Answer to Question #176380 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

Sucrose;Sucrose contains two sugars (fructose and glucose) joined by their glycosidic bond in such a way as to prevent the glucose isomerizing to aldehyde, or the fructose to α-hydroxy-ketone form. Sucrose is thus a non-reducing sugar, which does not react with Benedict's reagent.

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.