Problem 89 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

An aldose is converted into its next higher homologue by (a) Amadori rearrangement (b) Wohl's method (c) Killiani's reaction (d) Mutarotation

Step-by-Step Solution

Answer

The correct answer is (c) Killiani's reaction.

Step 1: Understand the Question

We need to determine which method is used to convert an aldose into its next higher homologue. A 'higher homologue' refers to a compound in which the carbon chain length is increased by one.

Step 2: Identify Methods

Let's look at each method: - (a) **Amadori rearrangement** is generally associated with the conversion of an aldose to a ketose, not for chain lengthening. - (b) **Wohl's Method** involves the reduction of sugars to a lower homologue, not a higher one. - (c) **Killiani's Reaction** (or **Killiani-Fischer Synthesis**) is known for chain lengthening by one carbon atom in an aldose. - (d) **Mutarotation** involves the change in optical rotation due to the conversion between different anomers of a sugar, not for increasing chain length.

Step 3: Conclusion

The method that results in the conversion of an aldose to its next higher homologue is Killiani's reaction (Killiani-Fischer Synthesis).

Step 4: Verify the Process

The Killiani reaction involves the addition of hydrogen cyanide (HCN) to an aldose to form a cyanohydrin, followed by hydrolysis and reduction steps. This process increases the carbon chain by one and converts the aldose into its next higher homologue.

Key Concepts

Killiani-Fischer Synthesis

The Killiani-Fischer Synthesis is a classic method in organic chemistry used to extend the carbon chain of aldoses by one carbon atom. This process is particularly important for understanding how sugar structures can be manipulated and varied in size. The synthesis is named after the chemists who developed it, Heinrich Kiliani and Hermann Emil Fischer. They paved the way for more comprehensive studies of carbohydrates.

This synthesis begins with the formation of a cyanohydrin using aldose and hydrogen cyanide (HCN). The cyanohydrin then undergoes hydrolysis, which converts the nitrile group into a carboxylic acid. Finally, the carboxylic acid is reduced to form the aldehyde of the next higher aldose.​ By using this sequence, we effectively add one carbon atom to the sugar molecule. The technique is essential in carbohydrate chemistry, allowing for the synthesis of rare sugars and the exploration of structural variations of these vital biomolecules.

Aldose

Aldoses are a type of monosaccharide, or simple sugar, that contains an aldehyde group. They are crucial components in numerous biological processes and serve as building blocks for more complex carbohydrates. Aldoses such as glucose play a fundamental role in energy metabolism.

In an aldose, the aldehyde group is located at the end of the carbon chain, and all other carbons are typically connected to hydroxyl groups. This structure makes aldoses reactive, especially in processes such as oxidation and reduction. The structural versatility of aldoses also means they can be transformed into other sugar forms, like ketoses, which is an essential concept in carbohydrate chemistry. Understanding aldoses and their reactions is central to mastering organic chemistry, especially when studying energy cycles and synthesis pathways in cells.

Chain Lengthening

Chain lengthening is a chemical process that increases the number of carbon atoms in a molecule, specifically in the carbon backbone of a sugar. This process is significant in the synthesis of larger, more complex carbohydrates from simpler sugars, allowing chemists to explore a variety of carbohydrates not naturally found in nature.

In the context of aldoses, chain lengthening typically involves the use of the Killiani-Fischer Synthesis. By adding another carbon atom, the aldose transforms into its next higher homologue. This transformation opens up the potential to study different sugar configurations and properties.

With chain lengthening, researchers can synthesize new sugars that might have unique biological roles or properties, advancing areas like drug development and understanding of metabolic pathways.

Cyanohydrin Formation

Cyanohydrin formation is a key step in the Killiani-Fischer Synthesis. It involves the reaction of an aldose with hydrogen cyanide (HCN), resulting in the formation of a cyanohydrin. This reaction is crucial because the cyanohydrin serves as an intermediate that allows the extension of the carbon chain in aldoses.

This process starts with the addition of HCN across the carbonyl group of the aldose. The newly formed cyanohydrin possesses both a hydroxyl and a nitrile group. This dual functionality enables further chemical reactions, such as hydrolysis, to convert the nitrile group into a carboxylic acid, eventually leading to chain extension.

Cyanohydrin formation is not exclusive to carbohydrates. It is a widely used reaction in organic chemistry for constructing carbon-carbon bonds, making it an essential tool in synthesis strategies for complex molecules.