Projection Experiment: Test for Sugars with Molisch Test

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Test for Sugars with Molisch Test

Peter Keusch

German version

Chemicals:

glucose

sucrose

ethanol 96%

1-naphthol

conc. sulfuric acid

Test solution: 2 g 1-naphthol dissolved in 20 mL EtOH 96% (MOLISCH REAGENT)

Hazards and safety precautions:

Ethanol is highly flammable.

1-Naphthol: Eye, skin and respiratory irritant. Causing liver damage.

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Safety glasses, gloves and good ventilation required.

Experimental procedure:

1 mL of glucose and sucrose solution, respectively, are pipetted into a test tube (T1 and T2, respectively). 3 drops of MOLISCH REAGENT are added to each of the sugar solutions. After mixing, 1 mL of concentrated sulfuric acid is added to each of the two test tubes. Using a Pasteur pipette sulfuric acid is carefully added down the inner sides of each tube, without mixing, to form a bottom layer.

Results:

A positive test for carbohydrate is indicated by a purple ring forming at the interface between the denser sulfuric acid and the less dense test solution above.

Photo

Discussion:

· In the presence of conc. H₂SO₄ sugars are dehydrated forming furfuryl derivates. Furfural is derived from the dehydration of pentoses and pentosans, while 5-hydroxymethyl furfural is produced from hexoses and hexosans (1).

Hydroxymethylfurfural is attacked by 1-naphthol, whose hydroxyl group has a strong electron donating resonance effect which increases the electron density in the benzene ring. 1-naphthol acts as a nucleophile and is added to the positive polarized carbon of the aldehyde group of 5-hydroxymethyl furfural. After an intramolecular proton migration water is eliminated and a C-C double bond is formed (2).

Now a further nucleophilic addition of 1-naphthol molecule takes place. After an intramolecular proton migration two protons are eliminated leading to a purple furfuryl dipheylmethane dye (3).

Although this test will detect compounds other than carbohydrates (i.e. glycoproteins), a negative result indicates the absence of carbohydrates.

General experimental instructions and index of experiments