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Understanding Stereochemistry in Carbohydrates Lecture

Learn about stereochemistry in carbohydrates, chiral carbon atoms, enantiomers, D and L isomers, and identifying sugar isomers. Understand the spatial arrangement of atoms in 3D space and the different forms of monosaccharides.

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Understanding Stereochemistry in Carbohydrates Lecture

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  1. CarbohydratesLecture 2

  2. Stereochemistry • Stereochemistry – study of arrangement of atoms in 3D space • Carbohydrates with identical functional groups linked in the same order but with different spatial orientation

  3. Are these compounds the same?

  4. Non-superimposable mirror images

  5. Different Compounds • Non-superimposable so different compounds • One carbon atom is chiral (not symmetric) • Non-superimposable mirror images are called enantiomers

  6. Stereochemistry • There are two facts about the molecule that contains one (or more) chiral center: (1) it is optically active and (2) it occurs in more than one isomeric form. • All monosaccharides except dihydroxyacetone are optically active isomeric forms because they contain one or more chiral carbon atoms in their structures. • They are able to rotate the plane of polarized light either clockwise (right) or counter clockwise (left).

  7. Chiral Carbon Atom Single asymmetric carbon

  8. Configuration about the chiral center • Configuration: the spatial relative arrangement of the substituents around the chiral center. • There are two configurations about each chiral center, the molecule can exist in two isomeric forms. • Accordingly, glyceraldehyde (the simplest aldose) has two different stereoisomers because it has one chiral center at the middle carbon atom. • The two forms are mirror images of each other so they are called enantiomers. By convention, one of these two forms is designated the D isomer, the other the L isomer. • In general, a molecule with n chiral centers can have 2n stereoisomers. Glyceraldehyde has 21 = 2 stereoisomers.

  9. D and L Isomers The −OH group on the • Chiral carbon farthest from the carbonyl group determines an L or D isomer. • Left is assigned the letter Lfor the L-isomer. • Right is assigned the letter Dfor the D-isomer. • A dextrorotatory compound is indicated by a plus sign (+) before its name, and a levorotatory compound is indicated by a minus sign (–) before its name. For example, D-glucose is dextrorotatory, it is written as (+)D-glucose, while D-Fructose is levorotatory written: (-)D-fructose.

  10. Examples of D and L Isomers of Monosaccharides D-glucose D-ribose L-galactose

  11. Check Identify each as the D or L isomer. A. B. C. __-ribose __- threose __- fructose

  12. Solution Identify each as the D or L isomer. A. B. C. L-ribose L-threose D-fructose

  13. It is important to know that the carbons of a sugar are numbered beginning from the carbonyl group (the functional group). • Stereoisomers that are mirror images of each other are called enantiomers. • Pairs of stereoisomers that are not mirror images of each other are called diastereomers. • Sugars that differ only by the configuration about one C atom are known as epimers of one another. • D-glucose and D-mannose are epimers with respect to C2, whereas D-glucose and D-galactose are epimers with respect to C4. However, D-mannose and D-galactose are not epimers of each other because they differ in configuration about two of their carbon atoms.

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