What is Molisch's test?

Conc. Sulphuric acid dehydrates sugars to furfuryl derivates. Furfural is derived from the dehydration of pentoses and pentosans, while hydroxymethyl furfural is produced from hexoses and hexosans. α-Naphthol reacts with the cyclic aldehydes(furfural) to form purple-coloured condensation products (furfuryl-diphenyl-methane-dyes). Although this test will be positive for compounds other than carbohydrates (i.e. glycoproteins), a negative result indicates the absence of carbohydrates. A red-violet layer at the interface between the acid (bottom) and aqueous (upper) layers is a positive test for carbohydrates. This test is called Molisch’s test and it can be considered as a test for the presence of pentoses and hexoses.

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