Problem 54 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Identify, \(\mathbf{D}\left(\mathrm{C}_{6} \mathrm{H}_{12}\right)\), an optically active hydrocarbon which on catalytic hydrogenation gives an optically inactive compound, \(\mathrm{C}_{6} \mathrm{H}_{14}\)

Step-by-Step Solution

Answer

The compound is 3-methyl-1-pentene.

Step 1: Understand the Problem

We need to identify the compound, \( \mathrm{C}_{6} \mathrm{H}_{12} \), which is optically active and becomes inactive \( \mathrm{C}_{6} \mathrm{H}_{14} \) upon hydrogenation. It suggests a compound with a chiral center involved in the double bond that loses chirality after hydrogenation.

Step 2: Recognize Optical Activity

Optical activity occurs when a compound contains a chiral center, an atom bonded to four different groups. Here, \( \mathrm{C}_{6} \mathrm{H}_{12} \) needs such a center, potentially made through structural arrangement.

Step 3: Analyze the Possible Structures

\( \mathrm{C}_{6} \mathrm{H}_{12} \) indicates either a cycloalkene or an alkene. Since it should be optically active, a structure like 3-methyl-1-pentene, involving an asymmetric carbon atom and a double bond, is possible.

Step 4: Consider Hydrogenation Reaction

Hydrogenation of \( 3\text{-}\text{methyl-1-pentene} \) will add \( \mathrm{H}_{2} \) across the double bond, reducing it to a single bond and yielding \( \mathrm{C}_{6} \mathrm{H}_{14} \). This eliminates the chiral center from the double bond.

Step 5: Define the Final Structure

With hydrogenation leading to a none chiral alkane \( \text{(hexane)} \), the compound starts as 3-methyl-1-pentene wherein the double bond is vital for chirality, which is absent post-reaction.

Key Concepts

Chiral Center

A chiral center is a key component in understanding optical activity. It refers to a carbon atom bonded to four different groups. This asymmetry allows the molecule to exist in two non-superimposable mirror images, known as enantiomers. These enantiomers can rotate plane-polarized light differently, hence being optically active. In our exercise, identifying an optically active hydrocarbon suggests the presence of at least one chiral center. The compound \( \mathrm{C}_6 \mathrm{H}_{12} \) must be able to rotate this light, indicating that its structure includes a chiral center. Double bonds in alkenes don't permit chirality directly, but they can influence the formation of chiral centers when special arrangements of atoms occur near them.

Catalytic Hydrogenation

Catalytic hydrogenation is a chemical reaction where hydrogen is added across a double bond, typically in the presence of a catalyst like palladium, platinum, or nickel. This process turns an unsaturated compound, such as an alkene, into a saturated compound, an alkane. For example, the reaction of our compound, 3-methyl-1-pentene, involves the addition of hydrogen molecules across the carbon-carbon double bond.

• The double bond in \( \mathrm{C}_6 \mathrm{H}_{12} \) makes it unsaturated.
• Upon hydrogenation, it is converted into \( \mathrm{C}_6 \mathrm{H}_{14} \), a fully saturated alkane.

The crucial part of this reaction with regard to optical activity is that it can remove a chiral center if it was part of the double bond or in its immediate vicinity. With the saturation of the double bond, the molecule loses its ability to exhibit optical activity, turning it into an optically inactive compound.

Structural Isomerism

Structural isomerism refers to molecules with the same molecular formula but different structural arrangements of atoms. This distinctly affects their physical and chemical properties, including optical activity. In the case of \( \mathrm{C}_6 \mathrm{H}_{12} \), numerous structural isomers exist, including alkenes with different placements of the double bond or cycloalkanes. For a compound to be optically active, like in our example, it should be an alkene that carries a subtle arrangement allowing for chirality.

• 3-methyl-1-pentene is one such structural isomer of \( \mathrm{C}_6 \mathrm{H}_{12} \) exhibiting optical activity by having a chiral center created by the arrangement of a methyl group and double bonds.
• Upon hydrogenation, different isomers of \( \mathrm{C}_6 \mathrm{H}_{14} \) can be formed, none of which possess a chiral center due to saturation.

This concept is critical in organic chemistry as it defines how a simple change in atom connectivity can affect optical properties and other characteristics.