Question

Which of the following will have a meso isomer also? (a) 2-chlorobutane (b) 2,3 -dichlorobutane (c) 2,3 -dichloropentane (d) 2-hydroxypropanoic acid

Short answer

2,3-dichlorobutane (option b) can have a meso isomer.

Step-by-step solution

Understand the Concept of Meso Isomer

A meso isomer is a type of stereoisomer that is superimposable on its mirror image due to an internal plane of symmetry. This means a compound with multiple chiral centers can have a meso form if there is symmetry.

Analyze Each Option for Meso Isomerism

- **Option (a): 2-chlorobutane** has only one chiral center and thus cannot have a meso isomer because meso compounds need at least two chiral centers. - **Option (b): 2,3-dichlorobutane** has two chiral centers. If we look at the structure, it has an internal plane of symmetry when the substituents are aligned, making it a meso compound. - **Option (c): 2,3-dichloropentane** also has two chiral centers but doesn't have an internal plane of symmetry due to the extra methylene group, which makes it incapable of being a meso compound. - **Option (d): 2-hydroxypropanoic acid** is lactic acid, which has only one chiral center, thus cannot have a meso form.

Conclusion

From the above analysis, only 2,3-dichlorobutane (option b) has the potential to form a meso isomer because it contains two chiral centers and an internal plane of symmetry when substituents are aligned.

Key concepts

Stereoisomer

Stereoisomers are fascinating molecules that have the same molecular formula but differ in the orientation of their atoms in space. These differences do not affect the connectivity of the atoms but rather their spatial arrangement. There are two main types of stereoisomers: enantiomers and diastereomers. Both types highlight the 3-dimensional nature of molecules.

Enantiomers are non-superimposable mirror images of each other, much like your left and right hands. They have identical physical properties but can differ greatly in their biological activities.

Diastereomers, on the other hand, are not mirror images. They have different physical and chemical properties, and this includes the fascinating category known as meso compounds. Meso compounds are unique as they are achiral, despite having chiral centers. This is due to an internal plane of symmetry that renders them superimposable on their mirror images.

Chiral Centers

A chiral center is an atom in a molecule that is attached to four different substituents, leading to a non-superimposable mirror image, much like how our hands are mirror images but not identical. Chiral centers are the cornerstone of stereochemistry.

Consider a carbon atom that is bonded to four different groups. This configuration allows for two possible arrangements of the substituents, resulting in two stereoisomers, or enantiomers.

For a molecule to exhibit meso characteristics, typically, it must have multiple chiral centers. However, the presence of chiral centers alone does not guarantee a meso form. For it to be considered meso, such centers must be arranged in such a way that the molecule still possesses an internal plane of symmetry.

Internal Plane of Symmetry

An internal plane of symmetry is a critical feature in determining whether a compound is meso. This imaginary plane divides the molecule into two equal halves that are mirror images of each other.

In stereochemistry, a meso compound has this plane of symmetry, even though it contains chiral centers. This symmetry effectively makes the compound achiral.

Take for example 2,3-dichlorobutane. Despite having two chiral centers, this molecule can have an arrangement where the chlorine atoms are on opposite sides, allowing for a mirror plane through the center of the molecule. This makes the compound meso, capable of being superimposed on its mirror image.