In carbohydrates, if there is more than 1 asymmetrical carbon atom, how do we determine the D and L-forms?

Question

Lucy Alessi · Accepted Answer

You look at the stereochemistry of the highest-numbered chiral centre in a Fischer projection.

If the #"OH"# in the highest-numbered chiral carbon (closest to the bottom) is on the right, we have a D-sugar.

If the #"OH"# is on the left, we have an L-sugar.

For example, the Fischer projection of D-glucose is

on #"C-5"# is on the right.

Do not be fooled into thinking that to get the structure of L-glucose, you just invert the configuration at #"C-5"# to get

(from www.easyochem.com)

D-Glucose and L-glucose are enantiomers — nonsuperimposable mirror images of each other.

Easton Jones · Answer

undefined The configuration of the highest numbered asymmetric carbon atom determines whether a carbohydrate is in the D or L-form. If the hydroxyl group on the highest numbered asymmetric carbon is on the right in a Fischer projection, it is in the D-form; if on the left, it is in the L-form.