BackCyclic Monosaccharides and Haworth Projections
Study Guide - Smart Notes
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Cyclic Monosaccharides
Introduction to Cyclic Structures
Monosaccharides, the simplest carbohydrates, can cyclize to form ring structures. The most stable rings are typically five-membered (furanose) and six-membered (pyranose) rings.
Furanose: Monosaccharide with a five-membered ring.
Pyranose: Monosaccharide with a six-membered ring.
These cyclic forms are prevalent in biological systems due to their stability.
Formation of Cyclic Structures
Monosaccharides cyclize when a hydroxyl group reacts with the carbonyl group (aldehyde or ketone) within the same molecule, forming a hemiacetal (from an aldehyde) or hemiketal (from a ketone).
This reaction creates a new chiral center at the anomeric carbon, leading to two possible isomers: α (alpha) and β (beta) anomers.
Example: D-glucose cyclizes to form α-D-glucopyranose and β-D-glucopyranose.
Haworth Projections
Haworth projections are a common way to represent the cyclic structures of monosaccharides. They provide a simplified, planar view of the ring, making it easier to visualize the orientation of substituents.
Thicker lines indicate the part of the ring closest to the viewer.
Substituents above or below the ring correspond to their positions in the Fischer projection.
Haworth projections are not true representations of the three-dimensional structure, as rings are not planar in reality.
Fischer Projection | Haworth Projection |
|---|---|
Linear structure, vertical carbon chain | Cyclic, planar ring with substituents above/below |
"Right" on Fischer = "Down" on Haworth | "Left" on Fischer = "Up" on Haworth |
Converting Fischer to Haworth Projections
Groups on the right in a Fischer projection point down in a Haworth projection.
Groups on the left in a Fischer projection point up in a Haworth projection.
Example: Converting D-glucose from Fischer to Haworth projection:
"Up-Lefting" and "Down-Righting" rules apply.
Assigning Numbers to the Carbons of Cyclic Monosaccharides
Numbering starts at the anomeric carbon (the carbon derived from the carbonyl group during cyclization).
The anomeric carbon is adjacent to the ring oxygen.
Other carbons are numbered sequentially around the ring.
Example: Numbering the carbons in a cyclic glucose ring: 1 (anomeric), 2, 3, 4, 5, 6 (outside the ring as CH2OH).
Practice Problems
Identify the correct carbon numbering in cyclic sugars.
Determine the correct Haworth projection for a given monosaccharide.
Key Terms and Definitions
Anomeric Carbon: The carbon derived from the carbonyl group during cyclization; it becomes a new stereocenter.
Hemiacetal/Hemiketal: Functional groups formed when an alcohol reacts with an aldehyde or ketone, respectively.
α-Anomer: The OH group on the anomeric carbon is trans (opposite side) to the CH2OH group.
β-Anomer: The OH group on the anomeric carbon is cis (same side) to the CH2OH group.
Relevant Equations
General cyclization reaction for an aldose (hemiacetal formation):
General cyclization reaction for a ketose (hemiketal formation):
Summary Table: Pyranose vs. Furanose Rings
Type | Number of Atoms in Ring | Example |
|---|---|---|
Pyranose | 6 | Glucose |
Furanose | 5 | Fructose |
Additional info: In biological systems, the pyranose form of glucose is the most common due to its stability. Haworth projections are a simplified way to represent these rings, but the actual molecules adopt chair or envelope conformations in three dimensions.
