Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following \begin{tabular}{ll} \hline Column-I & Column-II \\ \hline (a) Glucose & (p) Reduces Fehling's \\ solution \\ (b) Fructose & (q) Exhibits mutarotation \\ in mild alkaline \\ medium. \\ (c) Mannose & (r) Produces tetra acetate \\ derivative on treatment \\ with acetic anhydride \\ and pyridine. \\ (d) A-methyl & (s) Get oxidized by \(\mathrm{Br}_{2} /\) \\ glucopyranoside & \(\mathrm{H}_{2} \mathrm{O}\) \\ & (t) With \(\mathrm{NaBH}_{4}\) gives mai- \\ tol and sorbitol \end{tabular}

Step-by-Step Solution

Answer

(a) matches with (p), (b) with (q), (c) with (r), and (d) with (s).

Step 1: Understand the Functional Groups

Identify the functional groups present in glucose, fructose, mannose, and α-methyl glucopyranoside. This will help determine which properties they exhibit in reaction with specific reagents in Column-II.

Step 2: Analyze Each Pair

Start analyzing each pair in Column-I with the options in Column-II based on their chemical properties and the reactions they undergo.

Step 3: Match Glucose

Glucose is an aldohexose and a reducing sugar, which means it can reduce compounds such as Fehling's solution. Therefore, (a) matches with (p).

Step 4: Match Fructose

Fructose is a ketohexose, but it also exhibits some reducing properties due to tautomerization, participating in mutarotation. Thus, (b) matches with (q).

Step 5: Match Mannose

Mannose is an aldohexose like glucose and can also exhibit reducing properties, so it undergoes acetic anhydride reaction to form tetra acetate. Therefore, (c) matches with (r).

Step 6: Match α-methyl Glucopyranoside

α-methyl glucopyranoside is a non-reducing sugar derivative, known to be oxidized by Br₂/H₂O without tautomerization or formation of mutarotation. Thus, (d) matches with (s).

Key Concepts

Glucose Properties

Glucose is one of the most well-known monosaccharides and is a fundamental carbohydrate in biology. It is an aldohexose, which means that glucose is a hexose (a six-carbon sugar) containing an aldehyde group at its first carbon. This specific structure allows glucose to participate in various chemical reactions.

One important property of glucose is its ability to act as a reducing sugar. A reducing sugar contains a free aldehyde group or a free ketone group that can donate electrons to other molecules, thus reducing them. In the context of the exercise, glucose's capability to reduce Fehling's solution highlights this property.

Fehling's solution is used to differentiate between aldehyde and ketone groups and is also utilized to assess the presence of reducing sugars. Glucose reduces blue Cu²⁺ ions in Fehling's solution to a red-brown precipitate of Cu₂O, demonstrating its reducing nature.

Further, glucose can undergo mutarotation, a process involving the change in rotation of polarized light as it settles into an equilibrium mixture of its alpha and beta forms in solution. This occurs due to the ring opening and closing, allowing for interconversion between different anomeric forms.

Fructose Reactions

Fructose is another essential simple sugar found in many plants. As a ketohexose, it has a ketone group at its second carbon. While fructose contains a ketone group, it still exhibits some reducing properties.

The reducing features stem from a reaction mechanism known as tautomerization, where fructose converts into an enol form, eventually allowing it to behave like an aldehyde. This conversion is crucial because it facilitates fructose to engage in reactions typical for reducing sugars.

Furthermore, fructose can engage in mutarotation in mildly alkaline mediums. Mutarotation in fructose refers to the change in optical rotation that occurs when it transitions between different ring forms. This reaction may not be visible in all sugar types, specifically those that do not form tautomers efficiently.

These unique properties of fructose allow it to be a versatile participant in chemical reactions, enhancing our understanding of carbohydrate chemistry.

Mannose Derivatives

Mannose, much like glucose, is an aldohexose, sharing the six-carbon structure with an aldehyde group. However, slight variations in its structure impart unique properties and reaction tendencies.

Mannose is known to form derivatives when reacting with specific reagents. One such notable reaction is its conversion to a mannose tetraacetate derivative. This transformation occurs when mannose is treated with acetic anhydride in the presence of a base such as pyridine.

In this reaction, each of the hydroxyl (–OH) groups on mannose is acetylated, forming an acetate functional group, resulting in tetra acetated mannose. This elaborates on how mannose's structure allows it to partake in such specific derivatization reactions.

By exploring mannose and its derivatives, we gain insight into the versatility and adaptability of sugars in modifying their structure for different functional and chemical properties.

α-methyl Glucopyranoside Oxidation

α-methyl glucopyranoside is an interesting derivative of glucose. It is formed when the hydroxyl group on the first carbon of glucose is replaced with a methoxy (-OCH₃) group.

This transformation makes α-methyl glucopyranoside a non-reducing sugar. Being non-reducing means it lacks the free aldehyde group typical in reducing sugars, preventing it from undergoing the same reactions, such as tautomerization or mutarotation.

However, α-methyl glucopyranoside does undergo oxidation reactions, specifically with bromine water (Br₂/H₂O). In this reaction, the sugar can be oxidized without any change in its stereochemistry or configuration, reflecting its stability and resistance to mutarotation.

This reaction is significant because it emphasizes the chemical behavior of sugar derivatives and how minor changes in a molecule's structure can lead to distinct chemical properties.