Achiral

Definition

Achiral refers to a molecule or object that is not chiral, meaning it is superimposable on its mirror image. Achiral molecules lack the necessary structural features, such as the presence of a stereogenic center, that would give rise to non-superimposable enantiomers.

5 Must Know Facts For Your Next Test

1. Achiral molecules do not possess any stereogenic centers and, therefore, cannot exist as enantiomers.
2. The lack of a stereogenic center in achiral molecules means they have a plane of symmetry, allowing them to be superimposable on their mirror image.
3. Achiral molecules are often described as having a symmetric or planar structure, with substituents arranged in a way that makes the molecule superimposable on its mirror image.
4. In the context of 5.1 Enantiomers and the Tetrahedral Carbon, achiral molecules do not exhibit the phenomenon of chirality and cannot form enantiomers.
5. Achiral molecules are also incapable of forming diastereomers, as discussed in 5.6 Diastereomers, since they lack the necessary structural features to give rise to non-superimposable stereoisomers.

Review Questions

• Explain how the presence or absence of a stereogenic center determines whether a molecule is chiral or achiral.
  • The presence of a stereogenic center, such as a tetrahedral carbon atom with four different substituents, is the key structural feature that gives rise to chirality in a molecule. Molecules with a stereogenic center can exist as non-superimposable mirror-image enantiomers. In contrast, achiral molecules lack a stereogenic center and, therefore, are superimposable on their mirror image, meaning they cannot form enantiomers. The absence of a stereogenic center is the defining characteristic of an achiral molecule.
• Describe the relationship between achiral molecules and the formation of diastereomers.
  • Diastereomers are a type of stereoisomer that are not mirror images of each other and, therefore, have different physical and chemical properties. The formation of diastereomers requires the presence of at least two stereogenic centers within a molecule. Since achiral molecules lack any stereogenic centers, they are incapable of forming diastereomers. The lack of a stereogenic center in achiral molecules means they cannot exist in multiple non-superimposable spatial arrangements, which is a prerequisite for the existence of diastereomers as discussed in 5.6 Diastereomers.
• Explain how the addition of water (H$_2$O) to an achiral alkene can result in the formation of a chiral product.
  • In the context of 8.12 Reaction Stereochemistry: Addition of H$_2$O to an Achiral Alkene, the addition of water to an achiral alkene can lead to the formation of a chiral product. This is because the addition of water introduces a new stereogenic center, which was not present in the original achiral alkene. The resulting alcohol product will have a new chiral center and can exist as a pair of enantiomers, even though the starting alkene was achiral. This demonstrates how a reaction can transform an achiral molecule into a chiral product by the creation of a stereogenic center during the course of the transformation.

Related terms

Chiral: Chiral describes a molecule or object that is not superimposable on its mirror image, resulting in the existence of non-identical enantiomers.

Stereogenic Center: A stereogenic center is an atom within a molecule that has four different substituents attached, giving rise to the potential for the formation of enantiomers.

Enantiomers: Enantiomers are a pair of non-superimposable mirror-image molecules that have the same molecular formula and connectivity but differ in their three-dimensional arrangement of atoms.