Lab 4 - Chemical properties of carbohydrates Flashcards
What is Fehlings reaction for?
Used to distinguish reducing sugars from the non-reducing once
Monosaccharides
Aldose and ketose
What will give a positive result on the Fehlings test and why?
Monosaccharides (aldose and ketose) having a free aldehyde or ketone groups have reducing properties and will therefore give a positive result on the Fehlings reaction.
What are the reagents for Fehlings test?
- CuSO4 (Fehlings reagent I)
- An alkaline solution of potassium sodium tarte (Fehling reagent II)
What happens during the Fehlings test?
In the presence of reducing sugars, copper (II) hydroxide formed by mixing the reagents I and II, is reduced to red copper oxide (I) (Cu2O)
Benedict’s reaction
Characteristic reaction of reducing sugars
Reducing sugars cause the reduction of _____ to _____
Cu2+ to Cu+
Benedict blue colored reagent
Alkaline solution containing the complex copper(II) citric acid anions
Barfoead’s reaction
Used to distinguish monosaccharides from reducing disaccharides. Only monosaccharides are oxidized.
Disaccharides have to be hydrolysed into monosaccharides for it to get a positive reaction.
What does the Barfoed’s reagent comprise?
Copper(II) acetate in lactic acid
What happens in Barfoead’s reaction?
Cu2+ ions are reduced to Cu+ ions and the brick-red precipitate of copper(I) oxide (Cu2O) is formed.
When does the pentoses undergo dehydration?
Pentoses in concentrated hydrochloric acid undergo dehydration. (Bial’s reaction)
Bial’s reaction
Used to distinguish pentoses from hexoses
What color does the hexoses (apart from glycose) form in Bial’s reaction?
Brown complex but the reaction is not characteristic for them
What color do you get in the Bial reaction?
Green colored product
