Question

Which of the following statement(s) is/are correct? (a) The smallest aldose to form a cyclic hemi- acetal must have 4 carbon atoms. (b) D- glucose and D- fructose can be differentiated using Benedict's solution (c) When D- galactose is oxidised by \(\mathrm{HNO}_{3}\), it gives a meso isomer. (d) Aldoses react with Fehling's solution and \(\mathrm{PhNHNH}_{2}\) but not with \(\mathrm{NaHSO}_{3}\).

Short answer

Statements (a), (c), and (d) are correct.

Step-by-step solution

Analyze Statement (a)

An aldose is a monosaccharide that contains an aldehyde group. To form a cyclic hemi-acetal, the molecule must have at least 4 carbon atoms. This is because the formation of a cyclic structure involves the reaction between an aldehyde group and a hydroxyl group within the same molecule. The smallest possible aldose that can do this is erythrose (a tetrose), which has 4 carbon atoms. Therefore, statement (a) is correct.

Evaluate Statement (b)

Benedict's solution is a reagent used to test for the presence of reducing sugars. D-glucose is a reducing sugar, whereas D-fructose is also considered a reducing sugar because it can tautomerize to form D-glucose in solution. Thus, both D-glucose and D-fructose will give a positive test with Benedict's solution and cannot be directly differentiated using this test. Statement (b) is incorrect.

Verify Statement (c)

When D-galactose is oxidized by \( \mathrm{HNO}_3 \), it oxidizes both the aldehyde group and the primary alcohol group to form a dicarboxylic acid. The product for D-galactose upon oxidation in this manner is galactaric acid, which is a meso compound. This means it has an internal plane of symmetry, making statement (c) correct.

Assess Statement (d)

Aldoses can be oxidized by Fehling's solution to form carboxylic acids, and they can form hydrazones with \( \mathrm{PhNHNH}_2 \). However, \( \mathrm{NaHSO}_3 \) is typically used to form bisulfite addition compounds with ketones and aldehydes but it is not a distinct reactivity test for aldoses. Therefore, statement (d) should be considered correct if it implies the usual circumstances.

Key concepts

Aldoses

Aldoses are a specific type of monosaccharides that contain an aldehyde group. These sugars can be categorized based on the number of carbon atoms present. For instance, **tetrose aldoses** have four carbon atoms, such as erythrose, which is capable of forming cyclic structures.
Aldoses are significant due to their ability to participate in various chemical reactions. The presence of the aldehyde group allows them to be involved in oxidation reactions and to act as reducing agents.

• Common examples of aldoses include glucose, galactose, and erythrose.
• The simplest aldose is glyceraldehyde, a triose, with three carbon atoms.
• Aldoses play a crucial role in biochemistry, serving as building blocks for more complex carbohydrates.

Reducing Sugars

Reducing sugars are sugars that contain either an aldehyde or a ketone group, which allows them to reduce certain chemicals, such as copper ions in Benedict's and Fehling's solutions. These solutions are used in tests to identify the presence of reducing sugars in substances.
In basic conditions, reducing sugars can convert to other forms, making testing versatile. For example, while **D-glucose** naturally has an aldehyde group making it directly a reducing sugar, **D-fructose** needs to first convert to D-glucose or mannose to showcase its reducing properties.

• Reducing sugars are crucial in food chemistry, affecting browning reactions and flavor development during cooking.
• The ability to reduce metal ions is because of the free electron from the carbonyl group that can donate electrons.
• They have significant roles in biological systems, including energy metabolism and cellular signaling.

Oxidation of Sugars

The oxidation of sugars involves the conversion of the aldehyde or alcohol functional groups to carboxylic acids. The process is critical in both biological pathways and industrial applications.
An example is the oxidation of D-galactose using **nitric acid (HNO₃)**, which results in the formation of a dicarboxylic acid known as galactaric acid. This sugar oxidation can yield meso compounds, characterized by having internal symmetry.

• Oxidized sugars, such as aldonic acids and uronic acids, play roles in biological structures like glycosaminoglycans and plant cell walls.
• Knowledge about sugar oxidation helps in diagnostics and the synthesis of pharmaceuticals.
• These reactions illustrate how sugars can be chemically altered to serve different functions.

Cyclic Hemi-Acetal Formation

Sugars can exist in cyclic forms due to the ability of their aldehyde or ketone groups to react with the alcohol group within the same molecule. This intramolecular reaction forms a **hemi-acetal**.
The formation of cyclic hemi-acetals occurs in sugars with at least four carbon atoms, such as tetroses and pentoses. In aqueous solutions, sugars like glucose predominantly exist in a stable cyclic form, enhancing their biological function and reactivity.

• The cyclic version of glucose, known as glucopyranose, is crucial in phenomena such as blood sugar regulation and the formation of polysaccharides like cellulose and starch.
• Cyclic hemi-acetals can further engage in condensation reactions, leading to the formation of glycosidic bonds.
• This ability significantly influences the sugar's interaction with other molecules and its incorporation into biochemical structures.