Stereoisomers and Diastereoisomers in Organic Chemistry

Steroisomers

• Same structural formula but with different spatial configuration
• Enantiomers: four different atoms or groups of atoms are attached to an asymmetric carbon
  • Example: D and L glyceraldehyde, which are mirror images of each other
• Epimers: isomers that differ due to the H and OH configuration of carbons 2, 3, or 4
  • Example: D-glucose and D-mannose are epimers at C-2, and D-glucose and D-galactose are epimers at C-4

Diastereoisomers

• D-glucose and D-mannose are epimers at C-2, and D-glucose and D-galactose are epimers at C-4
• D-galactose and D-mannose are diastereoisomers, as they differ in more than one carbon

Anomers

• Result from cyclization, adding an additional asymmetric carbon
• The C-1 in a ring structure can become the asymmetric center of the ring, resulting in alpha- and beta-configurations of the sugar

Optical Isomerism

• The presence of asymmetric carbons or chirality influences the optical activity of compounds
• Example: D and L enantiomers of glyceraldehyde have identical properties, but differ in optical activity
• Optical activity is measured by the rotation of the plane of polarized light
  • L rotates clockwise, while D rotates counter-clockwise
• Examples: D(+) glucose, D(-) fructose

Structural Isomerism

• Isomers with the same molecular formula but different structural or spatial arrangements
• Classified into two classes: structural isomers and optical isomers (stereoisomers)
• Examples: erythrose (aldose) and erythulose (ketose), ribose and ribulose, xylose and xylulose

Pyranose and Furanose Ring Structures

• Monosaccharides in solution do not exist in open-chain structures, but rather cyclize into rings
• Hexoses form 6-membered rings, while ketohexoses and aldopentoses form 5-membered rings
• Haworth structures are used to represent the ring structures

Haworth vs Chair Formation Structure

• The 6-membered ring is not planar, but rather exists in a chair formation

Optical Activity of Compounds

• A solution of glucose can have different physical properties depending on the percentage of α- and β-isomers
• The specific rotation of the β- and α-isomers can be used to calculate the percentage of each isomer in a solution

Dissacharides

• Natural carbohydrates usually contain more than one monosaccharide unit
• Periodic acid (HIO4) is used to cleave C-C bonds and analyze carbohydrates

Periodate Oxidation

• Periodic acid cleaves C-C bonds if both carbons have hydroxyl groups or if one carbon has a hydroxyl group adjacent to another carbon with an amino group, keto, or aldo oxygen
• Every cleavage results in an oxidation
• The carbon participating in the cleavage reaction is oxidized to the next level (e.g., alcohol to aldehyde, and aldehyde to carboxylic acid)