5.6 Diastereomers

Stereoisomers are molecules with the same chemical formula but different 3D arrangements of atoms. They come in two main types: enantiomers (mirror images) and diastereomers (non-mirror images). Understanding these is key to grasping molecular structure and reactivity.

Knowing how to calculate and represent stereoisomers is crucial. The number of possible stereoisomers depends on chirality centers, while Fischer and Newman projections help visualize their 3D structures. This knowledge is essential for predicting molecular behavior and understanding biological processes.

Stereoisomers

Enantiomers vs diastereomers

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• Enantiomers
  • Non-superimposable mirror images of each other (like left and right hands)
  • Opposite configurations at all chirality centers (R and S)
  • Identical physical properties except for the direction of plane-polarized light rotation (clockwise or counterclockwise)
  • Exhibit optical activity due to their chiral nature
• Diastereomers
  • Not mirror images of each other (like cis and trans isomers)
  • Different configurations at one or more but not all chirality centers
  • Different physical properties such as melting points, boiling points, and solubilities (can be separated by physical means)

Stereoisomer quantity calculation

• Number of possible stereoisomers for a molecule determined by the formula: $2^n$, where $n$ is the number of chirality centers (also called stereocenters)
  • Molecule with 2 chirality centers will have $2^2 = 4$ possible stereoisomers (2 enantiomeric pairs)
  • Molecule with 3 chirality centers will have $2^3 = 8$ possible stereoisomers (4 enantiomeric pairs)
• Stereoisomers consist of:
  • Enantiomeric pairs: $\frac{2^n}{2} = 2^{n-1}$ (1 pair for 2 chirality centers, 2 pairs for 3 chirality centers)
  • Diastereomeric pairs: $\frac{2^n - 2^{n-1}}{2} = 2^{n-1} - 1$ (1 pair for 2 chirality centers, 3 pairs for 3 chirality centers)
• Meso compounds
  • Molecule has multiple chirality centers but is achiral due to an internal plane of symmetry (can be superimposed on its mirror image)
  • Meso compounds reduce the total number of stereoisomers by 1 (tartaric acid has 3 stereoisomers instead of 4)

Epimers and stereoisomer relationships

• Epimers
  • Subset of diastereomers that differ in configuration at only one chirality center (like glucose and galactose)
  • Closest stereoisomeric relationship among diastereomers (most similar in structure)
• Relationship to other stereoisomers
  • Enantiomers differ at all chirality centers, while epimers differ at only one (enantiomers are farther apart structurally)
  • Diastereomers that are not epimers differ at more than one but not all chirality centers (in between enantiomers and epimers)
• Importance of epimers
  • Similar physical properties due to their close structural relationship (harder to separate than other diastereomers)
  • Different biochemical activities or functions in biological systems (glucose and galactose metabolism)

Stereoisomer representations

• Fischer projections
  • 2D representation of 3D molecular structures, particularly useful for depicting stereoisomers
  • Horizontal lines represent bonds coming out of the page, vertical lines represent bonds going into the page
• Newman projections
  • Used to visualize different conformations of molecules, especially along carbon-carbon single bonds
  • Helpful in understanding configurational and conformational isomers