Answer to Question #176383 in General Chemistry for Pauline Ramos

Question

Which of the sugar solutions gave a negative test/positive test with Barfoed’s reagent? Why? What is the purpose of this test? How are carbohydrates differentiated by this test? Of what use is Barfoed’s reagent in identifying an unknown sugar? What is the reaction involved in this test?

Answer 1

Positive Barfoed’s test: development of brick red color ppt within 3-5 minutes

Negative Barfoed’s test: absence of red color

to detect reducing sugar

to distinguish monosaccharides from reducing disaccharides

Barfoed's test is used to detect the presence of monosaccharide (reducing) sugars in solution. Barfoed's reagent, a mixture of ethanoic (acetic) acid and copper(II) acetate, is combined with the test solution and boiled. A red copper(II) oxide precipitate is formed will indicates the presence of reducing sugar.

Barfoed's test is a chemical test used for detecting the presence of monosaccharides. It is based on the reduction of copper(II) acetate to copper(I) oxide (Cu₂O), which forms a brick-red precipitate. (Disaccharides may also react, but the reaction is much slower.)

R-CHO + 2Cu²⁺ + 2H₂O $\to$ R-COOH + Cu₂O + 4H⁺