Problem 167 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the multi-step conversion of an aldose into next higher aldose by Kiliani- Fischer synthesis, the reagent emplyed in the first step is (a) HCN (b) \(\mathrm{NH}_{2} \mathrm{OH}\) (c) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NHNH}_{2}\)

Step-by-Step Solution

Answer

The reagent employed in the first step of Kiliani-Fischer synthesis is HCN.

Step 1: Understanding Kiliani-Fischer Synthesis

Kiliani-Fischer synthesis is a method used to convert an aldose sugar into the next higher aldose. It involves the chain extension of the carbohydrate by adding one carbon atom.

Step 2: Identifying Initial Steps in the Synthesis

In the Kiliani-Fischer synthesis, the initial step involves converting an aldose into a cyanohydrin. Cyanohydrins are formed when hydrogen cyanide (HCN) is added to an aldehyde.

Step 3: Recognizing the Key Reagent

Since the first step of the reaction involves the formation of a cyanohydrin, the reagent used is hydrogen cyanide (HCN). This reacts with the aldose to add a carbon atom, thereby transforming it into a cyanohydrin.

Step 4: Conclusion

The correct reagent used in the first step of the Kiliani-Fischer synthesis is therefore hydrogen cyanide, denoted by the chemical formula HCN.

Key Concepts

Aldose Conversion

The term "aldose conversion" refers to the process of chemically modifying an aldose sugar. An aldose is a type of monosaccharide that contains an aldehyde group at the end of its carbon chain. During aldose conversion, particularly within the Kiliani-Fischer synthesis, this sugar is transformed into a higher-order aldose by increasing the number of carbon atoms. To achieve this, the conversion process typically begins with the formation of a cyanohydrin. This modification is crucial because it effectively extends the carbon chain, allowing the sugar to evolve into a more complex carbohydrate. Such conversions are important in a variety of biochemical and industrial applications where creating more complex carbohydrates from simpler ones is required. Aldose conversion relies on understanding the fundamental chemistry of sugars and involves bringing about subtle changes in molecular structure, which can lead to significantly different chemical properties and biological activities.

Cyanohydrin Formation

Cyanohydrin formation is a key step in the Kiliani-Fischer synthesis. It involves the reaction between an aldehyde group of an aldose sugar and hydrogen cyanide (HCN). This reaction results in the addition of a carbon and a nitrogen atom to the sugar, forming a cyanohydrin. An important aspect of this reaction is the nucleophilic addition mechanism, where the carbon of the cyanide ion attacks the electrophilic carbonyl carbon of the aldehyde group. This mechanism effectively adds a new carbon, increasing the length of the carbohydrate chain. These cyanohydrin intermediates are crucial because they serve as the stepping stone for further transformation into higher-order sugars. The introduction of the CN group allows further modifications that can extend the structure and complexity of the carbohydrate molecule. Understanding this process is important for students as it illustrates basic principles of organic chemistry and highlights the interconnection between structure and function in molecules.

Chain Extension in Carbohydrates

Chain extension in carbohydrates is a critical process in the synthesis of more complex sugars. During the Kiliani-Fischer synthesis, this is achieved by transforming simpler monosaccharides into elongated forms with an additional carbon atom. The process begins with cyanohydrin formation, as discussed, which sets the stage for the conversion into a longer carbohydrate chain. After forming a cyanohydrin, the next steps involve hydrolysis and subsequent reductions or rearrangements which ultimately convert the cyanohydrin to an aldose with a longer carbon chain. This chain extension is significant in the world of carbohydrate chemistry because it allows chemists to access a wider variety of monosaccharides from common precursors. By understanding how to extend carbohydrate chains methodically, researchers can design pathways to synthesize complex sugars that have specific applications in biochemical processes or material science.