Q.The prefixes D and L denote the configuration of the highest-numbered chiral carbon of a sugar (the chiral carbon nearest the terminal CH2OH), by comparison with glyceraldehyde: OH on the right of that carbon in the Fischer projection means D, on the left means L. The sign of optical rotation, (+) or (-), is measured separately and is NOT what fixes D or L. Three compounds are given as Fischer projections with their measured rotations: (I) glyceraldehyde, CHO / H-OH / CH2OH, which is (+); (II) an aldohexose, CHO / H-OH / HO-H / H-OH / H-OH / CH2OH, which is (+); (III) a 2-ketohexose, CH2OH / C=O / HO-H / H-OH / H-OH / CH2OH, which is (-). Which of these have the D configuration?
(A) I, II and III
(B) II and III
(C) I and II
(D) III
(A) I, II and III
(B) II and III
(C) I and II
(D) III

Concept

Do not confuse two different things:

• D/L configuration is a structural label. Draw the Fischer projection with the most oxidised carbon at the top; look at the chiral carbon nearest the bottom CH2OH. If its OH is on the right it is D (like D-glyceraldehyde); on the left it is L.
• (+)/(-) rotation is an experimental property (dextro/laevo) that has no fixed link to D or L.

Applying it to each

• I: glyceraldehyde with OH on the right of its single chiral carbon -> D-glyceraldehyde. (Its rotation happens to be +.)
• II: aldohexose whose reference carbon (C-5, next to CH2OH) has OH on the right -> D. (Rotation +.) …

Method: Assigning D/L Configuration Without Being Misled by Optical Rotation Sign

Core Concept

D/L configuration is a purely structural label, fixed by comparing the OH orientation at the reference carbon (the highest-numbered chiral carbon, i.e. the one nearest the terminal CH2OH) with glyceraldehyde. The (+)/(-) sign of optical rotation is an independent, experimentally measured property and must never be used to decide D or L.

Steps

1. Draw/identify the Fischer projection with the most oxidised carbon (CHO or C=O) at the top and CH2OH at the bottom.
2. Locate the reference carbon - the chiral carbon immediately above the terminal CH2OH (the highest-numbered stereocentre).
3. Check the OH orientation at that carbon only: right -> D configuration; left -> L configuration.
4. Ignore the (+) or (-) sign given for the compound - it plays no role in the D/L assignment.
5. Repeat independently for every compound being compared.

Applying it to this question

• Compound I (glyceraldehyde): its single chiral carbon has OH on the right -> D (its measured rotation of + is incidental). …