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

Short answer

Option (c) 2 and 3, with note of possible discrepancy in given options.

Step-by-step solution

Analyze Statement 1

Statement 1 claims that molecules which are not superimposable on their mirror images are achiral. This is incorrect; molecules that are not superimposable on their mirror images are actually chiral, not achiral. This makes Statement 1 false.

Evaluate Statement 2

Statement 2 states that a chiral molecule can have a simple axis of symmetry. This is false because if a chiral molecule had any axis of symmetry, it would be equivalent to its mirror image in some way and thus would not be chiral.

Verify Statement 3

Statement 3 notes that a carbon atom to which four different species are attached is a chiral centre. This is true—a carbon with four different substituents is typically a chiral centre due to being asymmetrical.

Consider Statement 4

Statement 4 suggests that a compound whose molecules are achiral exhibits optical activity. This is false; an achiral molecule does not exhibit optical activity as it can be superimposed on its mirror image.

Compare Statements to Options

Given the analysis: Statement 1 is false; Statement 2 is false; Statement 3 is true; Statement 4 is false. The correct statements according to the analysis are only Statement 3. Thus, the only option including the correct statement is (c) 2 and 3, but since option (c) includes a false statement, there is a discrepancy. If option c is the closest match assuming a potential typo in options given that all others are incorrect, it reflects the analysis closest.

Key concepts

Chiral Center

Chirality is a fascinating area of study in chemistry, primarily revolving around **chiral centers**. A chiral center, often referred to as a chiral carbon, is a central atom that is bonded to four different groups or atoms. This asymmetry is what grants the molecule its chiral nature, making it non-superimposable on its mirror image.
It's crucial to recognize that not any carbon will do; it must specifically have four unique substituents.

• A simple example is the carbon in lactic acid, which is bonded to a hydroxyl group (OH), a methyl group (CH₃), a hydrogen atom (H), and a carboxyl group (COOH).
• Because of its unique structure, these molecules will have two distinct forms, known as enantiomers, which are mirror images of each other.

Thus, identifying chiral centers is essential, as they are directly linked to the molecule's chirality.

Optical Activity

**Optical activity** refers to a molecule's ability to rotate the plane of polarized light. This property is directly related to the presence of chiral centers in a molecule. Only chiral molecules exhibit optical activity, as the structure causes light to bend in specific ways that achiral molecules do not.

• Optical activity is quantified by measuring the degree of rotation, which can be clockwise (dextrorotatory) or counterclockwise (levorotatory).
• Moreover, the extent of rotation depends on factors such as the wavelength of the light and the concentration of the chiral molecules.

Achiral molecules won't affect polarized light in this manner, as they can be superimposed with their mirror image, thereby cancelling any potential optical activity they might possess.

Symmetry in Molecules

**Symmetry in molecules** is a key concept in understanding why a molecule may or may not be chiral. A chiral molecule, by definition, lacks any plane of symmetry. If a molecule can be divided into two identical halves through some plane, axis, or point, it displays symmetry and is thus likely achiral.

• Enantiomers, the chiral counterparts, have no internal mirror plane— this lack of symmetry is what makes them unique.
• On the other hand, symmetric and achiral molecules line up perfectly with their mirror image, negating any chiral option they might have.

In contrast, all chiral molecules are inherently asymmetric, ensuring their non-superimposable nature with any mirror image, thus guaranteeing their chirality.