Benedict's Test: Reducing Sugars Detection

Benedict's Test

• Used to detect reducing sugars in biological fluids
• Originally called Fehling's test

Reducing Sugars

• Comprise all saccharides with free or potentially free aldehyde or ketone groups
• These groups are present in the anomeric carbon of individual sugars
• All monosaccharides have free aldehyde/ketone groups, making them reducing sugars
• Examples of reducing sugars include aldoses, ketoses, and monosaccharides with free reducible groups (CHO or C=O) and adjacent free hydroxyl groups

Isomerization (Lobry de Bruyn-Van Ekstein Reaction)

• Occurs when sugars are treated with dilute alkaline solutions
• Aldoses and ketoses isomerise to form the same endiol in dilute alkali
• Example: D-Glucose, D-mannose, and D-fructose form the same endiol in dilute alkali

Reaction with Benedicts Reagent

• Reducing sugars react with Cu2+ (cupric ion) in Benedicts Reagent to form Cu+ (cuprous ion)
• Cu+ is less soluble and forms a yellow/red solid precipitate (Cuprous oxide, Cu2O)
• The reducing sugar is oxidized to the corresponding carboxylic acid

Disaccharides

• Consist of monosaccharides joined covalently by O-glycosidic bonds
• Example: Maltose, lactose, and sucrose
• Maltose is a reducing disaccharide because the anomeric carbon of one glucose molecule is available for oxidation

Polysaccharides

• Examples: Starch, glycogen, and cellulose
• The end of the chain with a single free anomeric carbon is called the reducing end of the chain

Sucrose

• A disaccharide of glucose and fructose
• Contains no free anomeric carbon atom
• Not a reducing sugar and has no reducing end
• Abbreviated name: Glc a(1→2) Fru or Fru β(2→1) Glc
• Major intermediate product of photosynthesis
• Principle form of sugar transport in plants

Limitations of Benedict's Test

• Has a sensitivity limit of 0.1% to 0.2% sugar
• May be affected by urates and excess ammonia and ammonium salts