Diastereomers vs Enantiomers
There can be several structural formulas for a single molecular formula. These are known as isomers. Isomers can be defined as “different compounds that have the same molecular formula.” There are mainly two types of isomers called constitutional isomers and stereoisomers. For example, C4H10 can have following constitutional isomers.
Constitutional isomers are “isomers that differ because their atoms are connected in a different order.” Stereoisomers are isomers that differ from the spatial arrangement of the atoms. Diastereomers and enantiomers are the two types of stereoisomers.
Diastereomers are stereoisomers whose molecules are not mirror images of each other. For example, cis and trans isomers are Diastereomers.
In the above two compounds, the connectivity of atoms is same. Both compounds have a carbon- carbon double bond. And for each carbon, a chlorine atom and a hydrogen atom is 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. But in the trans isomer, hydrogen atoms are on either side of the carbon double bond. And 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.
Enantiomers are “stereoisomers whose molecules are nonsuperposable mirror images of each other. “ Enantiomers are only possible with molecules, which are chiral. Chiral molecule is the one that is not identical with its mirror image. For a molecule to be chiral, it should have one tetrahedral 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. So the molecule and the mirror image are said to be enantiomers. Following is an example of a compound that forms enantiomers.
The enantiomers are named using the R and S system. So, according to the system the molecule on the right hand side is (S)-2-butanol, and the other one is (R)-2-butanol. 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 separate only by their different behaviors towards the 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 said to be optically active. The equimolar mixture of two enantiomers is called a racemic mixture. Racemic mixture doesn’t show any rotation of the polarized light, therefore it is optically inactive.
What’s the difference between Diastereomers and Enantiomers?
- Diastereomers of a molecule are not mirror images of each other, but enantiomers are mirror images.
- Molecules with more than one stereocenter can be Diastereomers if they are not mirror images of each other. If there is only one stereo center, then that molecule has enantiomers.
- Diastereomers have different physical and chemical properties. But enantiomers have similar physical and chemical properties except, they have different optical properties towards the plane polarized light.