Monosaccharides Reactions Notes

Monosaccharides

Monosaccharides contain alcohols and either aldehydes or ketones. These functional groups undergo reactions such as oxidation, reduction, esterification, and nucleophilic attack, creating glycosides.

Oxidation and Reduction

• Oxidation of carbohydrates is a crucial biochemical reaction for energy production in the human body.
• Monosaccharides switch between anomeric configurations, briefly existing in an open-chain aldehyde form.
• Aldehydes can be oxidized to carboxylic acids, forming aldonic acids.
• Aldoses are considered reducing agents because they can be oxidized. Any monosaccharide with a hemiacetal ring is a reducing sugar.
• Oxidation of an aldose in ring form yields a lactone, a cyclic ester with a carbonyl group on the anomeric carbon.
• Lactones, like vitamin C, are essential in the human body.
• Tollen's reagent and Benedict's reagent are used to detect reducing sugars.
  • Tollen's reagent:
    • Must be freshly prepared, starting with silver nitrate $AgNO_3$.
    • $AgNO<em>3$ is mixed with NaOH to produce silver oxide $(Ag</em>2O)$.
    • Silver oxide is dissolved in ammonia to produce the Tollen's reagent $(Ag(NH<em>3)</em>2)^+$.
    • Tollen's reagent is reduced to produce a silvery mirror when aldehydes are present.
  • Benedict's reagent:
    • The aldehyde group of an aldose is readily oxidized.
    • Indicated by a red precipitate of $(Cu_2O)$.
• Glucose oxidase can be used to test specifically for glucose, as it does not react with other reducing sugars.
• Strong oxidizing agents like dilute nitric acid oxidize both the aldehyde and the primary alcohol on C6 to carboxylic acids.
• Ketose sugars are also reducing sugars and give positive Tollen's and Benedict's tests.
  • Ketones can tautomerize to form aldoses under basic conditions via keto-enol shifts.
  • In the aldose form, they react with Tollen's or Benedict's reagents to form carboxylic acids.
• Tautomerization involves rearrangement of bonds, typically by moving a hydrogen and forming a double bond.
  • The ketone group picks up a hydrogen, and the double bond moves between two adjacent carbons, resulting in an enol (a compound with a double bond and an alcohol group).
• Reduced sugars play an essential role in human biochemistry.
  • When the aldehyde group of an aldose is reduced to an alcohol, the compound is an alditol.
  • A deoxy sugar contains a hydrogen that replaces a hydroxyl group on the sugar.
    • Example: D-2-deoxyribose, found in DNA.

Esterification

• Carbohydrates have hydroxyl groups, enabling them to react with carboxylic acids and derivatives to form esters.
• Esterification is similar to the phosphorylation of glucose, where a phosphate ester is formed.
• Phosphorylation of glucose is a crucial metabolic reaction in glycolysis.
  • A phosphate group is transferred from ATP to glucose, phosphorylating glucose and forming ADP.
  • Hexokinase (or glucokinase in the liver and pancreatic beta islet cells) catalyzes this reaction.

Glycoside Formation

• Hemiacetals react with alcohols to form acetals.
• The anomeric hydroxyl group is transformed into an alkoxy group, yielding a mixture of alpha and beta acetals, with water as a leaving group.
• The resulting carbon-oxygen (C-O) bonds are called glycosidic bonds, and the acetals formed are glycosides.
• Example: reaction of glucose with ethanol.
• Equivalent reactions happen with hemiketals, forming ketals.
• Disaccharides and polysaccharides form via glycosidic bonds between monosaccharides.
• Glycosides derived from furanose rings are furanosides, and those from pyranose rings are pyranosides.
• Glycoside formation is a dehydration reaction; breaking a glycosidic bond requires hydrolysis.