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

The number of optical isomers formed is 0.

Step 1: Identify the Compound Structure

The compound in question is \((\mathrm{CH}_3)_2\mathrm{C} = \mathrm{CHCH}_3\). The central double-bonded carbon atom, \(\mathrm{C}=\mathrm{CH}-\), is the focus since it's the site of hydrogenation.

Step 2: Understand Hydrogenation

Hydrogenation involves the addition of hydrogen across a double bond, converting it into a single bond. Here, \((\mathrm{CH}_3)_2\mathrm{C}-\mathrm{CHCH}_3\) will become \((\mathrm{CH}_3)_2\mathrm{CH}-\mathrm{CH}_2\mathrm{CH}_3\) after hydrogenation.

Step 3: Determine Possibility of Optical Isomerism

Optical isomerism occurs when a carbon atom is attached to four different groups, making it a chiral center. After hydrogenation, the compound \((\mathrm{CH}_3)_2\mathrm{CH}-\mathrm{CH}_2\mathrm{CH}_3\) does not have any chiral centers, as all carbon atoms are attached to duplicate groups.

Key Concepts

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen atoms to a molecule. This process typically takes place in the presence of a catalyst, often a metal like platinum or palladium, which helps accelerate the reaction.
Hydrogenation is commonly used to convert unsaturated compounds, which have double or triple bonds, into saturated compounds, which only have single bonds.
In the context of organic chemistry, when we discuss hydrogenation concerning alkenes, we're talking about adding hydrogen across the double bond. This transforms the double bond into a single bond, effectively making the molecule "saturated".
For example, the compound \((\mathrm{CH}_3)_2\mathrm{C}=\mathrm{CHCH}_3\) contains a double bond. Through hydrogenation, this double bond is converted into a single bond, resulting in \((\mathrm{CH}_3)_2\mathrm{CH}-\mathrm{CH}_2\mathrm{CH}_3\).
This process is important because it can significantly change the properties of a molecule, making it more stable and less reactive. Understanding hydrogenation is crucial for analyzing how molecules behave and interact with each other.

Chiral Centers

A chiral center, also known as a stereocenter, is a carbon atom that is bonded to four distinct groups. This unique arrangement causes the molecule to be asymmetric.
Visualize a chiral center like your right hand; it cannot be superimposed perfectly on your left hand, creating a situation of non-superimposable mirror images or "enantiomers".
In organic molecules, the presence of a chiral center generally leads to optical isomerism. These isomers can rotate plane-polarized light in different directions.
If you have more than one chiral center in a compound, the number of potential isomers increases, which makes stereochemistry quite fascinating and complex.
However, not all molecules will have chiral centers. In our example, the hydrogenated compound \((\mathrm{CH}_3)_2\mathrm{CH}-\mathrm{CH}_2\mathrm{CH}_3\) has no chiral centers because all the carbon atoms are bonded to identical groups. This results in no optical isomerism.

Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms within a molecule. This arrangement can significantly influence the properties and reactions of a molecule.
A key component of stereochemistry is whether a molecule can exist as isomers, specifically optical isomers. Optical isomers are stereoisomers that differ only in the way they interact with plane-polarized light. When a molecule has chiral centers, these isomers can occur.
There are several forms of stereochemistry, with conformational, configurational, and geometric isomers being among them.

• Conformational Isomers: These are isomers that differ by the rotation around a single bond.
• Configurational Isomers: Isomers that can be changed into one another by breaking and reforming bonds.
• Geometric Isomers: Isomers that differ in arrangement across a double bond, like cis and trans forms.

In the case of the textbook exercise, hydrogenation removed the double bond, preventing the possibility of geometric isomers. Also, without any chiral centers, we do not have optical isomers present. Understanding stereochemistry is essential as it affects how molecules react and function in biological systems.