A monosaccharide A has a molecular weight of 150 Da. The two stereoisomers B and C, that are both op — Physical Chemistry Chemistry Question
Monosaccharides
A monosaccharide A has a molecular weight of 150 Da. The two stereoisomers B and C, that are both optically inactive, form when A reacts with NaBH4.
Draw the structures of A, B and C according to the Fischer projection.
Model Answer
A monosaccharide has the general formula Cn(H2O)n. Consequently, a molecular weight of 150 Da can only be reached by the formula C5(H2O)5. After reduction, B and C are the only two possible products that are optically inactive. These two products can only be formed from A as the common precursor. [VISUAL]
Determine at all stereocenters in A up to C the absolute configuration according to the CIP (R/S) nomenclature.
Model Answer
If two groups only differ in chirality, the (R)-center has a higher priority than the (S)-center. Based on the structures: In A: C2 is (S), C4 is (R). In B: C2 is (S), C3 is (S), C4 is (R). In C: C2 is (S), C3 is (R), C4 is (R).
Show all different stereoisomers of B and indicate their stereochemical relationship.
Model Answer
The stereoisomers of B (the 5-carbon alditols) and their relationships are mapped out as: Ribitol (meso) and Xylitol (meso) are diastereomers. L-Arabinitol and D-Arabinitol are enantiomers. Both L- and D-Arabinitol are diastereomers of both Ribitol and Xylitol. [VISUAL]