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SC9. Carbohydrates and Diastereomers

SC9. Carbohydrates and Diastereomers

  • Diastereomers occur in compounds containing more than one chiral center.

Threose is an example of a biological molecule (a carbohydrate) that contains two chiral centers.

Fischer projections are sometimes used in showing carbohydrates and other chain compounds with many chiral centers.  Fischer projections are Picasso-esque drawings in which the point of view alternates from one carbon to the next.  The chiral centers are easy to compare in Fischer projections because you simply have to decide whether groups are on the same side or opposite sides of the vertical line.

D-threose is chiral and it does have an enantiomer.  However, it also has other stereoisomers that are NOT its mirror image.  These isomers are called diastereomers.  The relationships between D-threose and its stereoisomers can be seen in wedge-dash projections.

Alternatively, these relationships can be viewed in Fischer projections.

  • 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.
  • D-threose is the enantiomer of L-threose.  The two are non-identical, but they are mirror images of each other.
 

Wire Frame

Ball & Stick

Spacefilling

Model SC9.1.  D-threose.

 

Wire Frame

Ball & Stick

Spacefilling

Model SC9.2.  L-threose.

 

Wire Frame

Ball & Stick

Spacefilling

Model SC9.3.  D-erythrose.

 

Wire Frame

Ball & Stick

Spacefilling

Model SC9.4.  L-erythrose.

  • When there is one chiral center present, two stereoisomers result.
  • When there are two chiral centers present, four stereoisomers result.
  • For each additional chiral center, the number of stereoisomers doubles.
Problem SC9.1. 

What are the absolute configurations of the two chiral centers in D-threose?  (You will need to number the carbons on which the chiral centers are found.  For example, if an S center is found on the second carbon along the chain and an R center is found on the fourth carbon along the chain, the configuration is 2S,4R. Note that you should number the chain from the highest-priority end, which has a carbon with the most bonds to oxygen.)

Problem SC9.2. 

What are the absolute configurations in L-threose?  What do you notice about the relationship between the configurations in D- and L- threose?

Problem SC9.3. 

What are the absolute configurations in L-erythrose?  What do you notice about the relationships between the configurations in L-erythrose and L- threose?

Problem SC9.4. 

Suppose a compound contained three chiral centers.

  1. How many possible stereoisomers would there be?
  2. How many different pairs of enantiomers would there be?
  3. Suppose you selected a pair of diastereomers from this group.  How many possible pairs could you choose from?
Problem SC9.5. 

Draw one enantiomer and one diastereomer for each of the following compounds.

Problem SC9.6. 

Draw one enantiomer and one diastereomer for each of the following compounds.