Enantiomers and Diastereomers
Enantiomers
Optically active chiral molecules which are non-super imposable on its mirror images are called enantiomers and the phenomenon is known as enantiomerism. Enantiomers differ in their configuration (R or S) at the stereogenic center. To convert between enantiomers, all chiral carbon atoms must change configuration. All stereocenters must be different for molecules to be enantiomers.
We can easily understand the chirality by comparing our hands (left hand and right hand). Our left hand and right hand are the best example of non-superimposable mirror image of each other. Each hand is therefore considered as chiral.
Properties of Enantiomers
The main properties of enantiomers are given below-
- Enantiomers always exist in pair.
- Enantiomers are non-super imposable on its mirror image.
- Enantiomers have same physical properties (like boiling point, melting point, solubility, density, viscosity, refractive index etc.)and chemical properties in achiral environment.
- Each enantiomers have opposite behavior with respect to plane polarized light, if one of them will rotate the plane polarized light towards right hand direction then definitely the other will rotate the plane polarized light towards left hand direction.
- Each enantiomers shows the same chemical reactivity with achiral reagent; however they have different reactivity with chiral reagent.
Diastereomers
Diastereomers are simply not enantiomers. They are compounds which have the same molecular formula and sequence of bonded elements but are not mirror image and nonsuperimposable. The must essential conditions for a molecule to be diastereomer there must be at least two asymmetric carbon.
To convert between diastereomers, only one chiral carbon atom must change configuration. At least one stereocenter must be different, and at least one stereocenter must be the same for molecules to be diastereomers.
Cis-trans isomers are diastereomers. All cis-trans isomers are diastereomers, but not all diastereomers are cis-trans isomers. For example, cis-butanoic acid and trans-butanoic acids are diastereomers because they are stereoisomers that are not mirror images of each other.
Properties of Diastereomes
The main properties of Diastereomers are given below-
- Diastereomers have different physical properties such as melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations.
- Diastereomers other than geometrical isomers may or maynot be optically active.
- Diastereomers show similar, but not identical chemical properties. The rates of reactions of the two diastereomers with a given reagent provided tha reagent is not rapidly active.
- Diastereomers can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc.
Enantiomers and Diastereoisomers
☛ Enantiomers differ only by their absolute stereochemistry (R or S etc).☛ Diastereoisomers differ by their relative stereochemistry.
A molecule can only have one enantiomer but any number of diastereoisomers.
Difference Between Enantiomers and Diastereomers
| Difference Between Enantiomers and Diastereomers | ||
|---|---|---|
| Parametes | Enantiomers | Diastereomers |
| Mirror Image | Yes | No |
| Superimposable | No | No |
| Pairing | Present in a paired form | Present separately |
| Chirality | Opposite configuration at all chiral centers | Opposite configuration at some, but not all, chiral centers |
| Optical Activity | Equal and opposite optical rotation | May or may not have equal and opposite optical rotation |
| Naming Convention | Named using R/S or D/L nomenclature | Named using cis/trans or E/Z nomenclature |
| Physical Properties | Identical (except optical activity) | Diferent |
| Chemical Properties | Identical in achiral environments, different in chiral environments | Diferent |
Also read Isomerism | Geometrical Isomerism | Optical Isomerism | Coformational Isomerism
