Sugar which does not give reddish brown precipitate with Fehling‟s reagent is:
- Lactose
- Maltose
- Sucrose
- Glucose
The Correct Option is C
Approach Solution - 1
To determine which sugar does not give a reddish-brown precipitate with Fehling's reagent, we need to understand the chemistry behind Fehling's test.
Fehling's solution is used to test for reducing sugars. It is an alkaline solution containing copper(II) sulfate. When a reducing sugar is present, it reduces the copper(II) ions to copper(I) oxide, which precipitates as a reddish-brown solid. Here's the reaction:
\(2Cu^{2+} + R-CHO + 4OH^- \rightarrow Cu_2O \ (s) + 2H_2O + R-COOH\)
Now, let's consider each option:
- Lactose - It is a reducing sugar because it has a free anomeric carbon, allowing it to reduce Fehling's solution, producing a reddish-brown precipitate.
- Maltose - Similarly, maltose is also a reducing sugar due to its free anomeric carbon, and it gives a reddish-brown precipitate with Fehling's reagent.
- Glucose - Glucose is a reducing sugar known for its ability to give a positive Fehling's test.
- Sucrose - This is a non-reducing sugar because the glycosidic bond in sucrose involves the anomeric carbon of both glucose and fructose, leaving no free anomeric carbon to act as a reducing agent. Therefore, sucrose does not reduce Fehling's solution and does not produce a reddish-brown precipitate.
Hence, the sugar which does not give a reddish-brown precipitate with Fehling's reagent is: Sucrose
Approach Solution -2
Step 1. Identify the Requirement for Fehling’s Test: Fehling’s reagent specifically reacts with reducing sugars that contain a free aldehyde or ketone group.
Step 2. Evaluate the Sugars: Sucrose: It is a non-reducing sugar because it lacks a free hemiacetal group due to its glycosidic linkage between glucose and fructose units. Lactose, Glucose, and Maltose: These are reducing sugars with free hemiacetal groups, allowing them to react with Fehling’s reagent and produce a reddish-brown precipitate.
Step 3. Conclusion: Since sucrose does not have a free aldehyde or ketone group, it does not give a positive test with Fehling’s solution.
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Concepts Used:
Aldehydes, Ketones, and Carboxylic Acids
Aldehydes, Ketones, and Carboxylic Acids are carbonyl compounds that contain a carbon-oxygen double bond. These organic compounds are very important in the field of organic chemistry and also have many industrial applications.
Aldehydes:
Aldehydes are organic compounds that have the functional group -CHO.
Preparation of Aldehydes
Acid chlorides are reduced to aldehydes with hydrogen in the presence of palladium catalyst spread on barium sulfate.
Ketones:
Ketones are organic compounds that have the functional group C=O and the structure R-(C=O)-R’.
Preparation of Ketones
Acid chlorides on reaction with dialkyl cadmium produce ketones. Dialkyl cadmium themselves are prepared from Grignard reagents.
Carboxylic Acid:
Carboxylic acids are organic compounds that contain a (C=O)OH group attached to an R group (where R refers to the remaining part of the molecule).
Preparation of Carboxylic Acids
Primary alcohols are readily oxidized to carboxylic acids with common oxidizing agents such as potassium permanganate in neutral acidic or alkaline media or by potassium dichromate and chromium trioxide in acidic media.