BackLecture #31 Carbohydrates: Structure, Classification, and Nomenclature
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Carbohydrates
Monosaccharide Structure and Classification
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the empirical formula Cn(H2O)n. The simplest carbohydrates are monosaccharides, which can be classified by the number of carbon atoms and the type of carbonyl group present.
Aldose: Monosaccharide with an aldehyde group at C1.
Ketose: Monosaccharide with a ketone group, usually at C2.
Hexose: Six-carbon sugar (e.g., glucose, fructose).
Pentose: Five-carbon sugar (e.g., ribose).
Example: Glucose is an aldohexose (six carbons, aldehyde group); fructose is a ketohexose (six carbons, ketone group).
Ring Forms: Furanose and Pyranose
Monosaccharides can cyclize to form ring structures. The two main types are:
Pyranose: Six-membered ring (five carbons and one oxygen).
Furanose: Five-membered ring (four carbons and one oxygen).
Formation of these rings occurs via intramolecular reaction between the carbonyl group and a hydroxyl group.
Example: Glucose commonly forms a pyranose ring, while ribose can form a furanose ring.
Nomenclature and Stereochemistry
The suffix -ose indicates a sugar. The prefixes aldo- and keto- specify the type of carbonyl group. The configuration (D or L) is determined by the orientation of the hydroxyl group on the chiral carbon furthest from the carbonyl group.
D-Form: Hydroxyl group on the right in Fischer projection.
L-Form: Hydroxyl group on the left in Fischer projection.
Example: D-glucose vs. L-glucose.
Open-Chain and Cyclic Forms
Monosaccharides exist in equilibrium between open-chain and cyclic forms. The open-chain form contains the carbonyl group, while the cyclic form results from hemiacetal (aldose) or hemiketal (ketose) formation.
Aldose: Aldehyde present in open-chain form.
Ketose: Ketone present in open-chain form.
Example: Glucose in solution is mostly in the cyclic pyranose form, with a small fraction in the open-chain form.
Summary Table: Classification of Monosaccharides
Type | Number of Carbons | Functional Group | Common Example |
|---|---|---|---|
Aldopentose | 5 | Aldehyde | Ribose |
Aldohexose | 6 | Aldehyde | Glucose |
Ketopentose | 5 | Ketone | Ribulose |
Ketohexose | 6 | Ketone | Fructose |
Key Equations
General formula for monosaccharides:
Hemiacetal formation (aldose cyclization):
Hemiketal formation (ketose cyclization):
Additional info:
The D/L system is based on the configuration of glyceraldehyde, the simplest chiral carbohydrate.
Pyranose forms are generally more stable than furanose forms for hexoses.
