Answer to Question #140971 in General Chemistry for Kim Ann

Question

Sugar solution. ( Molisch Test )( Bial's Test)
5% starch,5% galactose,5% fructose,5% lactose, 5% arabinose,5% glucose,5% maltose,5% sucrose,
water
a.Which of the sugar solution gave a negative/positive test with Orcinol regeant? Why? What is the purpose of this test? How are carbohydrates differentiated from this test? What is the reaction involved in this test?
b. Which of the sugar solutions gave a negative test/ positive test with Molisch regeant? Why? What is the purpose of this test? Why do many proteins give a positive test with Molisch regeant? What is the reaction involved in this test?

Answer 1

a)

Orcinol regeant determines the presence of pentoses.

Arabinose gives positive test on it. All others not.

This test is used to determine the presence of pentose in urine.

All sugars differenciate as pentoses (five C atoms) and all others.

In the presence of pentoses, the test reagent dehydrates pentoses to form furfural. Furfural, then, reacts with orcinol. The iron ion in the test reagent will produce a bluish product.

b)

Molisch reagent determines the presence of carbohydrates. All sugars from the list give positive test on Molisch reagent. Because they are all carbohydrates (consist of C, H, O nomatter mono-, di-, or poly- saccharides).

Molisch test is an indicator of the presence of carbohydrates in a given solution. Some proteins like glycoprotein give positive results to Molisch's test because they make a complex with carbohydrates.

In Molisch’s test, the carbohydrate undergoes dehydration upon the introduction of concentrated hydrochloric or sulphuric acid, resulting in the formation of an aldehyde. This aldehyde undergoes condensation along with two phenol-type molecules (such as ∝-naphthol, resorcinol, and thymol), resulting in the formation of a purple or reddish-purple coloured complex.