Which Compounds Give a Positive Fehling Test?
Fehling Test is essential in chemistry and helps students understand how reducing sugars and aldehydes can be identified using simple laboratory methods. It is commonly explored in organic chemistry and biochemistry practicals.
What is Fehling Test in Chemistry?
A Fehling Test refers to a classical chemical test used for distinguishing between reducing and non-reducing sugars and identifying the presence of aldehydes. This concept appears in chapters related to carbohydrates, organic chemistry, and qualitative analysis, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The Fehling Test does not test a single compound but uses a reagent. The main active chemical in Fehling’s reagent is copper(II) sulfate (CuSO4). The complete reagent is a mixture of Fehling’s A (aqueous copper(II) sulfate) and Fehling’s B (alkaline sodium potassium tartrate known as Rochelle salt). The solution is deep blue in color due to the copper(II) complex.
Preparation and Synthesis Methods
Fehling’s solution is freshly prepared in the lab by mixing equal volumes of:
- Fehling’s A: Dissolve 7 g CuSO4·5H2O in 100 mL distilled water.
- Fehling’s B: Dissolve 35 g sodium potassium tartrate and 10 g NaOH in 100 mL water.
- Mix the two fresh solutions together just before use.
Physical Properties of Fehling Test
Fehling’s A solution is blue due to copper(II) sulfate. Fehling’s B is colorless and alkaline. When mixed, they form a dark blue, alkaline reagent. During the test, a positive result is indicated by a brick-red precipitate (copper(I) oxide). The solution must be used fresh as the mixture is unstable on standing.
Chemical Properties and Reactions
The Fehling Test is based on a redox reaction. The copper(II) ions (Cu2+) in the reagent are reduced to copper(I) oxide (Cu2O), forming a red precipitate, while the aldehyde group is oxidized to a carboxylic acid.
General equation:
RCHO + 2Cu2+ + 5OH– → RCOO– + Cu2O (red ppt) + 3H2O
Frequent Related Errors
- Assuming all sugars or all aldehydes give a positive Fehling Test (aromatic aldehydes do not).
- Mixing Fehling’s A and B long before use (should always be freshly mixed for best results).
- Confusing negative result (blue) with absence of any carbohydrate (some non-reducing sugars exist).
- Applying the test to ketones—most do not react, except alpha-hydroxy ketones.
Uses of Fehling Test in Real Life
Fehling Test is widely used to detect reducing sugars in food samples and in laboratories. In clinical labs, it helps in detecting glucose in urine for diabetes screening. The test is also important in the food industry to monitor sugar content.
Relation with Other Chemistry Concepts
Fehling Test is closely related to Tollens Test and Benedict’s Test, both used for detecting aldehyde and reducing sugar groups via redox reactions and color changes, helping students build a conceptual bridge between analytical and organic chemistry.
Step-by-Step Reaction Example
1. Add equal volumes of Fehling’s A and B to prepare Fehling’s reagent.2. Add the test solution (e.g., glucose) to Fehling’s reagent in a clean test tube.
3. Heat the mixture in a boiling water bath for several minutes.
4. Observe the appearance of a brick-red precipitate (Cu2O) if the solution contains a reducing sugar or aldehyde.
5. Final Answer: Brick-red Cu2O confirms a positive Fehling Test.
Lab or Experimental Tips
Always mix Fehling’s A and B fresh before use. Use clean glassware and avoid contamination. Remember that only certain compounds (aliphatic aldehydes and most reducing sugars) give a positive Fehling Test. Vedantu educators recommend noticing the characteristic red precipitate carefully, as this helps avoid confusion with similar color changes from other reagents.
Try This Yourself
- Write the balanced redox reaction for glucose with Fehling’s reagent.
- Identify two non-reducing sugars that do not give a positive Fehling Test.
- Explain why benzaldehyde doesn’t respond to the Fehling Test.
Final Wrap-Up
We explored Fehling Test—its chemistry, stepwise procedure, molecular reaction, and practical applications. Remember, a brick-red precipitate is a quick clue for reducing sugars and aldehydes. For more in-depth discussions or exam-prep tips, visit Vedantu’s live classes and comprehensive notes designed for chemistry students.
Related Chemistry Topics
- Tollens Reagent
- Benedict's Test
- Carbohydrates
- Chemical Properties of Aldehydes and Ketones
- Qualitative Analysis
FAQs on Fehling Test: Principle, Procedure, and Results Explained
1. What is Fehling's Test and what does it detect?
Fehling's Test is a chemical test used to identify reducing sugars and differentiate them from non-reducing sugars. It primarily detects the presence of aldehydes, although some ketones (specifically alpha-hydroxy ketones) can also give a positive result. The test relies on the oxidation of the aldehyde group to a carboxyl group and the concomitant reduction of copper(II) ions to copper(I) oxide.
2. Which functional group gives a positive Fehling's Test?
The aldehyde functional group (-CHO) typically gives a positive Fehling's Test. However, some alpha-hydroxy ketones also react positively because they can tautomerize to an aldehyde form under alkaline conditions. Ketones in general do not react.
3. How is Fehling's Test different from the Tollens' Test?
Both Fehling's and Tollens' tests are used to distinguish aldehydes from ketones. However, they differ in their reagents and the resulting observations. Fehling's Test uses Fehling's solution (containing copper(II) ions) and produces a brick-red precipitate (Cu2O) in a positive test. Tollens' Test uses Tollens' reagent (ammoniacal silver nitrate) and produces a silver mirror on the test tube walls in a positive test.
4. What does a positive result in Fehling's Test look like?
A positive Fehling's Test results in the formation of a brick-red precipitate of cuprous oxide (Cu2O). This color change from the initial blue of Fehling's solution indicates the presence of a reducing sugar or aldehyde.
5. Can ketones be detected using Fehling's reagent?
Generally, no. Ketones usually do not react with Fehling's reagent. However, an exception exists for alpha-hydroxy ketones, which can tautomerize to an aldehyde form and thus give a positive test.
6. Which sugars respond to the Fehling's Test?
Many reducing sugars respond to Fehling's Test, including glucose, fructose, lactose, and maltose. Sucrose, a non-reducing sugar, does not react. The test's positive result indicates the presence of a free aldehyde or easily oxidized ketone group.
7. Why do aromatic aldehydes generally not give a positive Fehling's Test?
Aromatic aldehydes often do not give a positive Fehling's Test due to steric hindrance and the delocalization of electrons in the aromatic ring. These factors reduce the reactivity of the aldehyde group towards oxidation compared to aliphatic aldehydes.
8. What roles do copper(II) and tartrate ions play in Fehling's reagent?
Copper(II) ions (Cu2+) act as the oxidizing agent in Fehling's solution. Tartrate ions help to keep the copper(II) ions in solution, preventing the formation of insoluble copper(II) hydroxide. The tartrate ions form a complex with the copper(II) ions, which then reacts with the reducing sugar.
9. Is the Fehling's Test quantitative or qualitative, and can it be used to measure concentration?
Fehling's Test is primarily a qualitative test; it determines the presence or absence of reducing sugars. While it can be adapted for quantitative analysis (e.g., by titrating the Fehling's solution), it is not commonly used for precise concentration measurements due to the potential for interference from other substances.
10. Why do some reducing sugars not respond to the Fehling's Test?
Some reducing sugars may not respond to the Fehling's test due to factors such as steric hindrance, competing reactions, or the presence of other functional groups that interfere. The conditions of the test (e.g., temperature, pH) also play a critical role in determining reactivity.
11. How does pH influence the Fehling's Test outcome?
The Fehling's Test requires alkaline conditions for the reaction to occur effectively. A sufficiently alkaline environment (provided by the NaOH in Fehling's solution B) is necessary for the formation of the reactive copper(II) tartrate complex and for the oxidation of the aldehyde group. Under acidic conditions, the reaction is generally inhibited.
12. What are the limitations of Fehling’s Test?
Fehling’s test has several limitations: it cannot distinguish between different reducing sugars, it gives false positives with certain compounds, it is not very sensitive, and it is prone to interference by other substances present in the sample. Further, it is not suitable for aromatic aldehydes.
