The key difference between diastereomers and enantiomers is that diastereomers of a molecule are not mirror images of each other while enantiomers are mirror images of each other.
There can be several structural formulas for a single molecular formula. These are known as isomers. We can define isomers as “different compounds that have the same molecular formula.” There are mainly two types of isomers: constitutional isomers and stereoisomers. Diastereomers and enantiomers are the two types of stereoisomers.
CONTENTS
1. Overview and Key Difference
2. What are Diastereomers
3. What are Enantiomers
4. Side by Side Comparison – Diastereomers vs Enantiomers in Tabular Form
5. Summary
What are Diastereomers?
Diastereomers are stereoisomers whose molecules are not mirror images of each other. For example, cis and trans isomers are diastereomers. Here, the connectivity of atoms is the same.
Figure 01: Diastereomers
In the above example, both compounds have a carbon-carbon double bond. For each carbon, a methyl group and a hydrogen atom are connected. Cis and trans molecules differ only from the way atoms are arranged in the space. That is, in cis isomer, both hydrogens are on the same side of the carbon double bond. However, in the trans isomer, hydrogen atoms are on either side of the carbon double bond. Moreover, the two structures are not mirror images of each other. Therefore, they are diastereomers. However, cis and trans molecules are not the only kind of diastereomers we can find.
What are Enantiomers?
Enantiomers are stereoisomers whose molecules are non-superimposable mirror images of each other. Enantiomers are only possible with molecules that are chiral. A chiral molecule is a one that is not identical to its mirror image. For a molecule to be chiral, it should have one tetrahedral carbon atom with four different groups attached to it. This carbon atom is known as a stereocenter. Chiral molecules make mirror images that are not superposable. Thus, the molecule and the mirror image are enantiomers of each other. Following is an example of a compound that forms enantiomers.
Figure 02: Enantiomers
We usually name enantiomers using the R and S system. Enantiomers don’t have different boiling points, melting points, solubilities, different infrared spectra, etc. All these chemical and physical properties of enantiomers are similar because intermolecular forces are similar in both isomers. They become separated only by their different behaviours towards plane-polarized light. That is, enantiomers rotate the plane of the plane-polarized light in opposite directions. However, they rotate the light in equal amounts. Because of their effect on the polarized light, enantiomers are optically active. The equimolar mixture of two enantiomers is a racemic mixture. A racemic mixture doesn’t show any rotation of the polarized light; therefore, it is optically inactive.
What is the Difference Between Diastereomers and Enantiomers?
The key difference between diastereomers and enantiomers is that diastereomers of a molecule are not mirror images of each other while enantiomers are mirror images. Molecules with more than one stereocenter can be diastereomers if they are not mirror images of each other. However, if there is only one stereocenter, then that molecule has enantiomers. Diastereomers have different physical and chemical properties. But enantiomers have similar physical and chemical properties except for their different optical properties towards the plane-polarized light.
Summary – Diastereomers vs Enantiomers
Diastereomers and enantiomers are the two types of stereoisomers. The key difference between diastereomers and enantiomers is that diastereomers of a molecule are not mirror images of each other, but enantiomers are mirror images.
Reference:
1. Helmenstine, Anne Marie. “Enantiomer Definition.” ThoughtCo, Dec. 5, 2018, Available here.
Image Courtesy:
1. “Cis-trans-2-Buten” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia
2. “Thalidomide-enantiomers” By Klaus Hoffmeier – Own work (Public Domain) via Commons Wikimedia