Chirality: Videos & Practice Problems

Chirality refers to molecules whose mirror images are non-superimposable, leading to the concept of optical isomers or enantiomers. A chiral center is a carbon atom bonded to four unique groups, while achiral molecules lack this feature. Enantiomers can be drawn using two methods: creating a mirror image or inverting the spatial orientation of bonds. Chiral molecules are optically active, meaning they can rotate plane polarized light, a concept explored further in organic chemistry.

Chirality

Chirality is a fundamental property of certain molecules where their mirror images cannot be superimposed onto themselves. This concept can be illustrated with an analogy of a dog looking into a mirror; the reflection represents a chiral molecule, and if you attempt to align the two, they will not match due to their distinct spatial arrangements. This non-superimposable nature is crucial in understanding optical isomers, also known as enantiomers, which are a type of chiral molecule.

For a molecule to be classified as chiral, it must contain at least one chiral center, typically a carbon atom bonded to four unique groups. If a carbon atom is connected to fewer than four different groups, the molecule is termed achiral. For instance, a carbon connected to an -OH group, an -NH₂ group, and two identical -CH₃ groups is achiral because it lacks the necessary diversity in its attachments. Conversely, a carbon bonded to an -OH, an -NH₂, a hydrogen atom, and a -CH₃ group is chiral, as it has four distinct groups.

Chiral molecules are described as optically active, meaning they can rotate plane-polarized light. This property is significant in advanced organic chemistry, particularly in courses like Organic Chemistry I and II, where the implications of chirality and optical activity are explored in greater depth. For now, it is essential to grasp that chirality involves the concept of non-superimposable mirror images and the presence of chiral centers in molecules.

Chirality Example

In determining whether a molecule is chiral or achiral, it is essential to identify chiral centers, which are typically carbon atoms bonded to four distinct groups. A crucial point to remember is that if a carbon atom is involved in a double or triple bond, it cannot be chiral. This is because such carbons do not fulfill the requirement of having four unique substituents; they are limited in the number of bonds they can form.

For instance, consider a carbon involved in a double bond. This carbon forms three visible bonds, and the fourth bond is an implicit hydrogen atom, resulting in only three unique groups attached to it. Therefore, it cannot be classified as chiral. In the context of a benzene ring, we can analyze specific carbon atoms to determine chirality. For example, if we examine a carbon that is bonded to three hydrogens and one other group, it clearly does not meet the criteria for chirality.

However, if we find a carbon that is bonded to three distinct groups, such as a hydrogen, a benzene ring, and a methyl group (CH₃), this carbon has four unique substituents. Consequently, this carbon is classified as a chiral center, indicating that the overall molecule is chiral. Thus, the conclusion is that the molecule in question is indeed a chiral molecule.

Identify chiral centers in the provided optical isomers.

• A.
• B.
• C.
• D.

Answer: (B)

Identify molecule(s) capable of rotating plane polarized light.

• A. A and D
• B. B and C
• C. A and C
• D. C and D
• E. A, C, and D

Answer: (C)

Drawing Enantiomers

When drawing enantiomers of a chiral molecule, two primary methods can be employed to visualize these stereoisomers effectively. The first method involves creating a mirror image of the original molecule. Imagine a mirror placed vertically; the molecule reflects back its own structure. For instance, if the original molecule has a central carbon atom with an amine group (NH₂) at the top, the mirror image will also feature this carbon and the NH₂ group. However, the spatial arrangement of the other substituents will change. The hydrogen (H) and the methyl group (CH₃) will appear reversed in their orientation, with the H represented by a dashed wedge bond and the CH₃ by a solid wedge bond. This method emphasizes the concept that enantiomers are non-superimposable mirror images of each other.

The second method, known as the inversion method, keeps the original molecule stationary while altering the bonds to reflect the enantiomer's spatial orientation. In this approach, the bonds are inverted: a dashed bond becomes a wedged bond and vice versa. For example, if the original molecule has a dashed bond connecting the hydrogen, in the enantiomer, this bond would switch to a wedged bond, while the bond to the CH₃ group would change from wedged to dashed. This method highlights the importance of bond orientation in determining the identity of the enantiomer.

Understanding these two methods is crucial for accurately representing chiral molecules and their enantiomers, as the spatial arrangement of atoms significantly influences their chemical properties and interactions.

Chirality Example

In the study of chiral molecules, understanding enantiomers is crucial. Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, much like left and right hands. To visualize this, one can imagine a mirror placed beside the molecule. When the molecule reflects in the mirror, it creates an enantiomer.

For the first chiral molecule, consider a carbon atom with various substituents. When this molecule looks into the mirror, it sees its own reflection, which includes the same carbon backbone and substituents, but arranged differently. For instance, if the original molecule has a methyl group and two other groups attached to the carbon, the enantiomer will have these groups flipped in their spatial arrangement. It is essential to accurately depict the connections between the carbon atoms and the hydrogen atoms to illustrate the correct structure of the enantiomer.

In the second example, again envisioning a mirror, the molecule's structure remains intact, but the orientation of the substituents changes. If the original molecule has a bromine atom and an amine group (NH₂), the enantiomer will show these groups in a different spatial configuration. The hydrogen atoms will also be positioned differently in the mirror image, emphasizing the importance of stereochemistry in distinguishing between the two enantiomers.

By accurately drawing these mirror images, one can effectively represent the enantiomers of chiral molecules, which is vital for understanding their chemical behavior and interactions in various contexts, such as pharmaceuticals and biochemistry.

Provide the enantiomer using method 2. (Hint: chiral center is circled in red.)

• A.
• B.
• C.
• D.

Answer: (B)

Predict enantiomer for thalidomide compound given below.

• A.
• B.
• C.
• D.

Answer: (A)

Here’s what students ask on this topic:

What is chirality in chemistry?

Chirality in chemistry refers to a property of molecules where their mirror images are non-superimposable. This means that the molecule and its mirror image cannot be aligned perfectly, much like how your left and right hands are mirror images but cannot be superimposed on each other. Chirality often involves a carbon atom connected to four unique groups, known as a chiral center. Molecules with chiral centers are called chiral molecules or optical isomers, and they can rotate plane-polarized light, making them optically active.

What is a chiral center?

A chiral center, also known as a stereocenter, is a carbon atom within a molecule that is connected to four different or unique groups. This unique arrangement allows for the existence of non-superimposable mirror images, or enantiomers. If a carbon atom is connected to four different groups, it is considered chiral. If it is connected to any two identical groups, it is achiral. Chiral centers are crucial in determining the optical activity of a molecule.

How do you draw the enantiomer of a chiral molecule?

There are two methods to draw the enantiomer of a chiral molecule. The first method involves drawing the molecule as it would appear in a mirror, maintaining the central carbon and substituents but reversing the spatial orientation of the bonds. The second method, called the inversion method, keeps the molecule stationary and inverts the bonds at the chiral center, changing dashed bonds to wedged bonds and vice versa. Both methods result in the mirror image of the original chiral molecule.

What are optical isomers?

Optical isomers, also known as enantiomers, are chiral molecules that have one or more chiral centers. These molecules are non-superimposable mirror images of each other and are optically active, meaning they can rotate plane-polarized light. The direction in which they rotate light can be either to the right (dextrorotatory) or to the left (levorotatory). Optical isomers have identical physical properties except for their interaction with plane-polarized light and their reactions in chiral environments.

What makes a molecule achiral?

A molecule is considered achiral if it does not have a carbon atom connected to four different groups. For example, if a carbon atom is connected to two identical groups, such as two CH₃ groups, it is achiral. Achiral molecules do not have non-superimposable mirror images and do not exhibit optical activity. They are symmetrical and do not rotate plane-polarized light.