Problem 59 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Statement -1: Molecules that are not superimpossable on their mirror images are chiral. Statement -2: All chiral molecules have chiral centres. (a) If both Statement-1 and Statement-2 are correct, and Statement-2 is the correct explanation of the Statement-2. (b) If both Statement-1 and Statement-2 are correct, but Statement \(-2\) is not the correct explanation of the Statement-1. (c) If Statement-1 is correct but Statement-2 is incorrect. (d) If Statement-1 is incorrect but Statement-2 is correct.

Step-by-Step Solution

Answer

(c) Statement-1 is correct but Statement-2 is incorrect.

Step 1: Understanding Chirality

Chirality is a geometric property where a molecule cannot be superimposed on its mirror image, similar to how left and right hands are mirror images but not identical. This generally implies the molecule is chiral.

Step 2: Evaluating Statement-1

Statement-1 claims that molecules that are not superimposable on their mirror images are chiral. This statement is correct by the definition of chirality.

Step 3: Interpreting Chiral Centers

Chiral centers (also known as stereogenic centers) are typically carbon atoms with four different substituents, leading to non-superimposable mirror images.

Step 4: Evaluating Statement-2

Statement-2 claims that all chiral molecules have chiral centers. While many chiral molecules do have chiral centers, there are exceptions such as certain types of molecular chirality (e.g., axial chirality) where chirality exists without a traditional chiral center. Thus, this statement is not universally correct.

Step 5: Determining the Correct Explanation

Although both statements touch on aspects of chirality, Statement-2 fails to universally explain the reason behind chirality in all cases, as not all chiral molecules are due to the presence of chiral centers.

Step 6: Choosing the Correct Option

Since Statement-1 is correct and Statement-2 is incorrect, the appropriate choice given the options is (c).

Key Concepts

Understanding Superimposable Mirror Images

Superimposable mirror images are crucial in understanding molecular chirality. Imagine looking at your hand's reflection in a mirror. These images, when laid on top of each other, don't line up exactly. This is what it means for a structure to have non-superimposable mirror images. Similarly, a molecule is chiral if it cannot be superimposed on its mirror image. In chemistry:

• A molecule and its mirror image are mirror duplicates that can't overlap perfectly.
• This property is akin to left and right hands, which are mirror versions yet can't align when placed palm-to-palm.

Chirality often arises because a carbon atom in such a molecule is bonded to four different substituents. This leads to different spatial arrangements that are non-superimposable on their mirror images, giving rise to what we call chiral molecules.

Exploring Chiral Centers

Chiral centers, often found in carbon atoms, play a significant role in creating chirality. A chiral center is a carbon atom bonded to four distinct groups, making it asymmetric. This results in non-superimposable mirror images, thus making the molecule chiral. Important points about chiral centers include:

• They generally consist of carbon atoms because bond angles and covalent bonding provide the ideal setup for geometry-based differences.
• Each group connected to the chiral center must be different for the center to contribute to chirality.

However, not all chiral molecules have chiral centers due to different types of symmetry or axial chirality which don't rely on such centers. In conclusion, while chiral centers often explain chirality, they are not an absolute requirement for chirality in all molecular structures.

Stereogenic Centers and Their Role

Stereogenic centers are similar to chiral centers in that they influence a molecule's spatial configuration. The key aspect of a stereogenic center is that its presence causes different spatial arrangements, or stereoisomers, which can have distinct properties or behaviors. Here's what to remember about stereogenic centers:

• They include not only chiral centers but any atom that contributes to the molecule's stereochemistry.
• While all chiral centers are stereogenic centers, not all stereogenic centers cause chirality. Some result in different stereoisomers or geometric isomers.

The term 'stereogenic' is broader than 'chiral center' because it includes any source of spatial difference within a molecule. This understanding is important when examining molecular structures that might not have a clear-cut chiral center but are still capable of existing as different stereoisomers.