Problem 117 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements about chirality: 1\. molecules which are not superimposable on their mirror images are achiral 2\. a chiral molecule can have simple axis of symmetry 3\. a carbon atom to which four different species are attached is a chiral centre. 4\. a compound whose molecules are achiral exhibits optical activity Which of the statements given above are correct? (a) 1,2 and 4 (b) 2,3 and 4 (c) 2 and 3 (d) 1 and 4

Step-by-Step Solution

Answer

None of the options are correct; only Statement 3 is true.

Step 1: Analyze Statement 1

Statement 1 claims that molecules which are not superimposable on their mirror images are 'achiral'. However, by definition, molecules that are not superimposable on their mirror images are actually 'chiral'. Hence, Statement 1 is incorrect.

Step 2: Analyze Statement 2

Statement 2 states that a chiral molecule can have a simple axis of symmetry. A chiral molecule cannot have an internal plane or axis of symmetry as it would make the molecule superimposable on its mirror image. Therefore, Statement 2 is incorrect.

Step 3: Analyze Statement 3

According to Statement 3, a carbon atom to which four different species are attached is a chiral center. This statement is correct because a carbon with four different substituents forms a tetrahedral shape that is not superimposable on its mirror image, making it chiral.

Step 4: Analyze Statement 4

Statement 4 claims that a compound whose molecules are achiral exhibits optical activity. This is incorrect as only chiral compounds can exhibit optical activity. Achiral compounds do not rotate plane-polarized light.

Step 5: Identify Correct Statements

Based on the analysis, only Statement 3 is correct. None of the given options (a, b, c, d) correctly represent the situation that only Statement 3 is correct on its own.

Key Concepts

Understanding Chiral Centers

In chemistry, a chiral center is fundamental to the concept of chirality. Essentially, it's a carbon atom bonded to four distinct groups or atoms.
This asymmetry leads to a spatial arrangement where the molecule's mirror image cannot be superimposed on its original form.
This non-superimposability is key because it results in two distinct forms, known as enantiomers, each having different chemical properties.

• The presence of a chiral center ensures the molecule exhibits chirality, which is the foundation for distinct optical and chemical behaviors.
• Chiral centers create unique opportunities in drug development, as different enantiomers can have vastly different biological effects.

Recognizing chiral centers in molecules is crucial for predicting their physical and chemical properties, especially in stereochemistry, which studies how these properties affect chemical reactions.

The Role of Optical Activity

Optical activity is a fascinating property linked with chiral molecules. It refers to the ability of a substance to rotate the plane of polarized light.
Chiral compounds are specifically known for this, as their structure enables interaction with light in unique ways.

• Enantiomers of a chiral compound will rotate polarized light by equal amounts but in opposite directions, known as dextrorotatory (to the right, +) and levorotatory (to the left, -).
• This property is measurable using an instrument called a polarimeter, which can help in identifying the specific enantiomer present.

Understanding optical activity is crucial, especially in fields like pharmaceuticals and food science, where the direction and degree of light rotation affect the functionality and quality of products.

Symmetry in Molecules

Symmetry is a key concept when discussing molecule chirality. Chiral molecules lack a plane or axis of symmetry, setting them apart from achiral ones.
While achiral molecules can be superimposed on their mirror images owing to symmetry, chiral molecules cannot.

• A molecule with an internal mirror plane or rotational axis is inherently symmetrical, and thus, achiral.
• Presence of symmetry often affects the molecule's chemical reactivity and interaction with other molecules.

Understanding and analyzing molecular symmetry is essential for predicting molecular behavior and interactions, making it an invaluable tool in stereochemistry and beyond.