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=CCC(C)C (d) \(\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

The structure is \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\).

Step 1: Identify Number of Unsaturations

The formula for the hydrocarbon is given as \(\mathrm{C}_{6}\mathrm{H}_{10}\). Using the formula \(\text{Degree of Unsaturation} = \frac{2C + 2 - H}{2}\), we can find the number of rings and/or double bonds. For our compound, \(\text{DOU} = \frac{2 \times 6 + 2 - 10}{2} = 2\). This indicates the presence of two double bonds, one triple bond, or a ring and a double bond.

Step 2: Evaluate Reaction with Cu+ Solution

The hydrocarbon does not react with ammoniacal solution of \(\mathrm{Cu}^+\) ions, suggesting that it does not contain a terminal alkyne group.

Step 3: Identify Structure from Hydrogenation

Upon catalytic hydrogenation, the compound adsorbs 2 moles of hydrogen and forms 2-methylpentane. This suggests the presence of two double bonds in a linear conjugation—criteria mainly satisfied by a conjugated diene.

Step 4: Evaluate Hydration Reaction

The compound undergoes an addition reaction with water when treated with \(\mathrm{Hg}^{2+}/\mathrm{H}_2\mathrm{SO}_4\), indicating a system with accumulated double bonds such as \(\mathrm{cis}-\) or \(\mathrm{trans}-\) alkenes between carbon atoms.

Step 5: Determine Most Likely Structure

From characteristics gathered, the hydrocarbon likely contains two double bonds and is not an alkyne. Therefore, option (a) \(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) matches the answer; it has two double bonds and fits the observations from hydrogenation and hydration.

Key Concepts

Understanding the Degree of Unsaturation

To identify the structure of a hydrocarbon, the degree of unsaturation (DOU) is a crucial starting point. The degree of unsaturation provides information on the number of

• double bonds,
• triple bonds, and/or
• rings present in a compound.

For any hydrocarbon \( C_n H_m \) , the formula \( \text{DOU} = \frac{2n + 2 - m}{2} \) is used. Here, "n" is the number of carbons, and "m" is the number of hydrogens. For a compound such as \( \text{C}_6 \text{H}_{10} \), the DOU is computed as \( \frac{12 + 2 - 10}{2} = 2 \).
This means that our compound could contain:

• Two double bonds,
• One triple bond, or
• A ring and a double bond.

By analyzing DOU, students can start piecing together possible structural elements in a compound.

Exploring Catalytic Hydrogenation

Catalytic hydrogenation is a powerful tool for determining structures in organic chemistry. It involves the addition of hydrogen \( \text{H}_2 \) to unsaturated hydrocarbons in the presence of a catalyst, such as palladium. This reaction turns alkenes into alkanes and alkynes into alkenes or alkanes.In our problem, the hydrocarbon \( \text{C}_6 \text{H}_{10} \) absorbs 2 moles of hydrogen to form 2-methylpentane. This indicates that \( \text{C}_6 \text{H}_{10} \) most likely has two double bonds. Since adding two moles of hydrogen results in a fully saturated compound, the original hydrocarbon must possess two sites suitable for hydrogenation.
The reaction provides essential clues about the hydrocarbon's unsaturation levels and helps confirm structural hypotheses based on DOU calculations.

Investigating Alkyne Reactions

Understanding how hydrocarbons react or do not react can be a game-changer in identifying their structure. An important consideration is how triple-bonded carbons, known as alkynes, behave.Thankfully, there's a simple test involving ammoniacal silver nitrate (AgNO3) solutions to identify terminal alkynes. But in this problem, the hydrocarbon does not react with \( \text{Cu}^+ \) solutions, which are similar to the ammoniacal silver nitrate test. Thus, we can confidently determine that our compound is not a terminal alkyne.The absence of a reaction indicates either that the compound has no triple bonds or, if alkynes are present, none are terminal. This information steers us away from structures like option (d), which includes a terminal alkyne, focusing on other possibilities with double bonds.

Alkene Hydration Insights

Alkene hydration involves the addition of water (\( \text{H}_2\text{O} \)) across a double bond, typically catalyzed by strong acids like \( \text{H}_2\text{SO}_4 \). In this measured reaction, \( \text{Hg}^{2+} \) helps guide the process, suggesting the presence of alkenes with conjugated double bonds. Such setups may arise in alkenes arranged as conjugated systems or dienes.
The original compound, treated with \( \text{Hg}^{2+}/ \text{H}_2\text{SO}_4 \), adds one molecule of water, suggesting that it contained conjugated alkenes.Alkene hydration insight helps reinforce predictions made from DOU and hydrogenation reactions, allowing one to deduce or rule out candidate structures, like confirming the presence of two double bonds.

Strategies for Organic Chemistry Problem Solving

Organic chemistry can be confusing due to its complex structures and varied reactions, yet thoughtful, systematic approaches make it manageable.

• Start with the DOU: This gives a quick insight into possible structural characteristics.
• Use known reactions: Observing reactions—like catalytic hydrogenation—can tell you about the type and number of unsaturations.
• Conduct quick tests: Simple tests like reactions with \( \text{Cu}^+ \) allow you to determine the presence or absence of certain functional groups, like terminal alkynes.
• Work through hydration clues: Track how molecules like water add to alkenes to finalize structure possibilities.

Approaching problems with a clear, step-by-step strategy reduces errors and guides any student to success. Think about building each piece, step by step, until revealing a clear picture of the molecule's structure.