Fischer projections are a handy way to show 3D carbohydrate structures in 2D. They use horizontal and vertical lines to show bond directions, with the carbon chain vertical and the most oxidized carbon at the top. This makes it easy to see the layout of hydroxyl groups and other substituents.

These projections help us understand carbohydrate stereochemistry. By looking at the orientation of hydroxyl groups, we can determine D or L configurations and identify different types of isomers like enantiomers and epimers. This knowledge is crucial for understanding carbohydrate structure and function.

Fischer Projections and Carbohydrate Stereochemistry

Fischer projections of carbohydrates

• Fischer projections represent the three-dimensional structure of molecules on a two-dimensional surface
  • Horizontal lines depict bonds pointing towards the viewer (coming out of the page)
  • Vertical lines depict bonds pointing away from the viewer (going into the page)
• When drawing Fischer projections of carbohydrates, orient the carbon chain vertically with the aldehyde or ketone group at the top
  • Place the most oxidized carbon (aldehyde or ketone) at the top of the projection
  • Place the least oxidized carbon (often a CH2OH group) at the bottom
• Place substituents (hydroxyl groups) on either side of the carbon chain
  • Groups on the right side of the carbon chain are oriented towards the viewer
  • Groups on the left side of the carbon chain are oriented away from the viewer
• Examples of carbohydrates commonly represented using Fischer projections include glucose, fructose, and galactose
  • These are examples of monosaccharides, the simplest form of carbohydrates

Stereochemistry from Fischer projections

• Determine the configuration at each chiral center by the orientation of the substituents
  • If the hydroxyl group is on the right side of the carbon chain, the configuration is D
  • If the hydroxyl group is on the left side of the carbon chain, the configuration is L
• Determine the overall configuration of the carbohydrate by the configuration at the chiral center furthest from the aldehyde or ketone group
  • If the hydroxyl group at the chiral center furthest from the aldehyde or ketone is on the right, the sugar is a D-sugar (D-glucose)
  • If the hydroxyl group at the chiral center furthest from the aldehyde or ketone is on the left, the sugar is an L-sugar (L-glucose)
• The configuration at each chiral center contributes to the overall stereochemistry of the carbohydrate molecule
  • Enantiomers have opposite configurations at all chiral centers (D-glucose and L-glucose)
  • Diastereomers have opposite configurations at some, but not all, chiral centers (D-glucose and D-galactose)
  • Epimers are a specific type of diastereomers that differ in configuration at only one chiral center

Optical activity and isomerism in carbohydrates

• Carbohydrates with chiral centers exhibit optical activity, the ability to rotate plane-polarized light
• Different types of isomers in carbohydrates:
  • Enantiomers: mirror images with opposite configurations at all chiral centers
  • Diastereomers: non-mirror image stereoisomers
  • Epimers: diastereomers differing at only one chiral center
  • Anomers: cyclic forms of monosaccharides that differ in configuration at the anomeric carbon (C1)

Conversion of carbohydrate representations

To convert a Fischer projection to a Haworth projection (cyclic structure):

1. Cyclize the structure by forming a bond between the oxygen of the hydroxyl group on carbon 5 and the aldehyde carbon (carbon 1)
2. Groups that were on the right side of the Fischer projection (towards the viewer) will point downwards in the Haworth projection
3. Groups that were on the left side of the Fischer projection (away from the viewer) will point upwards in the Haworth projection

To convert a Haworth projection to a Fischer projection:

1. Open the cyclic structure by breaking the bond between the oxygen and carbon 1
2. Rotate the structure so that the carbon chain is vertical with the aldehyde or ketone group at the top
3. Groups that were pointing downwards in the Haworth projection will be on the right side of the Fischer projection
4. Groups that were pointing upwards in the Haworth projection will be on the left side of the Fischer projection

• Interconvert between chair conformations and Fischer projections by following a similar process
  • Cyclize the Fischer projection to form a Haworth projection
  • Convert the Haworth projection to a chair conformation by orienting the groups axially or equatorially