A student is asked to draw the Fischer projection of D-glucose. Which of the following statements correctly describes a characteristic feature of D-glucose in its Fischer projection?
- AThe -OH group on the lowest chiral carbon (C5) is on the right.
- BIt has an internal ether linkage, forming a cyclic hemiacetal.
- CIt is an aldoketose with a ketone group at C2.
- DThe -OH group on the lowest chiral carbon (C5) is on the left.
Show answer & explanationAnswer & explanation
Correct answer: A. The -OH group on the lowest chiral carbon (C5) is on the right.
D-glucose is defined by the configuration of its lowest chiral carbon. In a Fischer projection, if the -OH group on this carbon (C5 for hexoses) is on the right, it is a D-sugar. If it's on the left, it's an L-sugar. Glucose is an aldohexose, meaning it has an aldehyde group (not a ketone) and six carbons. While glucose does form a cyclic hemiacetal in solution, a Fischer projection is a linear representation.
Why the other options are wrong
- B. While true for glucose in solution, Fischer projections are linear representations and do not show cyclic structures.
- C. Glucose is an aldohexose (aldehyde, not ketone), and it is not an aldoketose.
- D. This describes an L-sugar, not a D-sugar.
D/L Configuration of Monosaccharides
The D or L configuration of a monosaccharide refers to the stereochemistry at the chiral carbon atom farthest from the carbonyl group. In a Fischer projection, if the hydroxyl group on this carbon is on the right, it's a D-sugar; if it's on the left, it's an L-sugar.
- Based on the lowest chiral carbon.
- Right -OH = D-sugar.
- Left -OH = L-sugar.
- Most naturally occurring sugars are D-isomers.
Memory trick: D-Right, L-Left, on the lowest chiral light.