Carbohydrate Classification
High-Yield Summary
- Carbohydrates are classified 4 ways: number of sugar units (mono-/di-/polysaccharide), carbon count (triose→hexose), functional group (aldose vs. ketose), and stereochemistry.
- Aldose: carbonyl is a terminal aldehyde. Ketose: carbonyl is an internal ketone. Glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose) are the simplest examples.
- 4 sugars to know cold: D-glucose (aldohexose), D-fructose (ketohexose, ketone at C2), D-galactose (epimer of glucose at C4), D-mannose (epimer of glucose at C2).
- Chiral center = carbon bonded to 4 different groups; possible stereoisomers = 2ⁿ (n = number of chiral centers).
- Enantiomers differ at every chiral center (mirror images); diastereomers differ at some but not all; epimers are diastereomers differing at exactly 1 center.
- D/L system: look at the chiral carbon farthest from the carbonyl in a Fischer projection — OH right = D, OH left = L. Most sugars in the body are D-sugars (most amino acids are L).
Key Terms
- Monosaccharide / disaccharide / polysaccharide
- 1 / 2 / many linked sugar units.
- Aldose
- Monosaccharide with the carbonyl as a terminal aldehyde.
- Ketose
- Monosaccharide with the carbonyl as an internal ketone.
- Chiral center
- A carbon bonded to 4 different groups.
- Stereoisomers
- Same molecular formula and connectivity, different 3D spatial arrangement.
- Fischer projection
- Flat drawing convention: vertical bonds point into the page, horizontal bonds point out of the page.
Enantiomers vs. Diastereomers vs. Epimers
| Type | Definition / Example |
|---|---|
| Enantiomer | Non-superimposable mirror images, differ at ALL chiral centers (D-glucose & L-glucose) |
| Diastereomer | Not mirror images, differ at some but not all chiral centers |
| Epimer | Diastereomer differing at exactly 1 chiral center (glucose & galactose at C4; glucose & mannose at C2) |
Assigning D vs. L (Fischer Projection)
- 1Draw the sugar as a Fischer projection (carbonyl near the top).
- 2Find the chiral carbon farthest from the carbonyl group (for aldohexoses like glucose, this is C5).
- 3Check which way its OH points: right → D-sugar; left → L-sugar.
- 4To find the enantiomer, flip every chiral center (not just the reference one).
Common MCAT Trap
- Epimers differ at exactly ONE chiral center — don't confuse with enantiomers (differ at ALL centers) or diastereomers generally (some, not all).
- The D/L label is decided ONLY by the chiral carbon farthest from the carbonyl — not by every stereocenter, and not the same rule as R/S (which uses CIP priority, not position).
- Galactose and mannose are both epimers of glucose, but NOT epimers of each other — they differ at 2 carbons (C2 and C4) relative to one another, making them diastereomers, not epimers.
Quick Recall
What distinguishes an aldose from a ketose?
How many chiral centers give 8 possible stereoisomers?
Are glucose and galactose enantiomers, diastereomers, or epimers?
In a Fischer projection, which direction does D-sugar's reference OH point?