Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will have a mesoisomer also? \(\quad[2004]\) (a) 2,3 -dichlorobutane (b) 2,3 -dichloropentane (c) 2 -hydroxypropanoic acid (d) 2 -chlorobutane

Step-by-Step Solution

Answer

2,3-dichlorobutane will have a mesoisomer.

Step 1: Understand the Definition of Mesoisomer

A mesoisomer is a compound that contains multiple stereocenters and is superimposable on its mirror image due to an internal plane of symmetry. It is a type of stereoisomer.

Step 2: Analyze 2,3-dichlorobutane

2,3-dichlorobutane has two chiral centers at the 2nd and 3rd carbon atoms. Draw both possible stereoisomers (R,R and S,S) and note any plane of symmetry. You'll find that one of these structures is superimposable on its mirror image due to a vertical plane between the centers, indicating a meso compound.

Step 3: Investigate 2,3-dichloropentane

2,3-dichloropentane also has two chiral centers at the 2nd and 3rd carbon atoms. However, due to the lack of perfect symmetry and odd number carbon backbone, it is not possible for any conformation to be superimposable on its mirror image, meaning it does not form a meso compound.

Step 4: Examine 2-hydroxypropanoic acid

This compound, also known as lactic acid, has only one chiral center and no possibility of having an internal plane of symmetry. Therefore, it cannot have a meso form.

Step 5: Check 2-chlorobutane

2-chlorobutane has only one chiral center at the second carbon atom. With no possibility of forming an internal plane of symmetry or having more than one stereocenter, it cannot be a meso compound.

Step 6: Conclusion

Based on the analyses, 2,3-dichlorobutane is the only compound from the list capable of having a mesoisomer due to its symmetry about the stereocenters.

Key Concepts

Stereochemistry

Stereochemistry is the branch of chemistry that studies the spatial arrangement of atoms within molecules. It plays a crucial role in determining how substances interact with each other. In particular, stereochemistry affects the physical and chemical properties of compounds. For example, the taste and smell of many natural and synthetic chemicals depend on their stereochemistry.
Stereochemistry deals with isomers, which are compounds with the same molecular formula but different structural arrangements. One type of isomer is the **stereoisomer**. These isomers differ only in the spatial orientation of their atoms. Understanding the stereochemistry of a molecule is essential for predicting its behavior in biological organisms and various reactions.

Chiral Centers

A chiral center, often referred to as a stereocenter, is an atom that has four different groups attached to it. This configuration results in non-superimposable mirror images, much like left and right hands. These mirror image isomers are known as **enantiomers**.
Identifying chiral centers in a molecule is key to understanding its stereochemistry. For instance, in 2,3-dichlorobutane, the second and third carbon atoms serve as chiral centers. This leads to multiple stereochemistry possibilities, including the formation of enantiomers.

• A molecule with one chiral center can have two stereoisomers.
• Multiple chiral centers increase the potential for various isomers, depending on symmetry and other factors.

Recognizing chiral centers helps predict how a compound might react in different environments, mainly because they usually exhibit unique interactions when compared to non-chiral molecules.

Stereoisomers

Stereoisomers are a subclass of isomers where the connectivity of atoms is the same, but their spatial arrangement in three dimensions differs. Within stereoisomers, there are more specific categories, such as **enantiomers and diastereomers**.
Enantiomers are mirror images that cannot be superimposed upon each other, while diastereomers are not mirror images. Both types behave differently in chiral environments, such as human bodies or specific solvents.

• Enantiomers: Display identical physical properties but interact differently with other chiral molecules.
• Diastereomers: Exhibit different physical and chemical properties.

A critical aspect of stereoisomers is their ability to exhibit optical activity. This means they rotate plane-polarized light, which can be used to distinguish between different stereoisomers in a laboratory setting. Understanding stereoisomers allows chemists to design drugs that can properly interact with biological systems.

Symmetry in Molecules

Symmetry in molecules is a fundamental concept when determining the potential for a molecule to exist as a meso compound. A meso compound is a type of stereoisomer containing multiple stereocenters but is achiral due to an internal plane of symmetry.
This symmetry effectively negates the optical activity that might be otherwise observed, as seen in compounds like 2,3-dichlorobutane. Here, the vertical plane between the chiral centers creates a mirror image that is superimposable, indicating a meso form.

• Molecules like 2,3-dichloropentane lack this symmetry and, therefore, cannot exhibit a meso isomer.
• Symmetry determines whether multiple chiral centers result in an overall chiral or achiral compound.

Understanding symmetry helps clarify why some molecules can have meso forms while others cannot, revealing deeper insights into their chemical and physical properties.