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🥼organic chemistry review

Plane of Symmetry

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

A plane of symmetry is a hypothetical plane that divides a molecule or object into two equal and mirror-image halves. It is an important concept in understanding the symmetry and stereochemistry of organic compounds, particularly in the context of meso compounds and the stereochemistry of addition reactions.

5 Must Know Facts For Your Next Test

  1. A molecule or object with a plane of symmetry can be divided into two identical halves that are mirror images of each other.
  2. The presence of a plane of symmetry in a molecule indicates that it is achiral, meaning it cannot be distinguished from its mirror image.
  3. Meso compounds, which have multiple stereogenic centers but are achiral, possess a plane of symmetry that passes through the molecule.
  4. During the addition of water (H$_2$O) to an achiral alkene, the plane of symmetry is maintained, resulting in the formation of an achiral product.
  5. The presence or absence of a plane of symmetry in a molecule can be determined by examining its overall shape and the spatial arrangement of its atoms.

Review Questions

  • Explain how the presence of a plane of symmetry in a molecule affects its stereochemistry.
    • The presence of a plane of symmetry in a molecule indicates that the molecule is achiral, meaning it cannot be distinguished from its mirror image. This is because the plane of symmetry divides the molecule into two identical halves that are superimposable. As a result, molecules with a plane of symmetry, such as meso compounds, do not exhibit optical activity and are not enantiomers.
  • Describe the role of a plane of symmetry in the stereochemistry of the addition of water (H$_2$O) to an achiral alkene.
    • When water (H$_2$O) is added to an achiral alkene, the plane of symmetry present in the starting material is maintained in the product. This is because the addition of water to an achiral alkene results in the formation of an achiral product, where the two hydroxyl groups (-OH) are added to the same face of the alkene. The preservation of the plane of symmetry ensures that the product is also achiral and does not exhibit optical activity.
  • Analyze how the presence or absence of a plane of symmetry in a molecule can be used to predict its stereochemical properties.
    • The presence or absence of a plane of symmetry in a molecule is a crucial factor in determining its stereochemical properties. If a molecule has a plane of symmetry, it is considered achiral, meaning it cannot be distinguished from its mirror image. This is the case for meso compounds, which have multiple stereogenic centers but are still achiral due to the presence of a plane of symmetry. Conversely, the absence of a plane of symmetry in a molecule indicates that it is chiral and can exist as a pair of enantiomers. Understanding the role of a plane of symmetry in the stereochemistry of organic compounds is essential for predicting their behavior and reactivity.