Table of contents

1. Matter and Measurements4h 31m
2. Atoms and the Periodic Table5h 23m
3. Ionic Compounds2h 18m
4. Molecular Compounds2h 18m
5. Classification & Balancing of Chemical Reactions3h 17m
6. Chemical Reactions & Quantities2h 27m
7. Energy, Rate and Equilibrium3h 46m
8. Gases, Liquids and Solids3h 25m
9. Solutions4h 10m
10. Acids and Bases3h 29m
11. Nuclear Chemistry56m
BONUS: Lab Techniques and Procedures1h 38m
BONUS: Mathematical Operations and Functions47m
12. Introduction to Organic Chemistry1h 34m
13. Alkenes, Alkynes, and Aromatic Compounds2h 12m
14. Compounds with Oxygen or Sulfur1h 6m
15. Aldehydes and Ketones1h 1m
16. Carboxylic Acids and Their Derivatives1h 11m
17. Amines39m
18. Amino Acids and Proteins1h 51m
19. Enzymes1h 37m
20. Carbohydrates1h 41m
21. The Generation of Biochemical Energy2h 8m
22. Carbohydrate Metabolism2h 22m
23. Lipids2h 26m
24. Lipid Metabolism1h 45m
25. Protein and Amino Acid Metabolism1h 37m
26. Nucleic Acids and Protein Synthesis2h 54m

15. Aldehydes and Ketones

Tollens' and Benedict's Test

GOB Chemistry

15. Aldehydes and Ketones

Tollens' and Benedict's Test

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Problem 79

Textbook Question

Mention at least two simple chemical tests by which you can distinguish between benzaldehyde and benzoic acid.

Verified step by step guidance

Step 1: Understand the chemical difference between benzaldehyde and benzoic acid. Benzaldehyde (C₆H₅CHO) is an aldehyde, while benzoic acid (C₆H₅COOH) is a carboxylic acid. This difference in functional groups will determine the chemical tests to distinguish them.

Step 2: Perform the Fehling's test. Aldehydes like benzaldehyde react with Fehling's solution to produce a red precipitate of copper(I) oxide (Cu₂O), while carboxylic acids like benzoic acid do not react. Add Fehling's solution to each compound and heat gently to observe the result.

Step 3: Conduct the sodium bicarbonate test. Carboxylic acids like benzoic acid react with sodium bicarbonate (NaHCO₃) to produce carbon dioxide gas (CO₂), which can be observed as effervescence. Aldehydes like benzaldehyde do not react with sodium bicarbonate. Add a small amount of NaHCO₃ to each compound and observe for bubbling.

Step 4: Optionally, perform the Tollens' test as an additional confirmation. Aldehydes like benzaldehyde react with Tollens' reagent to form a silver mirror on the test tube, while carboxylic acids like benzoic acid do not react. Add Tollens' reagent to each compound and observe the result.

Step 5: Record and compare the observations from the tests. Benzaldehyde will show a positive result in Fehling's and Tollens' tests but no reaction with sodium bicarbonate, while benzoic acid will show effervescence with sodium bicarbonate but no reaction in Fehling's or Tollens' tests.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Functional Groups

Benzaldehyde and benzoic acid are both aromatic compounds but differ in their functional groups. Benzaldehyde contains an aldehyde group (-CHO), while benzoic acid has a carboxylic acid group (-COOH). Understanding these functional groups is essential for predicting their chemical behavior and reactivity in tests.

Chemical Tests

Chemical tests are specific reactions used to identify the presence of certain functional groups in organic compounds. For instance, the Tollens' test can be used to distinguish benzaldehyde, which can be oxidized to a carboxylic acid, from benzoic acid, which does not react in the same way. Knowing these tests helps in differentiating between compounds based on their chemical properties.

Solubility and pH

The solubility of benzaldehyde and benzoic acid in water varies due to their functional groups. Benzoic acid is more soluble in water because it can ionize to form benzoate ions, affecting the pH of the solution. This property can be exploited in tests, such as measuring pH changes, to distinguish between the two compounds.

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Textbook Question

Which of the following will give a positive Tollens' test?

a. propanal

b. ethanol

c. ethyl methyl ether

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Textbook Question

Which of the following will give a positive Tollens' test?

a. 1-propanol

b. 2-propanol

c. hexanal

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Textbook Question

A fundamental difference between aldehydes and ketones is that one can be oxidized to carboxylic acids but the other cannot. Which is which? Give an example of a test to differentiate aldehydes from ketones.

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Textbook Question

Which of the following compounds will react with Tollens' reagent? With Benedict's reagent?

a. Cyclopentanon

b. Hexanal

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Textbook Question

A compound with the formula C₄H₈O is synthesized from 2-methyl-1-propanol and oxidizes easily to give a carboxylic acid. Draw the condensed structural formula and give the IUPAC name for the compound.

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Textbook Question

Compound A is a primary alcohol whose formula is C₃H₈O. When compound A is heated with strong acid, it dehydrates to form compound B (C₃H₆). When compound A is oxidized, compound C (C₃H₆O) forms. Draw the condensed structural formulas and give the IUPAC names for compounds A, B, and C.