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

Question

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

Step-by-Step Solution

Answer

(a) 2,3-dichlorobutane will have a mesoisomer.

Step 1: Understand the Concept of Meso Compounds

A meso compound is an achiral substance that has multiple chiral centers but is superimposable on its mirror image. This usually occurs due to an internal plane of symmetry that makes the overall molecule achiral despite the presence of chiral centers.

Step 2: Analyze Each Option for Chiral Centers

Identify whether each compound has chiral centers and an internal plane of symmetry. - **2,3-dichlorobutane**: Contains two chiral centers (C-2 and C-3). - **2,3-dichloropentane**: Contains two chiral centers (C-2 and C-3). - **2-hydroxypropanoic acid**: Contains one chiral center (C-2). - **2-chlorobutane**: Contains one chiral center (C-2).

Step 3: Look for Symmetry in Compounds with Multiple Chiral Centers

For a compound with multiple chiral centers to have a meso form, it must have an internal mirror plane. - **2,3-dichlorobutane**: Has an internal plane of symmetry in its eclipsed conformation. - **2,3-dichloropentane**: Lacks an internal plane of symmetry due to the additional carbon atom. Therefore, only 2,3-dichlorobutane fulfills the symmetry requirement.

Step 4: Conclusion

From the analysis, only 2,3-dichlorobutane has an internal plane of symmetry and multiple chiral centers which can lead to the formation of a mesoisomer.

Key Concepts

Chiral Centers

Chiral centers are specific atoms in a molecule where substituents are attached in such a way that the mirror images of the molecule cannot be superimposed. This is typically due to the presence of four different groups bonded to a carbon atom. In organic molecules, these carbon atoms are often responsible for giving the molecule its chiral nature. To identify a chiral center, look for:

• A carbon atom bonded to four different groups.
• The ability to produce non-superimposable mirror images.

Sometimes, compounds with multiple chiral centers can still be achiral if there's a plane of symmetry present, as seen in meso compounds.

Internal Plane of Symmetry

An internal plane of symmetry is an imaginary plane that divides a molecule into two mirror-image halves. In the context of meso compounds, this internal symmetry negates the potential chirality caused by multiple chiral centers. Here's how an internal plane impacts molecular symmetry:

• The molecule appears identical on either side of the plane.
• It makes the molecule superimposable on its mirror image.

When analyzing compounds for meso characteristics, always look for this internal mirror plane to determine if the molecule is truly achiral despite having chiral centers.

Achiral Substances

Achiral substances are molecules that lack chirality, meaning they do not have a non-superimposable mirror image. These substances can be superimposed on their mirror image, distinguishing them from chiral substances. Key characteristics of achiral substances include:

• The presence of an internal plane of symmetry.
• Equivalence of molecular halves across the mirror plane.

In organic chemistry, even molecules with chiral centers can be achiral if they are meso compounds, due to an overall symmetrical structure.

Mirror Image

In chemistry, a mirror image refers to a reflection of a molecule across an imaginary plane, like looking at the molecule in a mirror. For chiral molecules, these images are non-superimposable on each other, much like how left and right hands are mirror images but not identical. Important points about mirror images:

• In chiral molecules, they are distinct and non-superimposable.
• In achiral molecules, mirror images can be superimposed.

Recognizing the nature of a molecule’s mirror image is critical for understanding its chirality and whether it classifies as meso.

Organic Chemistry

Organic chemistry focuses on compounds primarily made of carbon and hydrogen, with many also containing elements like nitrogen, oxygen, and halogens. Chiral compounds are a significant area of study within organic chemistry due to their relevance in biological systems and materials science. A few key highlights in organic chemistry relevant to chirality include:

• Understanding the stereochemistry of carbon-based compounds.
• Recognizing the impact of chirality on chemical and biological reactions.

Mastery of these principles is integral to predicting molecular behavior and for the synthesis of new compounds with desired properties.