Fehling's Test for Reducing Sugars

Demonstration Experiment on Video

Fehling's Test

Objectives: Test for Reducing Sugars, Keto-Enol-Tautomerism, Copper-Tartrate-Complex

Peter Keusch

German version

Chemicals:
glucose
fructose
sucrose
potassium sodium tartrate
sodium hydroxide
CuSO₄ · 5 H₂0

Fehling I consists of 7 g of hydrated copper(II) sulfate dissolved in 100 mL of dist. water.
Fehling II is made by dissolving 35 g of potassium sodium tartrate and 10 g of sodium hydroxide in 100 mL of dist. water.

Fehling's reagent: Equal volumes of Fehling I and Fehling II are mixed to form a deep blue solution.

Apparatus and glass wares:
hot plate
thermometer
3 conical measures, graduated, 500 mL
3 glass stirring rods
1 beaker 1800 mL
3 beakers 150 mL
3 beakers 40 mL
measuring cylinder 100 mL

Hazards and safety precautions:

Sodium hydroxide is very corrosive. Causes severe burns. May cause serious permanent eye damage. Very harmful by ingestion. Harmful by skin contact or by inhalation of dust.

Copper(II) sulfate is harmful when swallowed.

Safety glasses, protective gloves, good ventilation.

Experimental procedure:

5 g of glucose, fructose and sucrose, respectively, are dissolved in 100 mL of dist. water. Three 500 ml conical measures are each filled with the sugar solutions. Each solution is made up to the 500 mL mark with dist. water warmed up to 60°C. Afterwards 8 mL of Fehling reagent are poured into each of the sugar solutions while stirring.

Results:

In two conical measures a yellow-red precipitate is formed. At first in the blue solution the precipitate may appear greenish. Finally the muddy green suspension yields brick red precipitate. Fructose reacts faster than glucose. In the third glass no change can be observed.

conical measure 1	glucose solution	brick red precipitate
conical measure 2	fructose solution	brick red precipitate
conical measure 3	sucrose solution	no change

Video clip (Download RealPlayer .rm file)

Discussion:

Fehling's tests for aldehydes are used extensively in carbohydrate chemistry. A positive result is indicated by the formation of a brick red precipitate. Like other aldehydes, aldoses are easily oxidized to yield carboxylic acids. Cupric ion complexed with tartrate ion is reduced to cuprous oxide.

Fig. 1: Redox reaction
· The sucrose does not react with Fehling's reagent. Sucrose is a disaccharide of glucose and fructose. Most disaccharides are reducing sugars, sucrose is a notable exception, for it is a non-reducing sugar. The anomeric carbon of glucose is involved in the glucose-fructose bond and hence is not free to form the aldehyde in solution.

Fig. 2: Sucrose

· On the other hand, glucose, a reducing sugar, reacts with Fehling's reagent to form an orange to red precipitate. Fehling's reagent is commonly used for reducing sugars but is known to be not specific for aldehydes. For example, fructose gives a positive test with Fehling's solution too, because fructose is converted to glucose and mannose under alkaline conditions. The conversion can be explained by the keto-enol tautomerism.

equation

Fig. 3: Conversion of fructose to glucose and mannose

The reduction of Fehling solution using fructose is not only to be attributed to the fact that the ketose is isomerized into an aldose. The treatment of fructose with alkali - e.g. Fehling solution - causes even decompostion of the carbon chain. More products with reducing capability are formed.

Fig. 4: Decomposition of fructose

Note: Fehling's test takes advantage of the ready reactivity of aldehydes by using the weak oxidizing agent cupric ion (Cu²⁺) in alkaline solution. In addition to the copper ion, Fehling's solution contains tartrate ion as a complexing agent to keep the copper ion in solution. Without the tartrate ions, cupric hydroxide would precipitate from the basic solution. The tartrate ion is unable to complex cuprous ion Cu⁺, so the reduction of Cu²⁺ to Cu⁺ by reducing sugars results in the formation of an orange to red precipitate of Cu₂O.

The structure of copper(II) D-tartrate has been determined to be:

Fig. 5: Copper-tartrate-complex

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