Identify aldehydes using Fehling’s test and Tollen’s test

 

Abstract

This experiment aims to identify the presence of aldehydes in a given organic compound solution using Fehling’s test and Tollen’s test. These chemical tests are based on the characteristic reactions of aldehydes with specific reagents, producing distinctive precipitates or color changes.

Introduction

Fehling’s test and Tollen’s test are classical chemical tests used to distinguish aldehydes from other organic compounds. In Fehling’s test, aldehydes react with Fehling's solution to produce a brick-red precipitate of copper(I) oxide. Tollen’s test involves the oxidation of aldehydes to carboxylic acids by silver ions in Tollen's reagent, resulting in the formation of a silver mirror on the inner surface of the reaction vessel.

Details of the Experiment

Procedure

1. Prepare Fehling’s solution and Tollen’s reagent according to standard laboratory procedures.
2. For Fehling’s test, add a small amount of the organic compound solution to Fehling’s solution in a test tube and heat gently.
3. For Tollen’s test, add a small amount of the organic compound solution to Tollen’s reagent in a test tube and heat gently.
4. Observe any color changes or formation of precipitates in both tests.
5. Record your observations.

Observations and Calculations

In Fehling’s test, the formation of a brick-red precipitate indicates the presence of aldehydes. In Tollen’s test, the formation of a silver mirror confirms the presence of aldehydes.

Conclusion

The presence of aldehydes in the given organic compound solution is confirmed by the positive results obtained from both Fehling’s test and Tollen’s test.

Precautions

• Handle all chemicals with care and wear appropriate safety gear.
• Perform the tests in a well-ventilated area to avoid inhalation of fumes.
• Use small quantities of organic compounds to minimize waste and reduce the risk of accidents.

Short Questions with Answers

1. What is Fehling’s test used for?
   Answer: Fehling’s test is used to identify the presence of aldehydes.
2. What color change indicates a positive result in Fehling’s test?
   Answer: Brick-red precipitate formation.
3. What is Tollen’s test used for?
   Answer: Tollen’s test is used to identify the presence of aldehydes.
4. What is the significance of the silver mirror formation in Tollen’s test?
   Answer: It confirms the presence of aldehydes.
5. What safety precautions should be followed during the experiment?
   Answer: Wear safety goggles and gloves, work in a well-ventilated area, and handle chemicals with care.
6. Explain the principle behind Fehling’s test.
   Answer: Aldehydes reduce copper(II) ions in Fehling’s solution to form a brick-red precipitate of copper(I) oxide.
7. What are the components of Tollen’s reagent?
   Answer: Tollen’s reagent contains silver ions complexed with ammonia.
8. How does Tollen’s test differentiate between aldehydes and ketones?
   Answer: Tollen’s test produces a silver mirror only with aldehydes, not with ketones.
9. What is the chemical formula of Fehling’s solution?
   Answer: Fehling’s solution consists of solutions of copper(II) sulfate, sodium hydroxide, and potassium sodium tartrate.
10. Describe the appearance of a positive result in Tollen’s test.
    Answer: A silver mirror forms on the inner surface of the reaction vessel.
11. Why is it necessary to heat the reaction mixture in both Fehling’s and Tollen’s tests?
    Answer: Heating accelerates the reaction rate and enhances the visibility of the results.
12. What is the purpose of potassium sodium tartrate in Fehling’s solution?
    Answer: It stabilizes copper(II) ions and prevents their precipitation in alkaline solutions.
13. How can false-positive results be avoided in Fehling’s and Tollen’s tests?
    Answer: By using pure reagents and performing control experiments.
14. What is the expected observation if the organic compound is not an aldehyde in both Fehling’s and Tollen’s tests?
    Answer: No color change or precipitate formation.
15. Why is it necessary to use small quantities of organic compounds in the tests?
    Answer: To minimize waste and reduce the risk of accidents.
16. What is the purpose of ammonia in Tollen’s reagent?
    Answer: It complexes with silver ions to form a stable complex, enhancing the sensitivity of the test.
17. Explain the role of sodium hydroxide in Fehling’s test.
    Answer: Sodium hydroxide provides the alkaline conditions necessary for the reaction to occur.
18. What is the expected observation if the organic compound is a ketone in both Fehling’s and Tollen’s tests?
    Answer: No precipitate formation or silver mirror.
19. How does the concentration of Fehling’s solution affect the sensitivity of the test?
    Answer: Higher concentrations increase sensitivity but may lead to false-positive results.
20. Why is it important to perform both Fehling’s and Tollen’s tests for confirmation?
    Answer: To ensure accurate identification of aldehydes and eliminate false-positive results.

Multiple Choice Questions (MCQs) with Answers

1. What is the purpose of Fehling’s test?
   • a) To identify alcohols
   • b) To identify aldehydes
   • c) To identify ketones
   • d) To identify carboxylic acids
   • Answer: b) To identify aldehydes
2. Which of the following indicates a positive result in Fehling’s test?
   • a) Formation of a silver mirror
   • b) Formation of a brick-red precipitate
   • c) No observable change
   • d) Evolution of gas bubbles
   • Answer: b) Formation of a brick-red precipitate
3. What is the purpose of heating the reaction mixture in Fehling’s test?
   • a) To evaporate the solvent
   • b) To dissolve the organic compound
   • c) To accelerate the reaction
   • d) To prevent contamination
   • Answer: c) To accelerate the reaction
4. Which of the following substances forms a silver mirror in Tollen’s test?
   • a) Alcohols
   • b) Aldehydes
   • c) Ketones
   • d) Carboxylic acids
   • Answer: b) Aldehydes
5. What is the function of ammonia in Tollen’s reagent?
   • a) To oxidize aldehydes
   • b) To complex with silver ions
   • c) To reduce silver ions
   • d) To stabilize the solution
   • Answer: b) To complex with silver ions