Problem 169 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

When methyl D-glucopyranoside is oxidised with periodic acid, how many moles of the oxidising agent are consumed per mole of the sugar? (a) 2 (b) 3 (c) 4 (d) 5

Step-by-Step Solution

Answer

The sugar consumes 3 moles of periodic acid per mole.

Step 1: Understanding the Structure

Methyl D-glucopyranoside is a hexose sugar derivative, specifically a pyranose form (six-membered glucose ring) with a methyl ether link at the anomeric carbon, C1.

Step 2: Identifying Oxidizable Bonds

Periodic acid cleaves vicinal diol groups, oxidizing them to form aldehydes and give off iodic acid. In the pyranose form of glucose, vicinal diols are present at C2-C3 and C4-C5 carbons, and also the hemiacetal oxygen forms a vicinal diol with C6's OH.

Step 3: Predicting Cleavage Sites

Since methyl D-glucopyranoside has two primary vicinal diol linkages (at C2-C3, C4-C5) and an additional diol-like structure involving the ring oxygen and C6 adjacent OH, we predict three cleavage sites.

Step 4: Calculating the Moles of Oxidation

Each cleavage by periodic acid uses one mole of oxidizing agent. With three cleavage sites in methyl D-glucopyranoside, three moles of periodic acid would be consumed per mole of the sugar.

Key Concepts

Oxidation Reactions

In organic chemistry, oxidation reactions involve the loss of electrons from an organic molecule and a subsequent increase in oxidation state. For sugars like methyl D-glucopyranoside, oxidation commonly implies the transformation of certain functional groups within the molecule.
In the case of periodic acid () oxidation, the reagent specifically targets vicinal diol groups. These are pairs of hydroxyl groups attached to adjacent carbons.
This reagent cleaves such diol groups to form aldehydes, effectively breaking carbon-carbon bonds. Periodic acid thus serves as a powerful oxidant that can deconstruct sugar molecules by attacking key diol sites.

• The reaction typically consumes a mole of periodic acid for each diol group cleaved.
• The concept of vicinal diols is crucial, as these structures are requisite substrates for oxidation via periodic acid.

Understanding oxidation reactions, particularly in the context of sugar molecules, helps elucidate how these molecules can be modified to yield smaller fragments.

Carbohydrate Chemistry

Carbohydrate chemistry focuses on the structure and reactions of monosaccharides like glucose, disaccharides, and larger polysaccharides. Methyl D-glucopyranoside, being a derivative of glucose, is structured as a ring known as a pyranose. Its ring form involves a specific arrangement where carbon atoms are linked in a six-membered formation with characteristic hydroxyl groups.
This compound features a methyl group, distinctive to its structure. It is attached to the anomeric carbon (C1), replacing the typical hydrogen in hydroxyl form, forming a methyl ether. This specific structure is central during oxidation reactions, especially when discerning the potential for oxidation.

• The presence of vicinal diols, typically located due to hydroxyl positions in the glucose ring, is important for determining how the molecule interacts with oxidizing agents like periodic acid.
• Understanding the carbohydrate chemistry structure allows chemists to predict where cleavages and reactions will occur.

Recognizing these structural nuances in carbohydrate chemistry is vital for leveraging oxidation reactions to dissect these molecules.

Organic Reaction Mechanisms

Organic reaction mechanisms deal with the detailed step-by-step process by which chemical reactions occur. With periodic acid oxidation, the mechanism involves the identification of vicinal diol groups which enable cleavage by the oxidizing agent.
The initial step is the reaction of periodic acid with vicinal diols in methyl D-glucopyranoside, leading to the cleavage of carbon-carbon bonds and the formation of aldehydes. During this step-by-step mechanism, the reagent not only cleaves but also promotes the formation of intermediate structures that form smaller fragments.

• Mechanisms explain the vital role of specific chemical bonds or groups in predetermined reactions.
• For methyl D-glucopyranoside, understanding the reaction mechanism aids in anticipating the formation of new products post-reaction.

Grasping these mechanisms provides insights into how complex organic molecules are transformed under various chemical conditions.