Which of the following is the correct structure of L-Glucose?

Solution

Chemistry ma’am · 3 steps

L-glucose is the mirror image of D-glucose, so flip every OH and check C5 points left.

1. Start from D-glucose. In the Fischer projection of D-glucose, with CHO at the top, the OH groups on C2 to C5 point right, left, right, right.
2. Take the mirror image. L-glucose is the enantiomer of D-glucose, so every stereocentre is reversed. Its OH groups on C2 to C5 point left, right, left, left. The OH on C5, the lowest stereocentre, points left, which is what makes it an L-sugar.

   Option	OH on C2, C3, C4, C5	What it is
   (a)	R, L, R, L	L-sugar, but C2 to C4 are not flipped (L-idose)
   (b)	L, L, L, L	L-allose
   (c)	L, L, R, R	D-mannose (C5 OH on the right)
   (d)	L, R, L, L	L-glucose

3. So. Only (d) is the exact mirror image of D-glucose at all four stereocentres, so (d) is L-glucose.

The idea to keep. D or L is decided only by the OH on the lowest stereocentre. But the enantiomer of a sugar needs every stereocentre flipped, not just that last one.

Answer from the NTA final answer key. Solution by PrepWiser.