Carbohydrates II: Monosaccharides Reactions

Study Notes

Monosaccharides with five or more carbon atoms predominantly exist in cyclic form.
Hemiacetals and hemiketals are produced from reactions between aldehydes or ketones with alcohols.
Inter- and intramolecular reactions can form hemiacetals or hemiketals in sugars, requiring sufficient carbon distance for stable ring formation.

Cyclization occurs through interactions between functional groups on distant carbons, such as C1 and C5.
Cyclization involving C2 and C5 leads to hemiketal formation.
The carbonyl carbon during cyclization becomes a new chiral center known as an anomeric carbon.
D-glucose has two anomeric forms: α-D-glucose (hydroxyl groups on opposite sides) and β-D-glucose (hydroxyl groups on the same side).
Cyclic hemiacetals in solution display mutarotation, a change in optical rotation due to the interconversion of anomers.

Anomers are cyclic monosaccharides that differ only at the anomeric carbon.
The α-form has the -OH of C1 and CH2OH of C5 on opposite sides, while the β-form has them on the same side.

In Fischer projections, the position of hydroxyl (-OH) groups determines the Haworth projection orientation.
If an -OH is right in Fischer, it points down in Haworth; left points up.

Aldoses with five or more carbon atoms create equilibrium between alpha, beta, and open-chain forms.
Monosaccharides may exhibit different characteristics based on their structure and functional groups.

Five key reactions include oxidation to acidic sugars, reduction to sugar alcohols, phosphate ester formation, amino sugar formation, and glycoside formation.
These reactions are relevant to glucose and similarly structured aldoses and ketoses.

The primary alcohol at C-6 of a hexose can undergo enzyme-catalyzed oxidation to form a uronic acid.
D-glucuronic acid, derived from glucose, detoxifies phenols and alcohols in the liver and is a component of acidic polysaccharides in connective tissues.
Weak oxidizing agents convert aldehyde groups to aldonic acids.

Reduction of the carbonyl group in monosaccharides yields alditols, or sugar alcohols.
Sorbitol, a common alditol, acts as a sweetener and moisturizing agent; it is found in various fruits and used medically for diabetics.

Reducing sugars can reduce silver or copper ions and include sugars with an aldehyde, hydroxyketone, or cyclic hemiacetal groups.
Tests for reducing sugars (Benedict’s, Fehling’s, Barfoed’s) facilitate identification based on precipitation outcomes.
The Barfoed test differentiates reducing monosaccharides from disaccharides.

Monosaccharides can form phosphate esters by reacting hydroxyl groups with inorganic oxyacids, essential for carbohydrate metabolism.

Replacement of a monosaccharide hydroxyl group with an amino group leads to amino sugars, significant in polysaccharide structures like chitin and hyaluronic acid.

Glycosides form when cyclic monosaccharides react with alcohols, converting a hemiacetal to an acetal.
Examples include glucosides from glucose and galactosides from galactose.

Glycoproteins consist of carbohydrate residues attached to proteins, playing critical roles in biological functions like antibody formation.
Human blood groups (A, B, AB, O) are defined by the oligosaccharide component on glycoproteins on erythrocyte surfaces. The terminal monosaccharide determines blood group type.