Aldehydes & Ketones Questions

Experiment 5: Carbohydrate Qualitative tests
Benedict's Test:
All reducing sugars give positive Benedict's Test
Principle: reducing sugars have free aldehyde or ketone group, it under go converting in
to enediol forms under hot alkaline condition. The ened

Organic Chemistry

Aldehydes & Ketones

Experiment 5: Carbohydrate Qualitative tests Benedict's Test: All reducing sugars give positive Benedict's Test Principle: reducing sugars have free aldehyde or ketone group, it under go converting in to enediol forms under hot alkaline condition. The enediol are strong reducing agents converting cupric ions (Cu+2) of the Benedict's solution into Cuprous ion which altimately oxide as a red precipitate red copper(1) oxide by aldehydes. Benedict's reagent contains blue copper(II) sulfate (CuSO4). 5H20 which is reduced to red copper(I) oxide by aldehydes, thus oxidizing the aldehydes to carboxylic acids. The copper oxide is insoluble in water and so precipitates. The color of the final solution ranges from green to brick red depending on how many of the copper (II) ions are present. The test is used to distinguish between monosaccharides and reduced disaccharides This reaction will detect reducing monosaccharides in the presence of disaccharides. Reagent uses copper ions to detect reducing sugars in an acidic solution. Barfoed's reagent is copper acetate in dilute acetic acid (pH 4.6) Reducing monosaccharides are oxidized by the copper ions in a weak acidic medium to form a carboxylic acid and a reddish ppt of Cu20 (cuprous oxide). Reducing disaccharides (lactose but not sucrose) undergo the same reaction but at slower rate. Therefore, boiling time is critical and positive test within ( 2min.) indicate the monosaccharides in case of disaccharides they are first converted into monosaccharide and then colored precipitate are formed. So the porolonged boiling may hydrolyses disaccharides to give a false positive test. Method: 1 ml of the solution to be tested + 2 ml of freshly prepared Barfoed's reagent. . . Place test tubes into a boiling water bath and heat for 2 minutes. Remove the tubes from the bath and allow to cool. Formation of a green, red, or yellow precipitate is a positive test for reducing monosaccharides. Do not heat the tubes longer than 3 minutes, as a positive test can be obtained with disaccharides if they are heated long enough.

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