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 (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) (c) C=C=CC(=C)C (d) \(\mathrm{HC}=\mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The correct structure is option (c).

Step 1: Understand the Problem

We need to determine the structure of a hydrocarbon with the formula \(\text{C}_{6}\text{H}_{10}\). This hydrocarbon does not react with ammoniacal \(\text{Cu}^+\), absorbs 2 moles of hydrogen during hydrogenation to form 2-methylpentane, and adds one molecule of water in the presence of \(\text{Hg}^{2+} / \text{H}_2\text{SO}_4\).

Step 2: Reaction with Hydrogen

The hydrocarbon absorbs 2 moles of hydrogen, indicating that there are two pi bonds present. This can mean either two double bonds or one triple bond in the structure.

Step 3: Non-Reactivity with \(\text{Cu}^+\)

The compound does not react with ammoniacal \(\text{Cu}^+\), suggesting it does not contain a terminal alkyne group. Terminal alkynes usually react with \(\text{Cu}^+\) ions.

Step 4: Hydration Reaction

The addition of water in acidic conditions to form 2-methylpentane suggests that the double or triple bond is located such that it can form a ketone or alcohol intermediate, eventually followed by rearrangement to 2-methylpentane.

Step 5: Analyze Given Structures

Let's evaluate the provided options: **(a)** Contains two double bonds but does not fit the reactivity and hydrogenation requirements. **(b)** Contains a triple bond but is a terminal alkyne (so it would react with \(\text{Cu}^+\)). **(c)** Has two double bonds leading to possible 2-methylpentane configuration upon hydrogenation and fits the reaction with water. **(d)** Is also a terminal alkyne.

Step 6: Determine Structure

In option **(c)**, the structure is such that it aligns with the absorption of 2 moles of hydrogen and hydration to result in 2-methylpentane. It is also consistent with not reacting with \(\text{Cu}^+\). Therefore, the correct structure is (c).

Key Concepts

Hydrocarbon Structure Analysis

Analyzing the structure of hydrocarbons like \( \text{C}_{6}\text{H}_{10} \) is key to solving organic chemistry problems. This hydrocarbon formula indicates it is unsaturated since its hydrogen atoms are fewer than in a fully saturated alkane (\( \text{C}_6\text{H}_{14} \)). This implies the presence of one or more pi bonds. To analyze structural possibilities, examine how different configurations adhere to the given chemical reactions and properties.

For \( \text{C}_{6}\text{H}_{10} \), identify isomers that comply with the experimental conditions, such as not reacting with ammoniacal \( \text{Cu}^+ \). This indicates the absence of a terminal alkyne group which would typically react with these ions. Assessing the structure involves looking at the connectivity of carbon atoms and the type of bonds (single, double, or triple). Consideration of all reactions given is crucial to narrowing down possibilities and selecting viable structural candidates.

Catalytic Hydrogenation

Catalytic hydrogenation is a pivotal reaction in organic chemistry where hydrocarbons absorb hydrogen in the presence of a catalyst, usually platinum, palladium, or nickel. This process leads to the reduction of multiple bonds, such as double or triple bonds, resulting in the formation of a saturated alkane.

In the case of our given hydrocarbon \( \text{C}_{6}\text{H}_{10} \), catalytic hydrogenation absorbs two moles of hydrogen, suggesting the presence of two pi bonds. This aligns with the knowledge of either two double bonds or one triple bond reducing into single bonds. The hydrogenation converts these unsaturated hydrocarbons, ultimately leading to the formation of 2-methylpentane. Therefore, hydrogenation not only provides insight into the presence and number of unsaturations but also assists in understanding the arrangement leading to specific structural conversions.

Reaction Mechanisms

Understanding reaction mechanisms helps in predicting and verifying chemical reactions step-by-step. For organic compounds, mechanisms detail how a reaction progresses from reactants to products, involving intermediate stages and transition states.

With \( \text{C}_{6}\text{H}_{10} \), several reactions provide clues related to probable mechanisms. The compound adds one molecule of water in the presence of \( \text{Hg}^{2+} / \text{H}_2\text{SO}_4 \). This hints at a hydration mechanism typically involving the formation of a carbocation intermediate facilitated by the acidic environment. This reaction helps determine the position of unsaturation closest to any resulting substituent after hydration.

Mechanisms can also explain why certain isomeric forms of a hydrocarbon, like (c) in this case, react favorably under given conditions while others do not, demonstrating the significance of understanding pathways in chemical conversions.

Pi Bonds Identification

Identifying pi bonds in hydrocarbons is essential to determine the degree of unsaturation. Pi bonds arise in double and triple bonds where overlapping p orbitals allow for additional bonding beyond the single sigma bonds.

In \( \text{C}_{6}\text{H}_{10} \), identifying pi bonds helps establish the amount of hydrogen that can be absorbed in reactions such as catalytic hydrogenation. The compound's reaction to absorb two moles of hydrogen suggests that two pi bonds are present, which can be represented as two double bonds or a single triple bond. By analyzing reactivity, such as non-reactivity with \( \text{Cu}^+ \), it's inferred that pi bonds are in a configuration that avoids reactions typical with terminal alkynes.

Recognizing pi bonds not only supports structural identification but also determines the nature of further chemical reactions these bonds may undergo.