Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of optical isomers formed by hydrogenation of the compound, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) are (a) 0 (b) 1 (c) 2 (d) 3

Step-by-Step Solution

Answer

(a) 0

Step 1: Understanding the Compound

The given compound is \((\mathrm{CH}_{3})_{2} \mathrm{C}=\mathrm{CHCH}_{3}\). This compound has a double bond in its structure which makes it unsaturated.

Step 2: Inspecting Hydrogenation of Unsaturations

Hydrogenation of the compound involves the addition of hydrogen across the double bond, converting it into a single bond.

Step 3: Identifying Chirality Post-Hydrogenation

After hydrogenation, the compound becomes \((\mathrm{CH}_{3})_{2} \mathrm{CH}-\mathrm{CH}_{2}\mathrm{CH}_{3}\). For optical activity, the compound must have a chiral center, which is typically a carbon atom with four different substituents.

Step 4: Assessing the Presence of Chiral Centers

In the new structure, the carbon previously part of the double-bond arrangement becomes \(\mathrm{CH}-\mathrm{CH}_{3}\), which means now it appears as \((\mathrm{CH}_{3})_{2}\mathrm{CH}\). However, this carbon does not have four different groups attached, hence, there are no chiral centers.

Step 5: Concluding Optical Isomers

Since the hydrogenated compound does not have any chiral centers, it cannot exhibit optical isomerism, leading to the conclusion that the number of optical isomers formed is 0.

Key Concepts

Understanding Chiral Centers

A chiral center, often referred to as a chiral carbon, is essential for optical isomerism in organic molecules. To spot a chiral center, you need to identify a carbon atom that is bonded to four distinct substituents. These distinct groups create asymmetry, allowing the molecule to have non-superimposable mirror images, similar to how left and right hands are mirror images but not identical. For example, consider a carbon atom in a molecule bonded to a hydrogen atom, an alcohol group (-OH), a methyl group (-CH₃), and an ethyl group (-C₂H₅). This diversity of attached groups makes it a chiral center. Post hydrogenation, the original exercise compound becomes o - After removing the double bond, observe new connections to see if at least one carbon has four unique groups. - Here, the carbon atoms end up having repetitive groups, like multiple -CH₃ groups, which eliminates chirality potential. As the lack of a chiral center in this scenario means the compound cannot form optical isomers, understanding the role of a chiral center helps reveal whether optical isomerism is possible in molecules.

Exploring the Hydrogenation Process

The hydrogenation process is fundamental in converting unsaturated compounds into saturated ones. During hydrogenation, hydrogen molecules are added to the compound, specifically targeting the double or triple bonds. This process is crucial: - **Double Bonds to Single Bonds**: In the context of the original exercise, hydrogenation converts the double bond to a single bond. - **Catalytic Influence**: Usually, a catalyst like palladium or nickel is used to facilitate the hydrogenation process. The compound from the exercise, ( CH₃ ) ₂C=CHCH₃, has a double bond, marking it as unsaturated. Through hydrogenation, the double bond is broken, creating new, single-bonded structures that are generally more stable due to a reduction in unsaturation. However, it is important to note that while hydrogenation stabilizes the compound, it may remove the potential for optical activity if no chiral centers arise because of it.

Saturated vs Unsaturated Compounds

Compounds in organic chemistry are often classified as saturated or unsaturated based on the presence of multiple bonds between carbon atoms. - **Unsaturated Compounds**: These contain at least one double or triple bond. Such bonds introduce kinks and alter the molecule's shape, often resulting in interesting chemical reactivity. - **Saturated Compounds**: These contain only single bonds between carbon atoms. With all possible bonds occupied, they tend to be more straight and stable compared to their unsaturated counterparts. In the original exercise, the compound ( CH₃ ) ₂C=CHCH₃ is unsaturated due to its double bond. - **Impact on Reactivity and Stability**: Unsaturated compounds are typically more reactive due to the presence of double or triple bonds, which can be sites for reactions such as hydrogenation. - **Transformation through Hydrogenation**: Hydrogenation changes an unsaturated compound into a saturated one by adding hydrogen atoms, making it more stable and less active chemically. Understanding the distinctions between saturated and unsaturated compounds is essential in predicting the reactivity and properties of organic molecules.