Enantiomers & Diastereomers
Identical twins look exactly alike - same height, same features, same bone structure. If you put one twin in front of a mirror, the reflection would look like the other twin. They are mirror images of each other. Now think about regular siblings. They share a family resemblance - same parents, similar features - but they are clearly not mirror images. One might be taller, one might have different eye color.
Enantiomers are the identical twins of stereochemistry: exact mirror images at every stereocenter. Diastereomers are the siblings: related by stereochemistry, but different at some positions while the same at others.
Enantiomers: Mirror-Image Partners
Definition: Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They have opposite configurations at EVERY stereocenter.
If a molecule has stereocenters with configurations (R,R), its enantiomer is (S,S). If it is (R,S,R), its enantiomer is (S,R,S). Every single stereocenter is flipped.
Properties of enantiomers:
Enantiomers share nearly all physical properties:
| Property | Same or Different? |
|---|---|
| Melting point | Same |
| Boiling point | Same |
| Density | Same |
| Solubility (in achiral solvents) | Same |
| IR spectrum | Same |
| NMR spectrum | Same |
| Refractive index | Same |
| Optical rotation magnitude | Same (but opposite sign) |
| Reaction with achiral reagents | Same rate |
| Reaction with chiral reagents | Different rate |
| Biological activity | Often different |
The one measurable physical difference between enantiomers in an achiral environment is the direction of optical rotation: one rotates plane-polarized light clockwise (+), the other rotates it counterclockwise (-), by exactly the same magnitude. We will cover this in detail in Section 2.7.
Diastereomers: Non-Mirror Stereoisomers
Definition: Diastereomers are stereoisomers that are NOT mirror images of each other. They differ at some stereocenters but are the same at others.
If a molecule has stereocenters with configurations (R,R), the possible stereoisomers are:
- (S,S) - enantiomer of (R,R)
- (R,S) - diastereomer of (R,R)
- (S,R) - diastereomer of (R,R)
Notice that (R,S) and (S,R) are enantiomers of each other, but both are diastereomers of (R,R).
Properties of diastereomers:
Unlike enantiomers, diastereomers have DIFFERENT physical properties:
| Property | Same or Different? |
|---|---|
| Melting point | Different |
| Boiling point | Different |
| Density | Different |
| Solubility | Different |
| Optical rotation | Different (both magnitude and sign) |
| Reaction rates | Different |
This is crucial for the MCAT: because diastereomers have different physical properties, they can be separated by standard techniques like distillation, crystallization, or chromatography. Enantiomers cannot be separated by these methods because their physical properties are identical in achiral environments.
Epimers: A Special Case of Diastereomers
Epimers are diastereomers that differ at exactly ONE stereocenter. This term comes up most often in carbohydrate chemistry.
Example: D-glucose and D-galactose are epimers - they differ only at carbon 4. All other stereocenters are identical. D-glucose and D-mannose are also epimers, differing only at carbon 2.
Epimers are a subset of diastereomers, so they have different physical properties and can be separated by standard methods.
Anomers: Another Special Case
Anomers are a type of epimer that differ specifically at the anomeric carbon (carbon 1 in aldoses, carbon 2 in ketoses) of cyclic sugars. When glucose cyclizes, the hydroxyl on the anomeric carbon can point either down (alpha anomer) or up (beta anomer) in a Haworth projection. Anomers are diastereomers of each other.
Relationships Between Multiple Stereocenters
For a molecule with two stereocenters, there are up to four stereoisomers ( = 4):
| Stereoisomer | Configuration | Relationship to (R,R) |
|---|---|---|
| Compound A | (R,R) | - (itself) |
| Compound B | (S,S) | Enantiomer |
| Compound C | (R,S) | Diastereomer |
| Compound D | (S,R) | Diastereomer |
Note: Compounds C and D are enantiomers of each other, but both are diastereomers of A and B.
How to Quickly Classify Two Stereoisomers
When the MCAT gives you two structures and asks for their relationship:
- Verify they have the same molecular formula and connectivity (if not, they are constitutional isomers or not isomers at all).
- Assign R/S at every stereocenter in both molecules.
- Compare the configurations:
- Opposite at ALL stereocenters = enantiomers
- Opposite at SOME, same at others = diastereomers
- Same at ALL stereocenters = identical compound (not isomers)
Biological Significance
In an achiral environment (a test tube with ordinary solvents), enantiomers behave identically. But the body is a chiral environment - every enzyme, receptor, and transport protein is built from L-amino acids, making them inherently chiral.
This means enantiomers of a drug can have:
- The same therapeutic effect (both active)
- Only one enantiomer active, the other inert
- One enantiomer active, the other toxic
- Both enantiomers active but for different conditions
The MCAT tests this concept because it connects organic chemistry to biology and pharmacology.