Problem 4 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Two aldopentoses \(\mathrm{X}\) and \(\mathrm{Y}\) give the same osazone derivative. \(\mathrm{X}\) is oxidised to an optically active aldaric acid by dilute nitric acid. Ruff degradation of \(Y\) gave a tetrose which was similarly oxidised to an optically active aldaric acid. Assign the structures of \(X\) and \(Y\) from the following list.

Step-by-Step Solution

Answer

The structures of X and Y are D-lyxose and D-arabinose, respectively.

Step 1: Identify osazone formation property

Osazone derivatives are formed by reactions with both the second and third carbon atoms of the sugar. Therefore, sugars that form the same osazone must have the same configuration at C-2 and C-3, but they may differ at C-4 and C-5.

Step 2: Oxidation to aldaric acids

The conversion to an optically active aldaric acid via oxidation with dilute nitric acid means the sugar is not symmetrical about the C-2 carbon (optically inactive sugars produce a meso-compound). Thus, the sugar must have the same configuration at C-4 and C-5. This rules out ribose, which would lead to an inactive acid.

Step 3: Consider Ruff degradation

Ruff degradation reduces the sugar's chain by one carbon, which affects C-1 and C-2, leaving C-3 as the new starting aldehyde carbon. The resulting tetrose that is oxidized to an optically active aldaric acid confirms the original sugar must have a non-symmetrical structure at C-3 and C-4 as well.

Step 4: Assign structures based on logic

Since X, when oxidized, forms an optically active aldaric acid, it must be D-lyxose (which has no mirror symmetry and would not lead to a meso compound). After Ruff degradation, the resulting tetrose (threose) also forms an active aldaric acid, confirming the structure. Similarly, Y must form the same osazone as X and give the same tetrose after degradation. Hence, Y is likely D-arabinose, fitting these criteria without identical optical inactivity.

Key Concepts

Aldopentoses

Aldopentoses are a type of monosaccharide, specifically sugars containing five carbon atoms and an aldehyde group. The term "aldo" refers to the presence of the aldehyde group, which is at the first position of the sugar carbon chain.
In the exercise, two specific aldopentoses, X and Y, are given which create the same osazone derivative. This indicates that these sugars possess the same stereochemistry at the second and third positions of their carbon chain but can differ at other positions.

• Aldopentoses are categorized based on their optical activity, usually noted as D or L forms.
• These optically active isomers will have a significant influence on their chemical reactions and properties.

In practical applications, understanding the specific arrangement of atoms in aldopentoses is crucial for predicting their behaviour in reactions like osazone formation.

Osazone Formation

Osazone formation is a classic chemical reaction utilized in carbohydrate chemistry to differentiate between sugars. When sugars react with phenylhydrazine, they form osazones.
This reaction is mainly between the carbonyl group of the sugar and the phenylhydrazine.
The distinctive feature of osazone formation is its targeting of the second and third carbon atoms.

• Osazone derivatives result when the first and second carbons react with phenylhydrazine, leading to crystals of characteristic shape.
• Since aldopentoses X and Y form the same osazone, this confirms that they share the same configuration at C-2 and C-3.

Recognizing the osazone shape helps in determining the identity of unknown sugars, as different sugars have unique osazone crystal forms.

Ruff Degradation

Ruff degradation is a chemical reaction method that shortens sugars by one carbon. This process involves oxidation and decarboxylation steps, effectively removing the first carbon atom from the sugar molecule.
In the given problem, Y undergoes Ruff degradation to form a tetrose, a four-carbon sugar.

• This transformation specifically affects the first and second carbons, leading to a new sugar where the previous C-3 becomes the new starting aldehyde carbon.
• The process is crucial when identifying sugar structure because it helps to determine the original arrangement of the sugar's atoms by assessing the oxidation results.

The exercise example shows how the formation of an optically active aldaric acid from this new tetrose confirms the existing sugar structure, ensuring it's not symmetrical.

Aldaric Acid Oxidation

Aldaric acid oxidation is a chemical reaction wherein aldoses are fully oxidized to form aldaric acids using agents like dilute nitric acid. This oxidation affects both the aldehyde group at the first carbon and the primary alcohol group at the last carbon.
A significant aspect of this reaction is determining whether the resulting aldaric acid is optically active or inactive (meso).

• An optically active compound indicates asymmetry; therefore, the sugar X forming an active aldaric acid suggests it isn't symmetrical over C-2, meaning it has distinct configurations at C4 and C5.
• Oxidation reactions also help to exclude sugars like ribose, which result in symmetrical, optically inactive products.

Understanding aldaric acid oxidation is essential for assigning specific structures to sugars, as it provides insight into the sugar's configuration and any symmetrical properties.