L6. FEHLING'S TEST: REDUCING SUGARS

L6: Fehling's test: reducing sugars

1. Introduction

Fehling solution is a chemical test used to differentiate between reducing and non-reducing sugars. 

Fehling's reagent has two separate solutions: Felhing A and felhing's B.

Felhing's A: is a blue aqueous solution of copper (II) sulphate.

Felhing's B: clear and colourless solution of potassium sodium tartrate and sodium hydroxide.

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 Felhing's solution sugars can be oxidized anf felhing's mixture can obtain this electrons.

2. Objectives

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

3. Hypotesis

The hypotesis is to identify which sugars perform felhing reactor's.

4. Material

• Test tube rack
• 10mL pipet
• distilled water
• 5 test tubes
• 5 spatula

• Lactose
• Maltose
• Glucose
• Sucrose
• Starch
• Felhibg's A and B solutions
• HCl

5. Procedure

First of all, we took 5 test tubes from the previous practice and we put 2 mL of Felhin's A and B Feling's solutions and put them to warn up. After 10 minutes we took the test tubes and we observed the results.

6. Observations

We have observed that glucose, maltose, lactose Felhing's reagent changed from blue to orange. And sucurose the mixture with felhing reagent didn't change the same with starch.

7. Conlusions

Monosaccharides  and some disaccharides have reducing power. However polysaccharides like starch isn't reducing sugar.

8. Questions

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

The OH from an anomeric C.

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

If we hydrolize starch, glucose are released and they react with Felhing reagent.

3. Would have you obtained a Felhing's positive test if you had hydrolyzed the sucrose (as you have done with starch)? why?

The sucrose has no free 0H therefore not react with the reagent Felhing, like the starch.

4.  What does " reducing sugars" term mean?

The solutions that react with the reagent Felhing are monosaccharides having a free OH, such as glucose, fructose, sucrose and maltose.