Chapter 31: Problem 97
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
Short Answer
Expert verified
The compound is (b) a monosaccharide.
Step by step solution
01
Understanding Ninhydrin Test
The ninhydrin test is used to detect amines and amino acids. A negative ninhydrin test indicates that the compound is not an amine or amino acid because it does not form a blue or purple complex with ninhydrin.
02
Understanding Benedict's Solution Test
Benedict's solution is used to detect reducing sugars, such as monosaccharides. A positive result, indicated by a color change to red or orange, suggests that the compound could be a reducing sugar, such as a monosaccharide.
03
Evaluating the Options
Based on the tests, the compound that gives a negative result with ninhydrin and a positive result with Benedict's test can be identified. Options (a) protein and (d) amino acid are eliminated due to the negative ninhydrin result, while option (c) lipid is unlikely to give a positive Benedict's test. This leaves option (b) monosaccharide as the possible compound.
04
Confirming the Answer
Given that monosaccharides do not have free amine groups to react with ninhydrin, they would indeed give a negative ninhydrin test. However, many monosaccharides are reducing sugars and would give a positive Benedict's test. Thus, the correct option is (b) monosaccharide.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Ninhydrin Test
The ninhydrin test is an important chemical reaction used primarily to detect free amino groups in compounds such as amino acids and amines. When a substance with free amino groups comes into contact with ninhydrin, a deep blue or purple compound, called Ruhemann's purple, is formed. This indicates a positive reaction. However, if there is no such interaction, the test yields a negative result, meaning that the tested compound doesn't have free amino groups.
In the context of our exercise, a negative ninhydrin test suggests that the compound in question is not a protein or amino acid, as these typically contain free amino groups that react to produce the characteristic color change. Instead, compounds like monosaccharides, which do not possess free amino groups, would not react with ninhydrin significantly, leading to a negative outcome. Thus, the results from the ninhydrin test are crucial in narrowing down the possible identities of an unknown compound.
In the context of our exercise, a negative ninhydrin test suggests that the compound in question is not a protein or amino acid, as these typically contain free amino groups that react to produce the characteristic color change. Instead, compounds like monosaccharides, which do not possess free amino groups, would not react with ninhydrin significantly, leading to a negative outcome. Thus, the results from the ninhydrin test are crucial in narrowing down the possible identities of an unknown compound.
Benedict's Solution
Benedict's solution is a chemical reagent commonly used to test for the presence of reducing sugars. Reducing sugars possess free aldehyde or ketone groups, which allow them to reduce the copper(II) ions in Benedict's solution to copper(I) oxide, resulting in a color change.
When Benedict’s test is conducted, a positive result is observed by the appearance of a reddish precipitate, indicating the presence of a reducing sugar. On the other hand, if the solution remains blue, no reducing sugars are detected.
A positive Benedict's test in our exercise suggests that the compound could be a reducing sugar, specifically a monosaccharide. Since proteins and lipids typically do not participate in reducing activities under these conditions, the test serves as an effective method to distinguish monosaccharides from other types of molecules. Therefore, Benedict's solution is invaluable for identifying compounds capable of acting as reducing sugars.
When Benedict’s test is conducted, a positive result is observed by the appearance of a reddish precipitate, indicating the presence of a reducing sugar. On the other hand, if the solution remains blue, no reducing sugars are detected.
A positive Benedict's test in our exercise suggests that the compound could be a reducing sugar, specifically a monosaccharide. Since proteins and lipids typically do not participate in reducing activities under these conditions, the test serves as an effective method to distinguish monosaccharides from other types of molecules. Therefore, Benedict's solution is invaluable for identifying compounds capable of acting as reducing sugars.
Reducing Sugars
Reducing sugars are carbohydrates that contain a free aldehyde group or a free ketone group. These groups are pivotal because they allow the sugar to act as a reducing agent, which means they can donate electrons to other molecules. This ability to reduce other substances is what makes them detectable in specific chemical tests, such as the Benedict's test.
Examples of reducing sugars include glucose, fructose, and maltose. These sugars interact with reagents like Benedict's solution to form a color change, which is indicative of their reducing nature. In practical terms, the presence of reducing sugars gives important information about carbohydrate composition in a mixture and is crucial for various biochemical analyses.
In the exercise, the compound's positive result with Benedict’s solution suggests it is a reducing sugar. This aligns with the properties of many monosaccharides, confirming their identity when reducing sugars are suspected.
Examples of reducing sugars include glucose, fructose, and maltose. These sugars interact with reagents like Benedict's solution to form a color change, which is indicative of their reducing nature. In practical terms, the presence of reducing sugars gives important information about carbohydrate composition in a mixture and is crucial for various biochemical analyses.
In the exercise, the compound's positive result with Benedict’s solution suggests it is a reducing sugar. This aligns with the properties of many monosaccharides, confirming their identity when reducing sugars are suspected.
Monosaccharides
Monosaccharides are the simplest form of carbohydrates and consist of single sugar molecules. They are classified based on the number of carbon atoms they contain: trioses have three, tetroses four, pentoses five, and hexoses six carbon atoms. Common examples include glucose and fructose.
These molecules are incredibly important in biochemistry because they serve various roles, including being a source of energy and serving as building blocks for more complex carbohydrates like disaccharides and polysaccharides. One of their defining features is the presence of a free aldehyde or ketone group, making them capable of acting as reducing sugars.
In the context of our exercise, the compound that tested negative with ninhydrin but positive with Benedict's solution was identified as a monosaccharide. This categorization is consistent since monosaccharides do not possess free amino groups to react with ninhydrin, yet they offer free aldehyde or ketone groups capable of functioning as reducing agents.
These molecules are incredibly important in biochemistry because they serve various roles, including being a source of energy and serving as building blocks for more complex carbohydrates like disaccharides and polysaccharides. One of their defining features is the presence of a free aldehyde or ketone group, making them capable of acting as reducing sugars.
In the context of our exercise, the compound that tested negative with ninhydrin but positive with Benedict's solution was identified as a monosaccharide. This categorization is consistent since monosaccharides do not possess free amino groups to react with ninhydrin, yet they offer free aldehyde or ketone groups capable of functioning as reducing agents.
