Question

What do you mean by D and L configurations of carbohydrates? Draw the structural formula of D-Glucose and D-Fructose.

Hint:

Do not confuse D/L notation with optical activity (+ or –). D and L are about configuration, not about dextrorotation or levorotation.

Answer 1

Step 1: D and L Configurations.
The D and L notations of carbohydrates are based on the configuration of the asymmetric carbon atom farthest from the carbonyl group (the penultimate carbon).
- If the OH group on the penultimate carbon is on the right → D-configuration.
- If the OH group on the penultimate carbon is on the left → L-configuration.
Step 2: Example.
- D-Glucose and D-Fructose both belong to the D-series, meaning that the OH group on the penultimate carbon is on the right-hand side in their Fischer projections. Step 3: Structures.
D-Glucose (Fischer projection): \[ \begin{array}{c} CHO
[-4pt] |
[-4pt] H - C - OH
[-4pt] |
[-4pt] HO - C - H
[-4pt] |
[-4pt] H - C - OH
[-4pt] |
[-4pt] H - C - OH
[-4pt] |
[-4pt] CH_2OH \end{array} \] D-Fructose (Fischer projection): \[ \begin{array}{c} CH_2OH
[-4pt] |
[-4pt] C{=}O
[-4pt] |
[-4pt] HO - C - H
[-4pt] |
[-4pt] H - C - OH
[-4pt] |
[-4pt] H - C - OH
[-4pt] |
[-4pt] CH_2OH \end{array} \] Conclusion:
D and L configurations are based on the arrangement of the OH group on the penultimate carbon atom, and both D-Glucose and D-Fructose belong to the D-series.