Carbohydrates Overview

Carbohydrates are organic compounds (C, H & O), the most abundant in nature (cellulose: 100 billion tons annually)
They are primary products of photosynthesis.
Carbohydrates are polyhydroxy aldehydes or ketones, or give either after hydrolysis.
The general formula for carbohydrates is Cn(H₂O)n.
Acetic acid (CH₃COOH) is not a carbohydrate.
Deoxy-sugars and amino-sugars aren't considered carbohydrates.
Carbohydrates can be found in bound form (polysaccharides, mucopolysaccharides, glycoproteins, proteoglycans, glycolipids, glycosides, nucleic acids) or as simple sugars.
Prefixes (tri-, tetra-, penta-) in a carbohydrate name indicates the number of carbon atoms in its chain.
Suffix (-ose) indicates the molecule is a carbohydrate.
Monosaccharides containing an aldehyde group are classified as aldoses (pentose, hexose), while those containing a ketone group are classified as ketoses.
Ketoses are indicated by the suffix –ulose (pentulose, hexulose).
Carbohydrates play a crucial role in energy production, biosynthesis of fats and proteins, roles in nucleic acids, and as components in supportive tissues (cellulose in plants, chitin in insects).
Carbohydrates are used in surgical dressings (cellulose) and treatment of ulcers (sucralfate), reconstructing cartilage (glucosamine), managing weight (chitosan), and treating diarrhea (pectin), laxatives (lactulose).

Monosaccharides react as alcohols (e.g., ester formation, ether formation, methylation), as carbonyl compounds (e.g., glycoside formation via reaction with methanol & HCl), as reducing agents (reduction to polyhydric alcohols/alditols), and via oxidation (oxidation into acids).
Reaction with Phenylhdrazine (Osazone formation) is key in identifying aldoses and ketoses, forming identifiable osazones.
Effect of acids depends on concentration and temperature. Both dilute and concentrated acids result in dehydration, and can form anhydro-sugars. Concentrated HCl produces furan derivatives from pentoses and hexoses. Concentrated sulfuric acid can produce further decomposition and degradation. Nitric acid oxidizes aldoses to dicarboxylic acids such as saccharic acids.
Effect of alkalis may also depend on its concentration, temperature, and presence of oxidizing agents.

Molisch's test: All carbohydrates produce a purple ring with a-naphthol and concentrated sulfuric acid.
Fehling's test: Monosaccharides and reducing disaccharides produce a brick-red precipitate (Cu₂O) when heated with equal parts of Fehling's solution.
Osazone test: Aldoses and ketoses react with an excess of phenylhydrazine and acetic acid to form crystalline osazones. This result determines which sugar is present, based on the formation of specific osazones. The test destroys the asymmetry at C-2, and only shows commonness in glucose, mannose, and fructose.
Resorcinol/Seliwanoff's test: Ketoses react to yield a red color with resorcinol and HCl when heated.
Phloroglucinol or Bial's Test: Pentoses react to yield a blue or red color with phloroglucinol and HCl when heated.
Keller-Kiliani test: 2-deoxy sugars react to yield a bluish-brown precipitate at the boundary between the layers.

Polysaccharides are tasteless polymers of monosaccharides.
They generally have complex structures and high molecular weight, ranging from 100 to 2000 units in length.
They are either linear or branched in structure and don't exhibit the properties of aldehydes or ketones.
They can originate from plant sources (e.g., cellulose, starch, pectin) and animal origins (e.g., glycogen).
Polysaccharides are further classified into homopolysaccharides (one type of sugar), heteropolysaccharides (more than one type of sugar), and derived carbohydrates.

Glycosides are natural products where a sugar molecule (glycone) is bonded to a non-sugar molecule (aglycone).
Common solvents in extracting glycosides are water and alcohol, requiring caution in procedures that avoid enzyme inactivation (boiling, drying at 100°C, etc) and using chemicals to remove other constituents.
They are classified by the chemical nature of the aglycone (e.g., phenol, alcohol, lactone) or by the presence of specific groups in the molecule (e.g., cyano-group, sulphur).
Glycosides are typically found in plants and can serve in energy storage or regulation, detoxification, or defenses.
Chemical properties associated with glycosides are those from the aglycone (characteristic tests). Reducing sugars will reduce Fehling's solution after hydrolysis.
Different ways of synthesizing glycosides include the involvement of phosphorylated sugar (activation by UDPG) followed by enzymatic reactions.

Isomers are molecules with the same molecular formula but different structural arrangement.
Enantiomers are non-superimposable mirror images of each other.
Diastereomers are non-superimposable non-mirror image isomers.
Epimers are diastereomers that differ in the configuration around only one chiral center.

The optical rotation of a carbohydrate solution gradually changes to a constant value when it is dissolved in water, due to the formation of different conformations.
A freshly prepared glucose solution has a specific rotation of +112.2°.
An aged glucose solution has a specific rotation of +52.7°.