Lecture Notes: Classes of Monosaccharides

Learning Objectives

• Classify monosaccharides as aldoses or ketoses and by the number of carbons (trioses, tetroses, pentoses, hexoses).
• Differentiate between D sugars and L sugars.

Classification of Monosaccharides

• Carbon Atom Count:
  • Trioses: 3 carbon atoms
  • Tetroses: 4 carbon atoms
  • Pentoses: 5 carbon atoms
  • Hexoses: 6 carbon atoms
• Functional Groups:
  • Aldoses: Contains an aldehyde group.
  • Ketoses: Contains a ketone group at the second carbon.
• Examples:
  • Aldohexose: Glucose
  • Ketohexose: Fructose

Structural Examples

• Ketopentose: Five carbons, second carbon is a carbonyl group.
• Aldotetrose: Four carbons, first carbon is part of an aldehyde group.

Stereochemistry

• Stereoisomers: Same structural formula but different spatial arrangement.
• Enantiomers: Non-superimposable mirror images.
  • Example: D-glyceraldehyde and L-glyceraldehyde.
• Chiral Carbon: Carbon atom with four different attached groups.

Fischer Projections

• Used to represent monosaccharides in two dimensions.
• Vertical Lines: Bonds pointing away.
• Horizontal Lines: Bonds coming toward.

D and L Sugars

• Related to D- and L-glyceraldehyde.
• Penultimate Carbon: Determines if a sugar is D or L, farthest from the carbonyl group.

Polarized Light and Optical Activity

• Plane-Polarized Light: All waves vibrate in a single plane.
• Optically Active Substances: Rotate the plane of polarized light.
  • Dextrorotatory: Rotates light clockwise (+).
  • Levorotatory: Rotates light counterclockwise (-).
• Polarimeter: Measures the extent of optical activity.

Summary

Monosaccharides are classified by carbon count and functional group type (aldose/ketose). They often contain chiral carbons leading to stereoisomerism, including enantiomers, which affect how they interact with polarized light. Fischer projections help in visualizing these structures.