Question

Discuss the chemical reactions that occurred between glucose, fructose, xylose and sucrose and the 4 reagents(Benedict, Bial, Seilwanoff and Molisch) which have resulted in color changes. Describe how it occurs in full using the chemical makeup of the reagents.

Answer 1

	Bial’s reagent Test for pentoses.components include orcinol,HCl,ferric chloride.Pentoses react to form furfural ,which gives colored substance.pentoses gives blue/green Positive for pentose sugar -xylose	Benedict’s -Free aldehyde or keto group in reducing sugars reduce cupric hydroxide in alkaline medium to red colored cuprous oxide.All monosaccarides are reducing .	Seilwanoff Color reaction specific for ketoses. Ketoses+conc HCl undergoes dehydration to yield 5-hydroxymethylfurfural derivatives that form deep red color complex with resorcinol. Faster rxn than aldoses	Molisch All pentose sugar and hexoses are dehydrated by conc. Sulphuric acid to form furfural or hydroxymethylfurfural, respectively. These products condense with α-naphthol to form purple colored condensation product.
glucose	Negative -hexose	positive	negative	positive
fructose	Negative -hexose	positive	positive	positive
xylose	positive	Positive	negative	positive
sucrose	Negative-dimer of hexose	Negative-disaccharides	positive	positive