Aldehydes, Ketones & Carboxylic Acids - Oxidation of Aldehydes & Ketones

• Oxidation refers to the process of adding oxygen or removing hydrogen from a substance.
• In the case of aldehydes and ketones, oxidation involves the conversion of a lower oxidation state (containing a carbonyl group) to a higher oxidation state (containing carboxylic acid).

Mild oxidation of aldehydes

• Aldehydes can be oxidized to form carboxylic acids using a mild oxidizing agent such as tollen’s reagent or Fehling’s solution.

Tollens’ test:

• Tollens’ reagent is a solution of ammoniacal silver nitrate (AgNO₃) that is used to distinguish between aldehydes and ketones.
• When an aldehyde is treated with Tollens’ reagent, it forms a silver mirror on the inner wall of the test tube. Ketones do not react with Tollens’ reagent.

Fehling test:

• Fehling’s solution is a blue solution of copper sulfate (CuSO₄) and sodium hydroxide (NaOH), which is used to distinguish between aldehydes and ketones.
• When an aldehyde is treated with Fehling’s solution, it forms a brick-red precipitate of copper(I) oxide (Cu₂O). Ketones do not react with Fehling’s solution.

Strong oxidation of aldehydes & ketones

• Aldehydes and ketones can be oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate (KMnO₄) or chromic acid (H₂CrO₄).

Oxidation of aldehydes to carboxylic acids:

• The oxidation of aldehydes to carboxylic acids involves the addition of an oxygen atom to the carbonyl group.
• This reaction can be carried out by treating the aldehyde with a strong oxidizing agent such as potassium permanganate (KMnO₄) in a basic solution.

Oxidation of ketones to carboxylic acids:

• Unlike aldehydes, ketones cannot be oxidized using mild oxidizing agents like Tollens’ reagent or Fehling’s solution.
• However, ketones can be oxidized to carboxylic acids using strong oxidizing agents like potassium permanganate (KMnO₄) or chromic acid (H₂CrO₄) in acidic conditions.

Mechanism of oxidation:

• The oxidation of aldehydes and ketones involves the breaking of a carbon-hydrogen (C-H) bond and the formation of a carbon-oxygen double bond (C=O).
• In the case of aldehydes, the carbon-oxygen double bond is further oxidized to form a carboxylic acid group (COOH) by the addition of another oxygen atom.

Examples:

1. Oxidation of ethanal (CH₃CHO) using Tollens’ reagent:

   CH₃CHO + 2AgNO₃ + 3OH^- –> CH₃COOH + 2Ag + 2H₂O + 2NO₃^-

2. Oxidation of propanone (CH₃COC₂H₅) with potassium permanganate in acidic conditions:

   CH₃COC₂H₅ + 8H₂O + 2KMnO₄ –> CH₃COOH + 2CO₂ + 2KOH + 6H₂O + MnO₂

Summary:

• Aldehydes can be oxidized to carboxylic acids using mild oxidizing agents like Tollens’ reagent or Fehling’s solution.
• Both aldehydes and ketones can be oxidized to carboxylic acids using strong oxidizing agents like potassium permanganate or chromic acid.
• The oxidation involves the breaking of a carbon-hydrogen bond and the formation of a carbon-oxygen double bond.
• Aldehydes undergo further oxidation to form carboxylic acids, while ketones directly convert to carboxylic acids.

11. Oxidation of Aldehydes & Ketones (Contd.)

• Apart from tollens’ reagent and Fehling’s solution, there are other oxidation methods for aldehydes and ketones.
• Aldehydes and ketones can also be oxidized using powerful organic oxidizing agents, such as potassium dichromate (K₂Cr₂O₇) and pyridinium chlorochromate (PCC).
• These organic oxidizing agents are milder than inorganic oxidizing agents like KMnO₄ or H₂CrO₄.

12. Oxidation using Potassium Dichromate (K₂Cr₂O₇)

• Potassium dichromate can be used to oxidize aldehydes to carboxylic acids.
• The reaction takes place in an acidic medium.
• The orange solution of potassium dichromate turns green after the reaction.
• The aldehyde is oxidized to the corresponding carboxylic acid, while the dichromate ion (Cr₂O₇^2-) is reduced to chromium(III) ions (Cr³⁺).

13. Oxidation using Pyridinium Chlorochromate (PCC)

• Pyridinium chlorochromate (PCC) is another mild oxidizing agent that can be used to oxidize aldehydes to carboxylic acids.
• The PCC complex contains CrO₃, HCl, and the organic base pyridine.
• PCC is particularly useful when only partial oxidation (to an aldehyde) is desired rather than the full oxidation to a carboxylic acid.

14. Side Reactions in Oxidation

• When aldehydes or ketones are oxidized, side reactions may also occur.
• Over-oxidation might lead to the formation of carbon dioxide and water.
• Incomplete oxidation can yield various other products, such as alcohols or alkenes, depending on the conditions and reactants involved.
• It is important to carefully control the conditions to achieve the desired oxidation product.

15. Examples: Oxidation of Aldehydes to Carboxylic Acids

• Example 1: Oxidation of formaldehyde (HCHO) using potassium dichromate (K₂Cr₂O₇) in the presence of sulfuric acid (H₂SO₄): HCHO + K₂Cr₂O₇ + H₂SO₄ → HCOOH + Cr₂(SO₄)₃ + K₂SO₄ + H₂O

• Example 2: Oxidation of propanal (CH₃CH₂CHO) using pyridinium chlorochromate (PCC): CH₃CH₂CHO + PCC → CH₃CH₂COOH + CrO₃ + HCl

16. Examples: Oxidation of Ketones to Carboxylic Acids

• Example 1: Oxidation of acetone (CH₃C(O)CH₃) using potassium dichromate (K₂Cr₂O₇) in the presence of sulfuric acid (H₂SO₄): CH₃C(O)CH₃ + K₂Cr₂O₇ + H₂SO₄ → CH₃COOH + Cr₂(SO₄)₃ + K₂SO₄ + H₂O
• Example 2: Oxidation of 2-pentanone (CH₃COCH₂CH₂CH₂CH₃) using pyridinium chlorochromate (PCC): CH₃COCH₂CH₂CH₂CH₃ + PCC → CH₃COOH + CrO₃ + HCl

17. Oxidation Mechanism of Aldehydes & Ketones

• The oxidation of aldehydes and ketones involves the transfer of electrons from the carbon-hydrogen (C-H) bond to the oxidizing agent.
• The oxidizing agent gains electrons, while the aldehyde or ketone molecule loses electrons.
• In the process, the carbonyl carbon undergoes a change in oxidation state from a lower state to a higher state.

18. Key Differences between Aldehydes & Ketones

• Aldehydes have the general formula RCHO, while ketones have the general formula R₂C=O.
• Aldehydes have a hydrogen attached to the carbonyl group, while ketones have two alkyl or aryl substituents attached to the carbonyl group.
• Aldehydes are more reactive towards oxidation reactions than ketones due to the presence of the hydrogen atom.

19. Uses of Aldehydes & Ketones

• Aldehydes and ketones have numerous applications in various industries and everyday life.
• Some common uses of aldehydes include formaldehyde as a disinfectant and preservative, acetaldehyde in the production of chemicals, and benzaldehyde in the flavor and fragrance industry.
• Ketones, such as acetone, are widely used as solvents, and some ketones are used in the production of pharmaceuticals and polymers.

20. Summary

• Aldehydes and ketones can be oxidized to form carboxylic acids.
• Mild oxidizing agents like Tollens’ reagent and Fehling’s solution can be used for aldehydes, while strong oxidizing agents like potassium permanganate and chromic acid are suitable for both aldehydes and ketones.
• Organic oxidizing agents like PCC and K₂Cr₂O₇ are milder alternatives to inorganic oxidizing agents.
• The mechanisms of oxidation involve the breaking of C-H bonds and the formation of C=O bonds.
• It is important to control the reaction conditions to achieve desired oxidation products and avoid side reactions.

21. Example: Oxidation of Benzaldehyde

• Benzaldehyde (C₆H₅CHO) can be oxidized using Tollens’ reagent to form benzoic acid (C₆H₅COOH).
• The reaction proceeds as follows: C₆H₅CHO + 2AgNO₃ + 3OH^- –> C₆H₅COOH + 2Ag + 2H₂O + 2NO₃^-

22. Example: Oxidation of Acetophenone

• Acetophenone (C₆H₅COCH₃) can be oxidized using potassium permanganate (KMnO₄) to form benzoic acid.
• The reaction proceeds as follows: C₆H₅COCH₃ + 4KMnO₄ + 4H₂O –> C₆H₅COOH + 4MnO₂ + 4KOH + 4H₂O

23. Example: Oxidation of Butanone

• Butanone (CH₃COCH₂CH₂CH₃) can be oxidized using chromic acid (H₂CrO₄) to form butanoic acid (CH₃CH₂CH₂COOH).
• The reaction proceeds as follows: CH₃COCH₂CH₂CH₃ + H₂CrO₄ + H₂O –> CH₃CH₂CH₂COOH + CrO₃ + H₂O

24. Example: Oxidation of Cyclohexanone

• Cyclohexanone (C₆H₁₀O) can be oxidized using potassium permanganate (KMnO₄) to form adipic acid (C₆H₁₀O₄).
• The reaction proceeds as follows: C₆H₁₀O + 2KMnO₄ + 4H₂O –> C₆H₁₀O₄ + 2MnO₂ + 2KOH + 2H₂O

25. Limitations of Oxidation Reactions

• The oxidation reaction of aldehydes and ketones requires specific conditions and reagents.
• Not all aldehydes and ketones are readily oxidized using the common oxidizing agents.
• Some aldehydes and ketones may undergo side reactions or produce other products instead of the desired carboxylic acids.
• Careful selection of oxidizing agents and reaction conditions is necessary for successful and selective oxidation.

26. Importance of Oxidation Reactions

• Oxidation reactions of aldehydes and ketones are important in various industries and scientific research.
• The production of carboxylic acids from aldehydes and ketones is crucial for the synthesis of pharmaceuticals, polymers, and other chemical compounds.
• Oxidation reactions also play a significant role in biochemical processes and metabolic pathways in living organisms.

27. Common Applications of Carboxylic Acids

• Carboxylic acids have diverse applications in various fields:
  • Acetic acid (CH₃COOH) is used in the production of vinegar, solvents, and plastics.
  • Formic acid (HCOOH) is employed as a preservative, disinfectant, and in the textile industry.
  • Citric acid (C₆H₈O₇) is used as a flavoring agent, pH regulator, and chelating agent.
  • Benzoic acid (C₆H₅COOH) has applications in the production of food preservatives, dyes, and pharmaceuticals.

28. Summary of Key Points

• Oxidation of aldehydes and ketones involves the conversion to carboxylic acids.
• Mild oxidizing agents like Tollens’ reagent and Fehling’s solution are suitable for aldehydes.
• Strong oxidizing agents like potassium permanganate and chromic acid can oxidize both aldehydes and ketones.
• Organic oxidizing agents such as PCC and K₂Cr₂O₇ provide milder alternatives.
• Care must be taken to control reaction conditions and avoid unwanted side reactions.
• Carboxylic acids have various applications in industries and research.

29. Practice Problems

1. Determine the product of the oxidation reaction of propanal with Tollens’ reagent.

2. Write the balanced chemical equation for the oxidation of 3-pentanone with potassium permanganate.

3. Predict the product of the oxidation of cyclohexanone with pyridinium