Problem 1 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following statements is correct? (a) The Ruff procedure lengthens an aldose chain and gives a single product. (b) (c) The Kiliani-Fischer procedure shortens an aldose chain and gives a single product. (d) The Kiliani-Fischer procedure lengthens an aldose chain and gives two epimers.

Step-by-Step Solution

Answer

Statement (d) is correct: The Kiliani-Fischer procedure lengthens an aldose chain and gives two epimers.

Step 1: Analyze the Ruff procedure

The Ruff procedure is a chemical reaction used for shortening the carbon chain of aldoses. This is opposite to lengthening, as it involves the removal of the terminal carbon as carbon dioxide. Therefore, statement (a) is incorrect since it suggests that it lengthens the chain and gives a single product.

Step 2: Analyze the Kiliani-Fischer procedure

The Kiliani-Fischer procedure is used to lengthen the carbon chain of aldoses. It involves the conversion of an aldose into a mixture of two epimeric cyanohydrins, which are then reductively hydrolyzed into two different aldoses. Thus, it lengthens the chain and results in two epimeric products, making statement (d) correct.

Step 3: Evaluate given statements

By reviewing the tasks performed by each procedure, we can confirm that the Ruff procedure shortens a chain, not lengthens it, and leads to one product, contradicting statement (a). The Kiliani-Fischer method indeed lengthens a chain and results in two epimers, validating statement (d). Statement (c) is incorrect as it mentions a shortening of the chain and a single product.

Key Concepts

Ruff procedure

The Ruff procedure is an essential concept in the realm of organic chemistry, particularly when dealing with aldose chain manipulation. Unlike what one might expect, this procedure does not lengthen but instead shortens the aldose chain. This is a significant distinction because it involves the removal of the terminal carbon atom.
Upon engaging in the Ruff procedure, a specific chemical reaction known as the oxidation occurs. This results in the formation of an aldonic acid. Following this, a further reaction causes the loss of carbon dioxide (a process referred to as decarboxylation), and thereby the aldose chain becomes one carbon shorter.
Here's a breakdown of the Ruff procedure benefits:

• Effective for converting higher aldoses into their lower counterparts.
• Widely used in synthetic routes for studying carbohydrates.

It's a fascinating example of how specific reactions in organic chemistry can completely transform a compound's structure.

Kiliani-Fischer procedure

The Kiliani-Fischer procedure plays a pivotal role in extending the length of an aldose chain. This process is used to synthesize longer carbohydrate molecules by adding carbon to form a new chiral center, thereby resulting in a chain extension.
Typically, this procedure starts with converting an aldose into an aldonic lactone with the use of hydrogen cyanide. This step produces cyanohydrins, a crucial intermediate in this reaction. The subsequent reaction converts these cyanohydrins into an aldose with an additional carbon atom. However, it is important to note one intriguing aspect of this process – it does not simply generate one product. Instead, it produces two diastereomers (epimers), which are aldoses differing in configuration at the newly formed chiral center.
This procedure is revered for its capabilities:

• Allows for the synthesis of sugars that were previously challenging to produce.
• Favors the creation of complex carbohydrates with potentially diverse functions.

In essence, the Kiliani-Fischer procedure serves as a remarkable tool in the toolkit of chemists managing carbohydrate transformations.

Aldose chain manipulation

Aldose chain manipulation refers to the diverse range of methods employed to alter the carbon chain length in aldoses. These techniques are instrumental in the study and synthetic replication of carbohydrates, which are fundamental biomolecules in organic chemistry.
The procedures such as the Ruff and Kiliani-Fischer methods exemplify this manipulation. Each procedure's distinctive reaction mechanism highlights the versatility in developing new configurations of aldoses.
Key manipulation techniques include:

• The Ruff procedure for aldose chain shortening.
• The Kiliani-Fischer procedure for aldose chain lengthening.

Such manipulations are crucial for:

• The synthesis of rare sugars that cannot be extracted from natural sources.
• The exploration of stereochemistry in carbohydrates, which can be pivotal for understanding their biological functions.

By undergoing these transformations, aldoses can be adapted for various research applications, showcasing their vast potential and importance in scientific developments.