Problem 3 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which statement about the pyranose form of mannose is not correct? (a) it exists as two anomeric stereoisomers. (b) it reacts with Tollen's reagent to give a silver mirror (i.e., it is a reducing sugars). (c) reaction with excess \(\mathrm{CH}_{3} \mathrm{I}\) and \(\mathrm{AgOH}\) gives a non-reducing penta-O-methyl derivative. (d) it resists reduction with aqueous sodium borohydride.

Step-by-Step Solution

Answer

Statement (d) is not correct; mannose does not resist reduction with NaBH_4.

Step 1: Understanding the Pyranose Form

Mannose is an aldohexose sugar and can form a stable six-membered ring known as a pyranose form, which is common for sugars like glucose and mannose.

Step 2: Analyzing Anomeric Isomers

When mannose forms a pyranose ring, the anomeric carbon (C1) can have two configurations (α and β), thus existing as two anomeric stereoisomers. Therefore, statement (a) is correct.

Step 3: Reacting with Tollen's Reagent

As a reducing sugar, mannose can undergo oxidation-reduction reactions. It can react with Tollen's reagent, causing the reagent to be reduced to silver, forming a silver mirror. Thus, statement (b) is also correct.

Step 4: Formation of Penta-O-Methyl Derivative

When mannose reacts with excess methyl iodide ( CH_3I ) and AgOH , all hydroxyl (OH) groups are methylated, including the hemiacetal hydroxyl at the anomeric carbon, converting it into a non-reducing sugar. This makes statement (c) true.

Step 5: Resistance to Reduction by NaBH₄

Aldoses like mannose can be reduced by sodium borohydride ( NaBH_4 ) to form corresponding alditols, such as mannitol. Thus, mannose does not resist reduction by NaBH_4 , making statement (d) incorrect.

Key Concepts

Anomeric Stereoisomers

Mannose, like other sugars, can form a structure known as the pyranose ring. This ring formation involves the conversion of the linear form of mannose into a cyclic form. This change creates a special kind of carbon atom called the anomeric carbon, which is typically the first carbon in the sugar chain. The anomeric carbon is unique because it can exist as one of two stereoisomers - these are known as the alpha (α) and beta (β) anomers.

These anomeric forms are termed as stereoisomers because they have the same molecular formula and follow the same connectivity of atoms, but differ in the spatial arrangement around the anomeric carbon. This difference in arrangement results in different physical properties and reactivities in the two isomers. In the case of mannose, when it forms a pyranose ring, the stereochemistry of the anomeric carbon becomes a point of variability, giving rise to two distinct compounds that are mirror images of each other: anomeric stereoisomers.

• The α form has the hydroxyl group at the anomeric carbon on the opposite side of the ring's CH₂OH group.
• The β form has the hydroxyl group on the same side as the CH₂OH group.

This variety and flexibility in structure play a vital role in the biological functions and reactions that sugars can partake in.

Reducing Sugars

A sugar is labeled as a reducing sugar if it can donate electrons to another molecule. This ability to reduce other compounds comes from the free aldehyde or ketone group in its open-chain form. Mannose, when present as a pyranose, retains the ability to temporarily open into a linear form showing a free aldehyde group.

This free aldehyde is crucial because it allows mannose to participate in oxidation-reduction reactions. For example, when mannose reacts with Tollen's reagent, the aldehyde group is oxidized while the Tollen's reagent itself is reduced. This reaction is characterized by the appearance of a silver mirror on the surface of the reaction vessel.

• The silver mirror test is a classic indicator of reducing sugars.
• It shows that such sugars can act as reducing agents, donating electrons to certain reagents.

Reducing sugars, like mannose, are essential in various chemical reactions and are pivotal in biochemical pathways such as glycolysis and the oxidative phase of pentose phosphate pathway.

Penta-O-Methyl Derivative

Through methylation, sugars like mannose can be converted into derivatives that possess different chemical properties than their original forms. When mannose undergoes a reaction with excess methyl iodide ( CH_3I ) in the presence of silver oxide ( AgOH ), all of its hydroxyl groups become replaced with methoxy groups.

This transformation results in a compound called the penta-O-methyl derivative. One of the key impacts of this conversion is that the sugar loses its reducing capability. This is because the hydroxyl group on the anomeric carbon, critical for opening the pyranose ring into an aldehyde form, is replaced. Therefore, the sugar derivative cannot participate in typical redox reactions that require a free aldehyde group.

• The penta-O-methyl derivative represents a non-reducing form of mannose.
• This conversion has practical applications, as such derivatives exhibit increased stability and altered solubility.

Derivatization of sugars is an important tool in carbohydrate chemistry as it allows researchers to modify the properties of sugars for various analytical and industrial purposes.