| Demonstration of Microscale Projection Experiments -
Chemistry en miniature Peter Keusch |
Hazards and safety precautions:
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 (see foto below). 
|  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 
|
