Fehling Test Reaction Principle Equation and Examples
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 in Organic Chemistry
1. What is Fehling test in chemistry?
The Fehling test is a chemical test used to detect the presence of aldehydes and reducing sugars using an alkaline copper(II) solution. It is commonly used in organic chemistry to distinguish aldehydes from ketones. In this test:
- Aldehydes reduce Cu2+ ions to Cu2O (brick-red precipitate).
- The solution changes from deep blue to a red solid.
- Ketones generally do not give a positive Fehling test.
2. What is the composition of Fehling solution?
The Fehling solution consists of two separate solutions: Fehling’s A and Fehling’s B. These are mixed before use:
- Fehling’s A: Aqueous copper(II) sulfate, CuSO4(aq).
- Fehling’s B: Alkaline sodium potassium tartrate (Rochelle salt) dissolved in NaOH(aq).
3. What is the principle of Fehling test?
The principle of the Fehling test is the reduction of alkaline copper(II) ions to copper(I) oxide by aldehydes. In alkaline medium:
- Aldehydes are oxidized to carboxylate ions.
- Cu2+ (blue) is reduced to Cu2O(s) (brick-red precipitate).
4. How do you perform the Fehling test?
The Fehling test is performed by heating the test solution with freshly prepared Fehling reagent. The steps are:
- Mix equal volumes of Fehling’s A and Fehling’s B.
- Add the test solution containing the suspected aldehyde or reducing sugar.
- Heat the mixture gently in a water bath.
- Observe for a brick-red precipitate of Cu2O, indicating a positive result.
5. What is the observation in a positive Fehling test?
A positive Fehling test produces a brick-red precipitate of copper(I) oxide, Cu2O. The observations include:
- Initial deep blue solution due to Cu2+ ions.
- On heating, formation of a red or orange precipitate.
- Indicates presence of an aldehyde group (–CHO) or reducing sugar.
6. Why do aldehydes give Fehling test but ketones do not?
Aldehydes give a positive Fehling test because they are easily oxidized to carboxylate ions, while most ketones resist oxidation under these conditions. Specifically:
- Aldehydes (R–CHO) are oxidized in alkaline medium.
- They reduce Cu2+ to Cu2O.
- Ketones (R–CO–R') lack a hydrogen on the carbonyl carbon, so they do not oxidize easily.
7. Which sugars give a positive Fehling test?
All reducing sugars give a positive Fehling test because they can form free aldehyde groups in solution. Examples include:
- Glucose
- Fructose (via enediol rearrangement in alkaline medium)
- Lactose
- Maltose
8. What is the balanced chemical equation for Fehling test with ethanal?
The balanced equation for the Fehling test with ethanal (CH3CHO) is: CH3CHO + 2Cu2+ + 5OH- → CH3COO- + Cu2O(s) + 3H2O
- Ethanal is oxidized to ethanoate ion (CH3COO-).
- Cu2+ ions are reduced to Cu2O(s).
- A brick-red precipitate confirms the presence of an aldehyde.
9. What is the difference between Fehling test and Tollens test?
The main difference between Fehling test and Tollens test is the oxidizing agent and the observation produced. Key differences:
- Fehling test: Uses alkaline Cu2+ solution; gives brick-red Cu2O precipitate.
- Tollens test: Uses [Ag(NH3)2]+; produces a silver mirror of Ag(s).
- Fehling’s solution is less sensitive than Tollens’ reagent.
- Tollens test works well for aromatic aldehydes, while Fehling test often does not.
10. What are the limitations of Fehling test?
The main limitations of the Fehling test are that it does not detect all aldehydes and may give false negatives with some compounds. Important limitations include:
- Aromatic aldehydes (e.g., benzaldehyde) usually do not respond.
- Most ketones give negative results.
- The reagent must be freshly prepared for accurate results.
- It is less sensitive compared to Tollens test.
