The key difference between absolute and relative configuration in stereochemistry is that the absolute configuration of a substituent in a molecule is independent of the atoms of groups elsewhere in the molecule whereas the relative configuration of a substituent is determined in relation to something else in the molecule.
Configuration refers to the arrangement of atoms or groups of atoms in a molecule. There are two types of configurations as absolute configuration and relative configuration. These terms are used specifically for organic compounds having substituents.
CONTENTS
1. Overview and Key Difference
2. What is Absolute Configuration in Stereochemistry
3. What is Relative Configuration in Stereochemistry
4. Side by Side Comparison – Absolute vs Relative Configuration in Stereochemistry in Tabular Form
5. Summary
What is Absolute Configuration in Stereochemistry?
Absolute configuration in stereochemistry is the arrangement of atoms or group of atoms that is described independently of any other atom or group of atoms in the molecule. This type of configuration is defined for chiral molecular entities and their stereochemical descriptions (e.g. R or S referring to Rectus and Sinister respectively). Often, we can obtain the absolute configuration for a chiral molecule that is in the pure forms using X-ray crystallography. However, there are some alternative techniques such as optical rotatory dispersion, vibrational circular dichroism, UV-visible spectroscopy, proton NMR, etc. The absolute configuration of a compound is relevant to the characterization of crystals.
Before 1951, it was not possible to obtain the absolute configuration for a molecule, but in 1951, Johannes Martin Bijvoet used X-ray crystallography to obtain the absolute configuration through resonant scattering effect. He used (+)-sodium rubidium tartrate in this experiment.
What is Relative Configuration in Stereochemistry?
Relative configuration in stereochemistry is the arrangement of atoms or group of atoms that is described relative to other atoms or group of atoms in the molecule. In other words, this term describes the position of atoms or group of atoms in space in relation to other atoms or group of atoms that are located elsewhere in the molecule. It is an experimentally determined relationship between two enantiomers even though we do not know the absolute configuration.
The absolute configuration was discovered in 1951. Before that time, the configurations were assigned relative to a standard (the standard compound was glyceraldehyde), which was chosen for the purpose of correlating to the configuration of carbohydrates.
What is the Difference Between Absolute and Relative Configuration in Stereochemistry?
The terms absolute and relative configurations are used specifically to describe organic compounds having substituents and stereochemical centres. The key difference between absolute and relative configuration in stereochemistry is that the absolute configuration of a substituent in a molecule is independent of the atoms of groups elsewhere in the molecule whereas the relative configuration of a substituent is determined in relation to something else in the molecule.
Below is a summary tabulation of the difference between absolute and relative configuration in stereochemistry.
Summary – Absolute vs Relative Configuration in Stereochemistry
The terms absolute and relative configurations are specifically used to describe organic compounds having substituents and stereochemical centres. The key difference between absolute and relative configuration in stereochemistry is that the absolute configuration in stereochemistry is the arrangement of atoms or group of atoms that is described independent of any other atom or group of atoms in the molecule, while the relative configuration in stereochemistry is the arrangement of atoms or group of atoms that is described relative to other atoms or group of atoms in the molecule.
Reference:
1. “Absolute Configuration.” Science Direct, Available here.
Image Courtesy:
1. “Absolute configuration en” By Qniemiec – Own work (CC BY-SA 3.0) via Commons Wikimedia