P6. Fehling's test: reducing sugars

Introduction:

From Fehling's solution we will identify whether or not a substance is reductive, ie if you have a free OH group at carbon 1 or 4 carbon.

Objectives:

• Identify reducing sugars.
• Comprehend redox reactions.
• Understand the relation between structure and reducing ability of some sugars.

Materials:

1. Test tube rack
2. 10mL pipet
3. Distilled water
4. 5 test tubes
5. 5 spatula
6. Lactose
7. Maltose
8. Glucose
9. Sucrose
10. Starch
11. Fehling's A and B solutions
12. HCl

Procedure:

• .Take five test tubes and we put 2 mL of distilled water. In each tube add a Glucose, Maltose another in another Sucrose, Lactose and another in the last starch. Add the Fehling's solution and hope for a while. Those who have changed color are reducers.

Comments:

• Sucrose does not react with Fehling's solution because it includes a furanose and pyranose.

Conclusions:

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 test because when it's free the sacharide will have the reducing power.

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

Starch has no reducing power as it is a polysaccharide, and has a very long chain of carbons.

3-Would have you obtained a Fehling's positive test if you had hydrolyzed the sucrose? Why? 

Yes. 

4-What does "reducing sugars" mean? 

Is the one that reacts positive to the Fehling's test.