Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

A hydrocarbon \(\mathrm{C}_{6} \mathrm{H}_{10}\) does not react with ammonical solution of \(\mathrm{Cu}^{+}\)ions; it adsorbs \(2 \mathrm{~mol}\) of hydrogen on catalytic hydrogenation to give 2 -methylpentane. The hydrocarbon adds one molecule of water on treatment with \(\mathrm{Hg}^{2+} / \mathrm{H}_{2} \mathrm{SO}_{4}\). The structure of given compound is

Step-by-Step Solution

Answer

The structure of the hydrocarbon is 3-Hexyne.

Step 1: Analyze the Possible Structures Based on Formula

The hydrocarbon has the formula \(\mathrm{C}_{6} \mathrm{H}_{10}\). This formula suggests it may be an alkene or a cycloalkane since it has fewer hydrogens than expected for a fully saturated hydrocarbon (\(\mathrm{C}_n\mathrm{H}_{2n+2}\)).

Step 2: Determine the Degree of Unsaturation

Calculate the degree of unsaturation (also known as double bond equivalence, DBE) for \(\mathrm{C}_{6} \mathrm{H}_{10}\) using the formula: \(\text{DBE} = C - \frac{H}{2} + \frac{N}{2} + 1\) (ignoring oxygen here as there isn't any in the given formula). This gives us DBE = \(6 - \frac{10}{2} + 1 = 2\). This indicates there are two \(\pi\) bonds or rings.

Step 3: Consider the Hydrogenation Information

The compound adsorbs \(2 \mathrm{~mol}\) of hydrogen, indicating it has two \(\pi\) bonds. Following hydrogenation, the product is 2-methylpentane, suggesting the presence of a six-carbon chain.

Step 4: Analyze Water Addition Information

The hydrocarbon adds one molecule of water in the presence of \(\mathrm{Hg}^{2+} / \mathrm{H}_{2}\mathrm{SO}_{4}\), indicating the presence of an alkyne group (a triple bond) as these conditions typically lead to hydration of alkynes forming ketones or aldehydes.

Step 5: Deduce the Structure Combining Information

Combining the results: the structure of the compound is likely a terminal alkyne given its inability to react with an ammoniacal solution of \(\mathrm{Cu}^{+}\), the ability to undergo hydrogenation forming a saturated alkane, and the ability to hydrate forming a ketone. The compound is 3-Hexyne, which upon hydrogenation forms 2-methylpentane and hydrates to form a methyl ketone.

Key Concepts

Degree of Unsaturation

Understanding the degree of unsaturation, often referred to as double bond equivalence (DBE), is crucial in identifying hydrocarbon structures. It provides insight into the number of rings or π bonds present in a hydrocarbon. The formula for DBE is:

• \(\text{DBE} = C - \frac{H}{2} + \frac{N}{2} + 1\)

Here, \(C\) represents the number of carbon atoms, \(H\) is the number of hydrogen atoms, and \(N\) is the number of nitrogen atoms, though nitrogen is not present in this particular problem.
In our hydrocarbon \(C_6H_{10}\), the computation for DBE results in 2. This means the molecule could possess either two double bonds, one triple bond, or a ring and a double bond. The degree of unsaturation is fundamental in narrowing down possible structures and confirming the presence of multiple bonds or rings within the molecule.

Alkynes

Alkynes play a vital role in hydrocarbon chemistry due to their distinctive feature—a carbon-carbon triple bond. This bond is a significant indication of unsaturation, meaning that the molecule has fewer hydrogen atoms than a saturated alkane.
In the exercise, the hydrocarbon has the formula \(C_6H_{10}\), indicating an unsaturation that could relate to a triple bond. Alkynes are linear around the triple-bonded carbons, influencing the molecule's geometry and interaction with other chemicals.
Furthermore, alkynes are crucial in several chemical reactions, prominently in hydration reactions where they transform into ketones or aldehydes. Understanding alkynes both in structure and reaction capabilities is essential for solving problems related to hydrocarbon chemistry.

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H₂) across unsaturated bonds in hydrocarbons, typically using a catalyst such as

• Palladium (Pd)
• Platinum (Pt)
• Nickel (Ni)

This reaction converts alkenes and alkynes into alkanes by saturating their double or triple bonds. In the exercise, 3-hexyne undergoes hydrogenation, where two moles of hydrogen add across the triple bond.
Consequently, this results in 2-methylpentane, representing a fully saturated six-carbon chain.
Hydrogenation is often used in organic chemistry to manage the level of saturation in molecules, directly affecting their stability and reactivity.

Hydration of Alkynes

The hydration of alkynes transforms them into carbonyl compounds, specifically ketones or aldehydes. This reaction is catalyzed by acidic conditions such as mercuric sulfate (

• \(\text{Hg}^{2+}\)

) and sulfuric acid (

• \(\text{H}_2\text{SO}_4\)

).
The addition of water to alkynes usually follows Markovnikov's rule, where the hydroxyl group attaches to the more substituted carbon of the now double-bonded carbons after initial rearrangement.
In the given exercise, the alkyne group in 3-hexyne undergoes hydration, resulting in the formation of a ketone. Understanding this transformation is essential because it reveals how triple bonds in alkynes can be chemically modified to derive useful and complex organic structures.