Question

Which of the following hexoses will form the same osazone when treated with excess phenyl hydrazine?

• A. D-glucose, D-fructose and D-galactose
• B. D-glucose, D-fructose and D-mannose
• C. D-glucose, D-mannose and D-galactose
• D. D-fructose, D-mannose and D-galactose

Hint:

To identify which sugars form the same osazone, check for isomeric sugars that possess the same functional groups, particularly the aldehyde group or similar reactivity.

Answer 1

The Correct Option is B

Hexoses are monosaccharides containing six carbon atoms. The reaction of hexoses with phenyl hydrazine to form osazones is a classic method for differentiating sugars. Osazones are formed when the sugars react with excess phenyl hydrazine at the carbonyl group(s), typically the first two carbon atoms (C-1 and C-2) in the hexose structure.

The key to determining which sugars will form the same osazone lies in their structural similarity around the first two carbon atoms:

• D-glucose: At C-1 and C-2, D-glucose has the configuration HO-CH₂-CHOH-CHOH-.
• D-fructose: As a ketose, it shares the C-2 configuration with D-glucose after rearranging. It has HO-CH₂-CO-CHOH-.
• D-mannose: It has a C-2 epimer of D-glucose with the same configuration: HO-CH₂-CHOH-CHOH-.

Both D-glucose and D-mannose have the same configuration at C-1 and C-2 as D-fructose once it is converted to an enediol form, allowing them to form the same osazone.

Sugar	C-1, C-2 Configuration
D-glucose	HO-CH2-CHOH-CHOH-
D-fructose	HO-CH2-CO-CHOH-
D-mannose	HO-CH2-CHOH-CHOH-

Therefore, D-glucose, D-fructose, and D-mannose will form the same osazone when treated with excess phenyl hydrazine. The correct answer is: D-glucose, D-fructose and D-mannose.

Answer 2

When excess phenyl hydrazine is used on sugars, they form osazone derivatives, which are important for distinguishing different carbohydrates. In the case of osazone formation, the reaction involves only the first two carbon atoms (C1 and C2) of the sugar.

The three sugars that form the same osazone are D-glucose, D-fructose, and D-mannose. Here's why:

1. D-glucose: The first two carbon atoms are involved when reacted with phenyl hydrazine, forming an intermediate product, which eventually leads to the osazone structure.
2. D-fructose: Interconversion happens where fructose changes its position of functional groups at C1 and C2, being analogous to glucose, eventually forming the same osazone structure.
3. D-mannose: Similar to glucose, when treated with phenyl hydrazine, mannose interchanges into a structure that involves the same first two carbon atoms (C1 and C2) as in glucose. These three sugars share the same configuration at the C3, C4, and C5 carbon atoms, leading to the same osazone structure.

The osazone formed by these sugars is identical due to the structural similarity of the first two carbon atoms after the reaction process.

Therefore, the correct option is D-glucose, D-fructose, and D-mannose.