Problem 97 - A Complete Resource Book in Chemistry for JEE Main

Question

A certain compound gives negative test with ninhydrin and positive test with Benedict's solution. The compound is (a) a protein (b) a monosaccharide (c) a lipid (d) an amino acid

Step-by-Step Solution

Answer

The compound is a monosaccharide (b).

Step 1: Identify Tests

Understand the tests involved: Ninhydrin test is used to detect amino acids and proteins, giving a purple color when positive. Benedict's solution is used to detect reducing sugars (such as monosaccharides) turning from blue to red, orange, or green when positive.

Step 2: Result Interpretation for Ninhydrin Test

Given that the compound gives a negative test with ninhydrin, it means the compound is not an amino acid or protein, as these typically give a positive result.

Step 3: Result Interpretation for Benedict's Test

A positive result with Benedict's solution suggests the compound is a reducing sugar, as it would cause a color change indicating the presence of monosaccharides or other reducing sugars.

Step 4: Conclusion

Since the compound gives a negative test with ninhydrin, it eliminates options (a) and (d), which relate to amino acids and proteins. The positive Benedict's test aligns with option (b), a monosaccharide, because it indicates the presence of reducing sugars.

Key Concepts

Ninhydrin Test

The ninhydrin test is a chemical test specifically used to detect the presence of amino acids and proteins. When ninhydrin is added to a solution containing amino acids, it reacts to form a deep blue or purple color. This reaction occurs because ninhydrin reacts with the free amino group (-NH2) present in amino acids.
The ninhydrin test is a popular choice in laboratories for identifying proteins in various samples. However, it will not react with compounds that lack amino groups, such as simple sugars or lipids, hence resulting in a negative test result.

• Positive result: Deep blue or purple color (indicating presence of amino acids or proteins).
• Negative result: No color change (indicating absence of amino acids or proteins).

Understanding the ninhydrin test is crucial because it helps differentiate between proteins or amino acids and other types of compounds.

Benedict's Solution

Benedict's solution is widely used in organic chemistry to test for the presence of reducing sugars. Reducing sugars are sugars that can donate electrons to another chemical, making them capable of reducing substances like Benedict's reagent.
When Benedict's solution, which is initially a clear blue liquid due to copper(II) sulfate, is added to a sample containing a reducing sugar, the solution undergoes a reaction when heated. The result can range from a green to yellow to dark red or brick red precipitate, depending on the amount and type of reducing sugar present.

• Positive result: Color change from blue to green, yellow, or red (indicating presence of reducing sugars).
• Negative result: No color change, remaining blue (indicating absence of reducing sugars).

This test is essential for distinguishing reducing sugars, such as glucose, from non-reducing sugars like sucrose.

Reducing Sugars

Reducing sugars play a crucial role in chemistry and biology as they are able to reduce other chemicals. This property makes them reactive and easy to identify using tests like Benedict's solution. A reducing sugar has a free aldehyde group (-CHO) or a free ketone group. In the presence of Benedict's reagent, they reduce copper(II) ions to copper(I) oxide, which precipitates as a colored compound.
Most monosaccharides, such as glucose and fructose, are classified as reducing sugars. In contrast, some disaccharides, like sucrose, do not have free aldehyde or ketone groups and are not reducing sugars.

• Examples of reducing sugars: Glucose, fructose, lactose.
• Importance: Used in energy metabolism, easily tested for with Benedict's solution.

Understanding the concept of reducing sugars is vital for identifying and analyzing carbohydrate molecules in various fields.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. These are the building blocks for more complex carbohydrates like disaccharides, oligosaccharides, and polysaccharides.
Monosaccharides generally have a sweet taste and are soluble in water, making them easily digestible energy sources. Their structure contains a single sugar molecule with a chemical formula of \( C_nH_{2n}O_n \). Each monosaccharide has multiple hydroxyl groups (-OH) and one carbonyl group (either a ketone or aldehyde) which allows them to function as reducing sugars.

• Characteristics: Simple structure, sweet taste, soluble in water.
• Examples: Glucose, fructose, galactose.

Monosaccharides are essential nutrients in biology, providing a primary source of energy and being crucial for cellular respiration processes.