D and L enantiomers are based on which carbon's hydroxyl group?

• A. On the first carbon
• B. On the penultimate carbon
• C. On the last carbon
• D. On the second carbon

Answer: (B)

D and L designations come from the orientation of the hydroxyl on the chiral carbon farthest from the carbonyl group. For aldoses, that means the carbon just before the last one (the penultimate carbon). In Fischer projections, if the OH on that carbon points to the right, the sugar is D; if it points to the left, it’s L. This convention uses D-glyceraldehyde as the reference. The last carbon is CH2OH and not a stereocenter, and the first carbon is the carbonyl carbon, so they don’t determine D/L. That’s why the hydroxyl on the penultimate carbon sets the D/L designation.