Overview
This lecture covers the structure, classification, stereochemistry, and cyclization of carbohydrates, focusing on monosaccharides and key concepts in sugar chemistry.
Structure and Classification of Carbohydrates
- Carbohydrates are molecules composed of carbon, hydrogen, and hydroxyl groups (hydrates of carbon).
- Small carbohydrates are called sugars; their names end with "ose" (e.g., glucose, sucrose).
- Monosaccharides are the building blocks (monomers) that join to form polysaccharides.
- Monosaccharides are classified by carbon number: triose (3), tetrose (4), pentose (5), hexose (6).
- Each carbohydrate contains either an aldehyde (aldose) or ketone (ketose) functional group.
Nomenclature and Stereochemistry
- Combine carbon number and functional group: e.g., aldohexose, ketopentose.
- Carbons with hydrogen and hydroxyl groups are chiral centers (stereogenic centers).
- Fischer projections are used to depict monosaccharides; horizontal lines are wedge bonds, vertical are dash bonds.
- The D/L configuration is determined by the chiral center farthest from the carbonyl group: D (OH right), L (OH left).
- Most naturally occurring sugars are D isomers.
Stereoisomers of Monosaccharides
- A molecule with n chiral centers has 2ⁿ possible stereoisomers.
- Aldotriose: 2 isomers; aldotetrose: 4 isomers; aldopentose: 8 isomers; aldohexose: 16 isomers.
- Swapping two groups on a chiral center inverts configuration; 90° rotations are not allowed in Fischer, but 180° are.
Cyclization and Anomers
- Monosaccharides exist in equilibrium between linear and cyclic forms; cyclic form is preferred.
- Cyclization occurs via intramolecular hemiacetal formation (alcohol attacks aldehyde/ketone within the same molecule).
- Attack can occur from either side, generating two anomers: alpha (OH down) and beta (OH up) at the anomeric carbon.
- Haworth projections are used for cyclic sugars; conventions specify placement of anomeric carbon and CH₂OH group.
Ring Types and Mutarotation
- Six-membered cyclic sugars are called pyranoses; five-membered are furanoses.
- Monosaccharides can interconvert between alpha and beta anomers (mutarotation).
- Beta-glucose is more stable due to the equatorial position of the anomeric OH.
- Some sugars (e.g., mannose) prefer the alpha anomer due to the anomeric effect.
Key Terms & Definitions
- Carbohydrate — molecule made of carbon, hydrogen, and hydroxyl, often with energy-storage or structural roles.
- Monosaccharide — simple sugar and carbohydrate monomer.
- Aldose — carbohydrate with an aldehyde group.
- Ketose — carbohydrate with a ketone group.
- Chiral center — carbon attached to four different groups, resulting in stereoisomerism.
- Fischer projection — a flat drawing style for sugars showing stereochemistry.
- D/L configuration — orientation of the terminal chiral center: D (right), L (left).
- Anomer — isomer differing at the anomeric carbon formed during cyclization (alpha or beta).
- Pyranose — six-membered cyclic monosaccharide.
- Furanose — five-membered cyclic monosaccharide.
- Mutarotation — change in optical rotation due to equilibrium between alpha and beta anomers.
Action Items / Next Steps
- Review Fischer and Haworth projection drawing conventions.
- Be able to assign D/L and alpha/beta designations for given sugars.
- Prepare for the next lecture on polysaccharides.