Question

1.) An unknown carbohydrate solution tested positive with molisch reagent, formed a red precipitate with barfoed's reagent within 3 minutes, and turned blue-green when mixed with bials reagent. what carbohydrate is this?

2.) Erythrose is a four-carbon aldose. Describe the results of testing it with the following reagents.

a.) Molisch

b.) Barfoed's

c.) Benedict's

d.) Seliwanoff's

Answer 1

Solution:

1)

When a monosaccharide reacts with a Molisch reagent, it rapidly reacts with the Molisch reagent. Disaccharides and polysaccharides reacts slowly with Molisch reagent. Also, the reagent forms furfural and derivates of furfural with pentoses and hexoses respectively.

When a monosaccharide reacts with a Barfoed's reagent, the aldehyde of the monosaccharide converts to the carboxylic acid oxidized by the Cu²⁺ ions in the solution. Formation of Cuprous Oxide gives us the red precipitate in the solution.

When a monosaccharide reacts with Bials reagent, the reagent dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion present in the test reagent to produce a bluish product. This reaction is specific for pentoses.

Hence the carbohydrate present here is a monosaccharide and a pentose.

2)

As it is shown earlier , Molisch's test is rapid in case of Monosaccharides, but they have to be pentoses and or hexoses, whereas Barfoed's test is specific for Monosaccharides only. Disaccharides undergo oxidation through the same steps but a lot slower. Since, erythrose is an aldose, therefore it will only undergo the Barfoed's test, since it has an aldehyde group only.

Now, Benedict's reagent is a chemical reagent commonly used to detect presence of reducing sugars, however other reducing substances also give a positive reaction. This includes all monosaccharides and many disaccharides, including lactose and maltose. It specifically searches for the aldehyde and alpha hydroxy ketone group to oxidize into the carboxylic acid group. Erythrose will undergo Benedict's test since it has aldehyde group giving us a brick red colored solution.

Seliwanoff's test tells us the distinction between an aldose and a ketose. This test relies on the principle that, when heated, ketoses are more rapidly dehydrated than aldoses. When added to a solution containing ketoses, a red color is formed rapidly indicating a positive test. When added to a solution containing aldoses, a slower forming light pink is observed instead. Therefore Erythrose will give us a pink colored solution.