Problem 173 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

When glucose is treated with an excess of \(\mathrm{HIO}_{4}\), the products formed are (a) one mole of oxalic acid (COOHCOOH), three moles of \(\mathrm{HCOOH}\) and one mole of \(\mathrm{HCHO}\) (b) five moles of \(\mathrm{HCOOH}\) and one mole of \(\mathrm{HCHO}\) (c) one mole of glyoxal (CHOCHO), one mole of HCHO and three moles of \(\mathrm{HCOOH}\) (d) six moles of \(\mathrm{HCOOH}\)

Step-by-Step Solution

Answer

Option (d): Six moles of formic acid ( \(\mathrm{HCOOH}\)).

Step 1: Understand the Reaction

When glucose is treated with excess periodic acid (\(\mathrm{HIO}_{4}\)), it undergoes oxidative cleavage at each glycol group (\(-CHOH-CHOH-\)) to form aldehydes or acids. This reaction specifically targets carbon–carbon bonds between glycolic groups.

Step 2: Identify Glycolic Cleavage

In glucose, there are several hydroxyl groups where oxidative cleavage can occur. Each glycol between two adjacent hydroxyl groups that are bonded to different carbon atoms will be cleaved by periodic acid.

Step 3: Determine Outcome for Each Cleavage

Each cleavage between glycols results in the formation of two carbonyl compounds. Typically, a glycol cleaves to form formic acid (\(\mathrm{HCOOH}\)) when the carbon is primary or secondary.

Step 4: Calculate Products from Glucose Cleavage

Glucose has 5 hydroxyl groups forming 4 cleavable glycol pairs. Each can produce formic acid for a terminal and internal carbon pair, while a primary aldehyde is formed from any terminal group. Summing all side reactions, we yield six moles of \(\mathrm{HCOOH}\) from these sequences.

Step 5: Validate Options

The reaction results examined show six moles of formic acid. Comparing this with the given options, option (d) states that the products are six moles of \(\mathrm{HCOOH}\). This matches our calculated outcomes.

Key Concepts

Glucose Oxidation

Glucose oxidation involves the transformation of glucose molecules by introducing oxygen or removing hydrogen, effectively changing its structure and breaking it down into smaller components. In organic chemistry, this process typically uses an oxidizing agent such as periodic acid (HIO). This approach is essential for analyzing carbohydrate structure and understanding the behavior of sugars under oxidative conditions.

When excess periodic acid is added to glucose, the reaction specifically targets carbon-carbon bonds between glycol groups, or adjacent hydroxyl groups. The process results in the breaking of these bonds, leading to the formation of smaller carbonyl compounds like aldehydes and acids. This oxidation is crucial for studying the molecular breakdown, as it allows chemists to discern the finer details of structural components in complex carbohydrates.

Periodic Acid Reaction

The periodic acid reaction is a well-known chemical test in organic chemistry used to study the breakdown of sugars and other compounds containing vicinal diols. A periodic acid molecule (HIO) enables the oxidation of these diols, resulting in cleaved products.

• Periodic acid effectively disrupts the carbon-carbon linkage between the hydroxyl groups.
• The reaction is selective towards glycol groups, ensuring precise cleavage where needed.
• This method is instrumental in determining the presence and position of diol groups in complex molecules.

By applying periodic acid reaction, students can predict the cleavage products of sugars, understanding the roles and relationships of hydroxyl group arrangements in defining sugar properties and reactions.

Organic Chemistry Problem

Organic chemistry problems dealing with oxidative cleavage often present a challenge to students. These problems require an understanding of the molecular structure and an ability to apply known reactions to deduce products. The use of periodic acid with glucose exemplifies this challenge but provides a clear pathway for learning.

In an exercise involving glucose and excess HIO, one must carefully identify each cleavable glycol group within the molecule. This involves:

• Analyzing the glucose structure and determining each cleavable pair.
• Predicting the resulting products from the oxidative cleavage at these sites.
• Verifying these products against the exercise’s options to derive the correct answer.

Through understanding the strategies behind these organic chemistry problems, learners can gain confidence in their problem-solving skills, leading to more accurate outcomes in their analysis.

Glycol Cleavage Mechanism

The glycol cleavage mechanism describes how specific hydroxyl pairs in glucose are oxidized by periodic acid to produce characteristic products such as formic acid (HCOOH) and aldehydes. Understanding this mechanism requires students to visualize the systematic steps leading from an intact glucose molecule to its oxidatively cleaved products.

Key features of this mechanism include:

• Periodic acid targeting the vicinal diols, which destabilizes the bond between adjacent hydroxyls.
• Formation of intermediate structures, leading to subsequent breakdown into smaller carbonyl compounds.
• Clarity in how each glycol pair's cleavage contributes to the final product makeup, such as multiple formic acid molecules.

Through careful study of the glycol cleavage mechanism, students can better grasp the chemical basis for sugar transformations, paving the way for further exploration in carbohydrate chemistry and related fields.