Why do disaccharides and polysaccharides give positive Molisch Test?

Answer 1

simply because they are carbohydrates, all carbohydrates produces positive result in molisch's test

All carbohydrates i.e. monosacchardes, dissacharides and polysaccharides are dehydrated (lose water) by a dehydrating agent (e.g. Sulphuric acid) they form aldehydes known as furfurral. The furfurral reacts with a component known as 1- naphthol present in Molisch reagent resulting in a reddish violet colour forming between the Reagent and the previous mixture of the carbohydrate and thedehydrating agent. It is important to note that the reaction is more rapid with simple sugars or monosaccharides

Answer 2

I think it is beter to discuss the principle of this test first.

Any monosaccharide can answer this test (gives positive result).

This happens through dehydration by sulfuric acid (H2so4); this dehydration results in production of furfural or one of its derivatives depending on the type of monosaccharide whether it's hexose ( gives 5-hydroxymethylfurfural) or pentose (gives furfural). Then this furfural reactes with 2 molecules of phenol (usually α-naphthol, though other phenols (e.g. resorcinol, thymol). The product of this reaction has red or purple colored (the positive ).

Now regarding your question if the positive means the presence of carbs (sugars). The answer is that it can be a sugar or [furfural (not a sugar or carbs)]. Actually I am not sure if there is another compund also gives positive result.

N.B. Disaccharide and polysaccharides must be hydrolyzed using HCl to produce monosaccharides before giving the result. If you get any negative result with Molisch, you can proceed to hydrolysis step then redo the test. if it still negative the substance you test is not a sugar. On the other hand, if it turns to positIve, that means it was either disaccharide or polysaccharide.

To conclude this, I can tell you that the negative result exclude the presence of the carbohydrates but positive can't give any evidence that the substance give the result is carbohydrate. In other words, a negative result by this reaction is a very good evidence of the absence of carbohydrates, but a positive test is indication to the probable presence of carbohydrate.

I think further tests must be done.

Answer 3

yes, with very few exceptions a positive Molisch's test confirms the presence of carbohydrates.