Re: Why do all sugars give a positive result in molisch's test\

Molisch's test detects the presence of carbohydrates and glycoproteins by exploiting the chemistry of the aldehyde group that all carbohydrates have in common.

The test consists of treating a sample with sulfuric acid, which dehydrates the sugars to hydroxymethylfurfural (HMF) (pentoses, like ribose or deoxyribose, are dehydrated to furfural). As with the sugars, these molecules are aldehydes, and when 1-napthol is added, each aldehyde (either HMF or furfural) reacts with two 1-napthol molecules to form a purple compound, which is the positive indicator of the presence of carbohydrate.

While hexose monosaccharides have the formula C₆H₁₂O₆, HMF has the forumla C₆H₆O₃, so you can see that 3 H₂O (water) have been lost. Similarly, while pentoses have the formula C₅H₁₀O₅, furfurl has the forumla C₅H₄O₂, and you can again see that 3 water have been lost.

After the dehydration, the aldehyde group present in the sugars remains unchanged, and it is this that reacts with the 1-napthol.

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