of acetylene, is a classic example of an aldehyde, a type of carbonyl compound.
• Carbonyl groups are pivotal in numerous biological and chemical transformations as they are electrophilic, influencing reactivity patterns in organic chemistry.
• The carbonyl bond itself is polar, where oxygen holds two lone pairs, making it a site for further reactions.