@@she simplest sugars, called monosaccharides, are polyhydroxy aldehydes or ketones and their monomers are usually formed by five or six carbon atoms. Glucose and sucrose are the representatives of sugars. The former occurs free in fruits, plants, honey, and in the blood and urine of animals, while the latter is the common table sugar. Starch is a polysaccharide, which is stored in the seeds, roots, and fibers of plants. Compare the reduction abilities of the three kinds of sugar and also their reactions with iodine.
Sugar, Starch-iodine reaction, Reducing sugar
[Ware] Heating tools, Test tube, Beaker (100 ml), Komagome pipet
[Reagents] CuSO4₯5H2O(copper(U) sulfate pentahydrate), KNaC4H4O6(potassium sodium tartrate, "Rochelle salt"), NaOH(sodium hydroxide), 1%-Glucose solution, 1%-Sucrose solution, 1%-Starch solution, 3mol/l-H2SO4(sulfuric acid), Na2CO3(sodium carbonate anhydrous, powder), Iodine and potassium iodide solution
@T@Reduction and hydrolysis of sugars
P@Preparation of Fehling's solution/ Solution T: Add 6.9 g of CuSO4₯5H2O to enough water to make 100 ml. Solution U: Add 34.6 g of KNaC4H4O6 and 10 g of NaOH to enough water to make 100 ml. Place 5 ml of both solution T and solution U in a test tube and mix them. This is "Fehling's solution".
|Fehling's solutionF Fehling's solution, which was invented by Hermann von Fehling, is one of the testing reagents for reducing sugars. The reaction process involves the oxidizing action of complexed copper(U) ion; a red precipitate (copper(T) oxide) indicates the oxidation of an aldehyde or other easily oxidizable substances.
Q@Place 3 ml each of 1%-glucose solution, 1%-sucrose solution, and 1%-starch solution in three separate test tubes. Add 1 ml of Fehling's solution to each test tube. Heat them and observe the changes. If the change is not apparent, then heat again for a few minutes.
R@In (a) different test tube(s), place 3 ml of solution/solutions for which the change was/were observed in Procedure Q. Add 1 ml of 3mol/l-H2SO4 to this/these and heat it/them gently without boiling for about 5 minutes.
S@Add Na2CO3 powder little by little to the mixture until generation of CO2 bubbles cease. Add about 1 mlof Fehling's solution to the reaction mixture. Heat it and observe the change.
@U@Starch-iodine reaction and hydrolysis of sugars
T@Place 1 ml of 1%-glucose solution, 1%-sucrose solution, and 1%-starch solution in three separate test tubes. Add 2-3 drops of iodine and potassium iodide solution to each.
U@Place 1 ml of sample solution/solutions for which the change was/were observed in Procedure T in another test tube(s). Add 2-3 drops of iodine and potassium iodide solution and 1 ml of 3mol/l-H2SO4 to this/these. Heat the mixed solution/solutions without boiling. Observe the change.
V@Place 1 ml of 1%-starch solution in a test tube and add 2-3 drops of iodine and potassium iodide solution.
W@Heat the mixture gently without boiling. Next, cool it with water and observe the change.
Questions for students
P@Write the chemical formula of the red precipitate formed by the reaction between reducing sugars and Fehling's solution.
Q@Why do the solutions of sucrose and starch show reductionability by heating with dilute sulfuric acid?
R@Account for the reason why the changes were observed in Procedure U.
S@Starch-iodine reaction occurs when the iodine molecule is taken in the hollow part of the spiral of starch molecule. Explain the color change with the course of heating and cooling of the reactant in Procedure W.