Question

Define the following terms: (a) Chiral (b) Achiral (c) Chiral carbon (d) Enantiomer

Short answer

Chiral objects are non-superimposable on their mirror images; achiral objects are superimposable. A chiral carbon is a carbon with four different atoms/groups. Enantiomers are non-superimposable mirror image isomers.

Step-by-step solution

Define Chiral

A chiral object or molecule cannot be superimposed on its mirror image. It typically has no plane of symmetry and exists in two enantiomeric forms.

Define Achiral

An achiral object or molecule can be superimposed on its mirror image. It has a plane of symmetry and does not exist in two enantiomers.

Define Chiral Carbon

A chiral carbon is a carbon atom that is attached to four different groups or atoms, making it an asymmetric center and contributing to the chirality of the molecule.

Define Enantiomer

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. They have identical physical properties except for the direction in which they rotate plane-polarized light and reactions in a chiral environment.

Key concepts

Chiral Molecules

Chiral molecules are fascinating entities in chemistry due to their unique properties. They lack the ability to be superimposed on their mirror images, similar to how your left and right hands are mirror images but cannot perfectly align when overlaid. This non-superimposability arises from the molecule's lack of a plane of symmetry. Without symmetry, the molecule can exist in two distinct forms known as enantiomers. Each form is a mirror image of the other, yet they are not identical. Understanding chirality is crucial when studying molecular behavior, especially in biological systems, where the chirality of a molecule can determine its function and interaction.

Achiral Molecules

Achiral molecules, on the other hand, can be perfectly superimposed on their mirror images. Unlike chiral molecules, they inherently possess a plane of symmetry. This symmetry ensures that both halves of the molecule are identical, resulting in a single, unified structure without any enantiomeric forms. For example, a simple molecule like ethane is achiral because it can be flipped, rotated, or mirrored, and still appear the same. Recognizing achiral molecules is essential in chemical analysis, as they tend not to interact the same way as chiral ones when subjected to chiral environments.

Chiral Center

The chiral center, often referred to as a stereocenter, is a key concept in understanding chirality. It typically involves a carbon atom bonded to four different atoms or groups. This unique arrangement promotes asymmetry, making the molecule chiral. The presence of a chiral center is what prevents the molecule from being superimposable on its mirror image. Identifying a chiral center is pivotal in stereochemistry since it dictates how the molecule behaves and interacts. Chiral centers are commonly found in organic compounds, particularly those of biological significance, such as amino acids and sugars.

Enantiomers

Enantiomers are a specific type of stereoisomer, forming when a molecule has one or more chiral centers. They are non-superimposable mirror images of each other akin to left and right hands. These molecules share identical physical properties—such as melting point and boiling point—but differ in how they interact with polarized light and chiral environments. One enantiomer will rotate plane-polarized light in one direction, while its partner rotates light in the opposite direction. Enantiomers are notably significant in pharmaceuticals since one form may be active or effective in treatment, while the mirror image may not be, or could even be harmful.