Problem 117 - A Complete Resource Book in Chemistry for JEE Main

Question

Glucose and fructose give the same osazone. One may, therefore conclude that (a) the structures of glucose and fructose have mirror image relationship (b) glucose and fructose have identical structures (c) the structures of glucose and fructose differ only in those carbon atoms which take part in osazone formation (d) glucose and fructose are anomers

Step-by-Step Solution

Answer

(c) the structures of glucose and fructose differ only in carbon atoms not involved in osazone formation.

Step 1: Understand the osazone reaction

When sugars react with phenylhydrazine, they form osazones. The reaction specifically involves the carbonyl group (C=O) present at the first two carbon atoms of the sugar molecule. Because of this, the rest of the carbon chain does not affect the osazone structure.

Step 2: Identify involved carbons in sugars

In monosaccharides, like glucose and fructose, the C1 and C2 carbons are involved in osazone formation. Glucose is an aldohexose with the carbonyl group at C1, while fructose is a ketohexose with the carbonyl group at C2.

Step 3: Analyze the consequence on sugar's structure

Since osazone formation only involves the first two carbons, the structural differences in the remaining carbons of glucose and fructose do not affect the type of osazone formed. This means that even if the sugars have differences elsewhere, they can still form identical osazones.

Step 4: Determine the correct conclusion

Based on the osazone formation, the correct conclusion is that glucose and fructose differ only in the arrangement of atoms that are not involved in osazone formation, specifically different arrangements at carbon atoms apart from C1 and C2.

Key Concepts

Glucose and fructose comparison

When exploring sugars like glucose and fructose, understanding their similarities and differences is key. Both glucose and fructose are simple sugars or monosaccharides. They share some similarities, but also have notable distinctions in their molecular structures.

Glucose is an aldohexose, which means it contains an aldehyde group (-CHO) at one end of its linear form. Fructose, on the other hand, is a ketohexose with a ketone group (C=O) located typically at the second carbon when in its linear form.

Despite these differences, glucose and fructose can form identical osazones, which are crystalline compounds formed during a reaction with phenylhydrazine. This odd occurrence happens because the reaction only involves the first two carbon atoms. Therefore, the differences in their later carbon structures do not affect this process.

• Glucose: Aldohexose with an aldehyde group at C1.
• Fructose: Ketohexose with a ketone group at C2.
• Both form the same osazone.

The osazone reaction relies only on similarities at specific points, proving that glucose and fructose are closely related, mirror-image isomers.

Carbonyl group in sugars

In sugars, the carbonyl group plays a crucial role in their biochemical behavior and reactions. The carbonyl group is composed of a carbon atom double-bonded to an oxygen atom (C=O). This functional group is present in both aldehydes and ketones.

In glucose, the carbonyl group appears as an aldehyde at the first carbon, forming what is known as an aldohexose. Meanwhile, in fructose, the carbonyl group is a ketone located at the second carbon, classifying it as a ketohexose.

This group is pivotal in reactions like osazone formation. Osazone formation occurs as this group reacts with phenylhydrazine, particularly involving the carbons where these groups reside. This does not change the overall sugar type, but it shows how reactions can target specific molecular sites while leaving other parts unchanged. These specific locations help determine how the sugar reacts with other compounds and what kinds of derivatives form.

• Carbonyl group is crucial in sugar reactivity.
• Glucose: carbonyl as an aldehyde (C=O at C1).
• Fructose: carbonyl as a ketone (C=O at C2).
• Key in osazone reaction involving these carbons.

Understanding carbonyl's role clarifies why glucose and fructose can produce the same osazone, despite other structural differences.

Monosaccharides structure

Monosaccharides are the simplest form of carbohydrates, and their structure is foundational in understanding their functions and reactions. These sugars are defined by their small size and comprise a single unit or molecule.

They are composed of carbon, hydrogen, and oxygen atoms. A typical monosaccharide will follow the general formula \( (CH_2O)_n \) where \( n \) can range between 3 and 7. In these structures, the presence of a carbonyl group dictates whether the sugar is an aldose or a ketose. An "aldose" will have the carbonyl group at the end of the carbon chain, as seen in glucose. A "ketose" will have it within the chain, like fructose.

Each monosaccharide has a specific molecular arrangement, which determines how it interacts with other molecules, including water solubility and sweetness. Monosaccharides can exist in either a straight-chain form or a ring structure. This ability to morph between forms is essential during metabolism and other chemical reactions they participate in.

• Single unit sugars with formula \( (CH_2O)_n \).
• Aldoses: Carbonyl group at the end (e.g., glucose).
• Ketoses: Carbonyl group within the chain (e.g., fructose).
• Ability to exist in linear and ring forms.

Understanding the basic structure of monosaccharides allows us to grasp why they are so versatile and crucial in biological systems.