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: Understanding Ninhydrin Test

The ninhydrin test is used to detect amino acids and proteins. If a substance gives a negative test with ninhydrin, it likely does not contain free amino acids or proteins.

Step 2: Understanding Benedict's Test

Benedict's solution is used to test for the presence of reducing sugars, like monosaccharides. If a substance gives a positive test with Benedict's solution, it contains reducing sugars.

Step 3: Analyzing the Properties

Since the compound gives a negative test with ninhydrin, it can be ruled out as a protein or an amino acid as they contain amino groups. A positive Benedict's test indicates the presence of a reducing sugar, suggesting the compound may be a monosaccharide.

Step 4: Conclusion

By process of elimination based on test results (negative ninhydrin, positive Benedict's), the compound cannot be a protein, amino acid, or lipid. It is most likely a monosaccharide.

Key Concepts

Understanding the Ninhydrin Test

The ninhydrin test is a well-known chemistry test used to detect amino acids and proteins. When a substance undergoes a ninhydrin test, it reacts with free amino groups.

• If the substance contains amino acids, it will typically turn a deep blue or purple upon reacting with ninhydrin.
• This color change indicates the presence of free amino groups, which are a key component of amino acids and proteins.

However, if the test result is negative, it suggests that the substance does not contain these free amino groups. Thus, it likely lacks amino acids or protein content. This makes it a reliable method for ruling out certain biomolecules, such as amino acids and proteins.

Understanding Benedict's Test

Benedict's test is designed to detect the presence of reducing sugars, which are a type of carbohydrate. Reducing sugars, such as monosaccharides, have free aldehyde or ketone groups that react with Benedict's reagent.

• The reagent itself is a blue solution composed of copper(II) sulfate.
• Upon encountering a reducing sugar, the copper is reduced to copper(I) oxide, a red or orange precipitate.
• The test solution changes color, signifying a positive result.

A positive Benedict's test is indicative of the presence of reducing sugars in the tested sample, confirming that the sample contains carbohydrates that can reduce other compounds.

The Role of Reducing Sugars

Reducing sugars are carbohydrates that possess free aldehyde or ketone groups, allowing them to act in reduction reactions.

• Common examples include glucose, fructose, and some disaccharides.
• These sugars can donate electrons to other molecules, a characteristic observed during Benedict's test.

In analytical chemistry, identifying reducing sugars is crucial because they play significant roles in various biological and chemical processes. If a compound tests positive in Benedict's test, it is an indicator of reducing sugars, which supports its classification as a carbohydrate such as a monosaccharide.

Amino Acids Detection and Its Chemistry

Amino acids are organic compounds characterized by the presence of both amino and carboxyl groups. These groups are essential for their detection through tests like the ninhydrin test.

• The amino group (-NH2) reacts with ninhydrin, leading to the appearance of a blue-purple color.
• This reaction also produces a compound known as Ruhemann's purple, specific to amino acid detection.

If a substance does not react with ninhydrin, it suggests the absence of free amino acids. This method proves useful for distinguishing amino acid-containing substances from others, highlighting the compound's biochemical nature and its potential classification.