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What is the meaning of periplanar?

Published in Chemical Conformation 3 mins read

Periplanar describes a molecular conformation where specific atoms or groups lie approximately in the same plane, signifying a "nearly planar" orientation.

This precise term, coined by Klyne and Prelog in 1960, is fundamental in the field of chemistry, particularly for defining the flexible arrangements of atoms within molecules. It allows for a slight deviation from perfect flatness, reflecting the dynamic nature of chemical structures.

Understanding Periplanar Conformations

The concept of periplanar is tied to specific ranges of torsional angles, also known as dihedral angles. A torsional angle measures the angle between two intersecting planes defined by four atoms along a chemical bond.

The definition of periplanar encompasses two primary types of arrangements:

  • Syn-Periplanar: This describes an arrangement where the relevant atoms or groups have a torsional angle of 0° ± 30°. In this conformation, the groups are on the same side of a reference plane and are nearly aligned with each other.
  • Anti-Periplanar: This refers to an arrangement where the torsional angle is 180° ± 30°. Here, the groups are on opposite sides of the reference plane but still maintain a near-linear alignment.

These ranges acknowledge that molecules are not rigid but can undergo rotations around their single bonds, leading to various conformations.

Etymology of "Periplanar"

The prefix "peri-" is derived from the Greek word meaning "near" or "around." When combined with "planar" (meaning "flat" or "in a plane"), the term "periplanar" effectively conveys the meaning "approximately planar" or "nearly in a plane." This etymology underscores the core concept of the term.

Periplanar Torsional Angle Ranges

The following table summarizes the key angle ranges for periplanar conformations:

Conformation Torsional Angle Range Description
Syn-Periplanar 0° ± 30° Groups are nearly aligned on the same side.
Anti-Periplanar 180° ± 30° Groups are nearly aligned on opposite sides.

Importance in Chemistry

The concept of periplanar is crucial for understanding molecular behavior and reactivity in various aspects of chemistry:

  • Reaction Mechanisms: Many organic reactions, especially elimination reactions like the E2 mechanism, proceed most efficiently when the departing groups adopt an anti-periplanar conformation. This optimal alignment allows for effective orbital overlap, facilitating the reaction.
  • Conformational Analysis: It provides a precise way to describe the preferred or accessible spatial arrangements (conformations) of molecules. Understanding these conformations is vital for predicting a molecule's stability, physical properties, and reactivity.
  • Stereochemistry: By defining flexible ranges for torsional angles, periplanar helps to explain and predict the stereochemical outcomes of reactions, where the three-dimensional arrangement of atoms is critical.

Distinguishing from Coplanar

While sometimes informally used interchangeably, it's important to recognize the subtle but significant distinction between periplanar and coplanar:

  • Coplanar: Implies perfect planarity, meaning the atoms lie exactly in the same plane (precisely 0° or 180° torsional angles).
  • Periplanar: Acknowledges approximate planarity, providing a more realistic and flexible definition for dynamic molecular systems that exhibit slight deviations from ideal geometry. This distinction highlights the nuanced precision in describing molecular structures.