Return to Contents Chapter 25 : Carbohydrates Ch 25 contents
Carbohydrate Stereochemistry Fischer projections are very commonly used to represent sugars as the provide a quick way of representing multiple stereocenters plus they can be related to the cyclic sugar structures reasonably easily. Note that Fischer projections of carbohydrates are typically drawn with the longest chain oriented vertically and with the more highly oxidised C (the carbonyl group) at the top. Here we see the Fischer projections of the simplest carbohydrate, glyceraldehyde in its (S)-(-)- and (R)-(+)- forms:
 
Fischer diagram of S-(-)-glyceraldehyderevealing the stereochemistry of the Fischer diagram
Fischer diagram of R-(+)-glyceraldehyderevealing the stereochemistry of the Fischer diagram
S-(-)-glyceraldehyde
or
L-glyceraldehyde
R-(+)-glyceraldehyde
or
D-glyceraldehyde
The D- and L- notation is based on these structures.  (Note that D- and L- means something quite different  to d- and l-).
By definition (R)-(+)-glyceraldehyde is D-glyceraldehyde and (S)-(-)-glyceraldehyde is L-glyceraldehyde.
Other carbohydrates are defined as either D- or L- by comparing the stereochemistry at the chirality center next to the -CH2OH group (i.e.  with the chirality center furthest from the carbonyl group or the highest numbered chirality center) with the chirality center in glyceraldehyde.
 
Study Tip ?
L-glyceraldehyde has the -OH group on the left and D-glyceraldehyde on the right in the Fischer projections of the structures.

A series of carbohydrates are shown below, decide whether they are D- or L- by comparing them to glyceraldehyde

 
L-glyceraldehyde
ANSWER
Haworth diagrams these are "flattened" diagrams used to represent the stereochemistry carbohydrates, note how the relative positions of the groups in the two structures are related.
 
Haworth diagram

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© Dr. Ian Hunt, Department of Chemistry, University of Calgary