Q.The prefixes 'D' and 'L' before a sugar's name give the configuration of its reference carbon - the highest-numbered chiral carbon, i.e. the chiral carbon next to the terminal CH2OH - by correlation with glyceraldehyde: OH on the right of that carbon in the Fischer projection means D, on the left means L. Consider an aldohexose drawn as a Fischer projection with CHO at the top and CH2OH at the bottom, in which the four chiral carbons, read from top to bottom (C-2, C-3, C-4, C-5), carry their OH groups on the left, right, left and left respectively. Predict whether this compound has the D or the L configuration.

Concept

The D/L system correlates the sugar's reference stereocentre with glyceraldehyde. Draw the molecule as a Fischer projection with the most oxidised group (CHO) at the top and CH2OH at the bottom. The reference carbon is the highest-numbered chiral carbon (the one just above the terminal CH2OH; C-5 in an aldohexose). If its OH points to the right, the compound is D; to the left, it is L. The configurations of the other chiral carbons do not affect the D/L label.

Applying it …

Method: Assigning D or L Configuration to an Aldohexose from Its Fischer Projection

Core Concept

The D/L label depends solely on the configuration of the reference carbon - the highest-numbered chiral carbon, immediately above the terminal CH2OH - compared with glyceraldehyde. OH on the right of that carbon means D; OH on the left means L. The configuration at every other chiral carbon is irrelevant to this label.

Steps

1. Orient the Fischer projection with CHO at the top and CH2OH at the bottom.
2. Number the chiral carbons from the top (C-2) down to the one just above CH2OH (C-5 for an aldohexose).
3. Identify only the OH orientation at that final reference carbon (C-5).
4. If it is on the right, assign D; if on the left, assign L.
5. Disregard the OH orientations at C-2, C-3 and C-4 for the purpose of the D/L label - they define which specific sugar it is, not whether it is D or L.

Applying it to this question …