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SC10. Diastereomers and Physical Properties

In a pair of diastereomers, some chiral centers are the same and some are opposite. The molecule is neither identical to nor the mirror image of its diastereomer.

Threitol is a metabolite closely related to carbohydrates.  D-threitol is the enantiomer of L-threitol.  The two are non-identical, but they are mirror images of each other.  Both compounds can be crystallized, forming needle-like crystals.  Both have melting points of 88-89oC.  D-threose has an optical rotation [a]D= - 4.0 (c, 7 in H2O), but L-threose has an optical rotation [a]D= + 4.6 (c, 6 in H2O).

  • Enantiomers have identical physical properties, except that the optical rotation is opposite.  

 

 

Wire Frame

Ball & Stick

Spacefilling

Model SC10.1.  D-threitol.

 

Wire Frame

Ball & Stick

Spacefilling

Model SC10.2.  L-threitol.

Erythritol is a diastereomer to both L-threose and D-threose.  It has one identical chiral center and one opposite one.  Erythritol is a solid with a melting point of 121oC and no optical rotation.  Erythritol has properties that are different from threitol.

 

 

Wire Frame

Ball & Stick

Spacefilling

Model SC10.3.  Erythritol.

  • Diastereomers have different physical properties.
  • They do not have the same overall shape.
  • They fit together in different ways, and experience different intermolecular attractions.

The fact that erythritol contains chiral centers but has no optical rotation is unusual.  Erythritol is chiral but not optically active.  It is a special case called a meso compound.

  • A meso compound contains two identical, opposite chiral centers.
  • Rotation of plane-polarized light is canceled between these two centers, just like in a racemic mixture.
  • Meso compounds are not optically active and do not have enantiomers, because the mirror image of a meso compound is itself.
 

Wire Frame

Ball & Stick

Spacefilling

Model SC10.4.  Erythritol in a conformation that shows its internal mirror symmetry.

Problem SC10.1.

What are the absolute configurations of

  1. D-threitol?
  2. L-threitol?
  3. erythritol?
Problem SC10.2. 

In problem SC10.1., you should have numbered the carbons corresponding to each chiral center when you were denoting the absolute configuration.  How did you know which end of the chain to start counting from?

Problem SC10.3.

From the following group of molecules, select:

  1. a pair that are the same compound.
  2. a pair that would have the same physical properties but opposite optical activities.
  3. a pair that have different physical properties.
  4. a compound that contains chiral centers but has no optical activity.