L5. Fehling's test reducing sugars

Introduction:

Fehling's solution is a chemical test used to different between reducing and non-reducing sugars. This test is based on the reaction of a functional group of sugar molecules with Fehling's reagent.

Fehling's A: is a blue aqueous solution of copper (II) sulphate. 
Fehling's B: clear and colourless solution of potassium sodium tartrate and sodium hydroxide.

When a sugar has reducing ability, the mixture turns from deep blue colour to green colour suspension with a red precipitate. Some sugars are capable of reducing copper II ions to copper I ions. This reducing ability is useful in classifying sugars. When the sugar to be tested is added to the Fehling's solution and the mixture is heated, some sugars can be oxidized (to lose electrons) and the Fehling's mixture can obtain this electrons (reduced)

Material: 

- Test tube rack
- 10 ml pipet
- Distilled water
- 5 test tubes
- 5 spatula
- Lactose
- Maltose
- Glucose
- Sucrose
- Starch
- Fehling's A and B
- HCl

Procedure: 

In this experiment you will first determine with sugars give a positive test Fehling's reagent and then, by testing the reaction of some organic molecules containing onlya single functional group, you should be able to deduce which functional group of sugar is reacting with Fehling's reagent:

1. Take 5 test tubes and label: G, M, S, L, ST.
2. Put 2 mL of distilled water inside each tuve.
3. With different spatulas put a small amount of each sugar. Dissolve the sugar.
4. Add 2mL of Fehling's A solution and then Fehling's B.
5. Place each test-tuve in a boiling water bath (250mLbeaker on a hotplate stirrer).
6. Observe what is happening.

Starch Hydrolysis:
Hydrolysis is the reaction of a compound with water. As you know, starch is a polymer, consisting of many units of α-D-glucose covalently linked together.

1. Place 2mL of 1% starch in a test tuve and add 0.5mL of 3M HCl. Mix and place this mixture in a boiling water bath for 10 minutes.
2. After 10 minutes, remove the tuve from the water bath and let it cool. Neutralize this solution with 1M NaOH and mix well.
3. Transfer 8-10 drops of this solution to a small test tuve.
4. Add 1mL of Fehling's A solution and 1mL of Fehling's B.
5. Heat for a few minutes in a boiling water bath.
6. Record your observations. Compare the results of this test with your results for unhydrolyzed starch in the step 1 of this experiment.
7. You can test the absence of starch with iodine solution too.       

Questions:

1-From your observations and the structures of the sugars given above, indicate which functional group in the sugar molecules reacts with Fehling's reagent.

 

The OH group is the one that reacts with Fehling's reagent because when it is free the sacharide will have the reducing power.

If the bond is monocarbonilic the sacharide will have reducing power, that's why all monosacharides have it (the OH from the C1 is always free).

 

2-Compare the results you obtained for the Fehling's test of starch and Fehling's test of hydrolyzed starch. Explain your results.

I haven't done the Fehling's test of hydrolized starch but i can deduce it:

In the Fehling's experiment the starch doesn't have a reducing power because the OH is not free but when the starch is hydrolyzed it turns into glucose and glucose has a free OH because it's a monosacharide. Also the last glucose of the starch chain will have a reducing power (the ones that are in the ends).

The starch components are alfa D glucoses: amylose alfa (1 -> 4) linear chain, and amylopectine alfa (1->6) ramifications.

3- Would have you obyined a Fehling's positive test if you had hydrolyzed the sucrose (as you have done with starch9? Why?  

If we hydrolize the sucrose, we break the o-glycosidic bond. The two monosaccharides that formed sucrose (glucose and fructose) would react with Fehling reagent and then the reaction will be positive becuase monosaccharides have reducing power.

4-  What does "reducing sugars" term mean?

A reducing sugar is the one that reacts positive to the Fehling's test. This means that they are capable of reducing coper II ions to copeer I ions. When the sugar to be tested is added to the Fehling's solution and the mixture is heated, some sugars can be oxidized (to lose electrons) and the Fehling's mixture can obtain the electrons (reduced).