11.2 Preparation of Benzoic Acids

In the previous section, we explored the acidity of carboxylic acids. Now, we will learn a practical method for preparing a specific and important carboxylic acid: benzoic acid (C₆H₅COOH). Benzoic acid is widely used as a food preservative and as an intermediate in the synthesis of many other organic compounds. One of the most common methods for its preparation is the oxidation of alkylbenzenes, such as toluene (methylbenzene).

Definition

• Benzoic Acid: An aromatic carboxylic acid with the formula C₆H₅COOH. It consists of a benzene ring with a carboxyl group (-COOH) attached.
• Alkylbenzene: A benzene ring with one or more alkyl groups attached (e.g., toluene, C₆H₅CH₃).
• Oxidation: A reaction in which a substance loses electrons (or gains oxygen / loses hydrogen). In this context, the alkyl side chain is oxidized to a carboxyl group.

Simple Explanation: Benzoic acid can be made by oxidizing the methyl group of toluene using hot alkaline potassium permanganate (KMnO₄). The methyl group is converted into a carboxyl group (-COOH).

Main Idea

The main idea is that benzoic acid can be prepared by the oxidation of an alkylbenzene (like toluene) when treated with hot alkaline potassium permanganate (KMnO₄). The mixture is then hydrolyzed with dilute acid to yield benzoic acid. The alkyl side chain is oxidized to a carboxyl group, while the aromatic ring remains intact.

Core Concept

The core concept is the oxidation of the benzylic carbon. The carbon atom directly attached to the benzene ring (the benzylic carbon) is more reactive than other aliphatic carbons because the resulting radical or cation is stabilized by resonance with the aromatic ring. Hot alkaline KMnO₄ is a strong oxidizing agent that can convert the benzylic carbon of an alkyl group into a carboxyl group (-COOH). The reaction is carried out under alkaline conditions, and the final product is acidified (hydrolyzed) with dilute acid to obtain the free carboxylic acid.

Detailed Explanation

The textbook states that benzoic acid can be prepared by the oxidation of alkyl benzene, when treated with hot alkaline KMnO₄. The mixture is finally hydrolyzed with dilute acid.

The Reaction

Starting Material: Toluene (methylbenzene), C₆H₅CH₃. Reagents: Hot alkaline potassium permanganate (KMnO₄ + OH^-), followed by dilute acid (H₃O⁺). Product: Benzoic acid, C₆H₅COOH.

The overall reaction is: C₆H₅CH₃ + 3[O] Alkaline KMnO₄→ C₆H₅COOH + H₂O

Mechanism Overview

The oxidation of the methyl group to a carboxyl group proceeds through several steps, ultimately converting the -CH₃ group to a -COOH group. The key steps involve:

• Oxidation of the Benzylic Carbon: The benzylic C-H bond is broken, and the carbon is oxidized. The intermediate species (radical or cation) is stabilized by the aromatic ring.
• Formation of Benzoic Acid Salt: Under alkaline conditions, the product is the benzoate ion (C₆H₅COO^-).
• Hydrolysis (Acidification): The reaction mixture is then acidified with dilute acid to convert the benzoate ion into benzoic acid (C₆H₅COOH).

Generalization

The reaction is general for alkylbenzenes. Any alkyl side chain that has at least one benzylic hydrogen can be oxidized to a carboxyl group. The alkyl chain is cleaved at the benzylic position. If the alkyl group is larger than a methyl group, the entire alkyl chain is oxidized to a carboxyl group (except the benzylic carbon, which becomes the carboxyl carbon).

Example: Oxidation of ethylbenzene yields benzoic acid. C₆H₅CH₂CH₃ Alkaline KMnO₄→ C₆H₅COOH

Important Notes

• The aromatic ring is resistant to oxidation by KMnO₄ under these conditions; only the alkyl side chain is oxidized.
• The use of hot alkaline KMnO₄ is crucial. Cold or neutral KMnO₄ may not fully oxidize the side chain.
• The final acidification step is necessary to protonate the benzoate ion and obtain the free benzoic acid.

Intermediate Level

Benzoic acid can be prepared by the oxidation of toluene using hot alkaline potassium permanganate (KMnO₄). The oxidation converts the methyl group (CH₃) of toluene into a carboxyl group (COOH), while the aromatic ring remains intact. The reaction is carried out under alkaline conditions, and the product is the benzoate ion, which is then acidified with dilute acid to yield benzoic acid. This method is general for alkylbenzenes, where the alkyl side chain is oxidized to a carboxyl group. The benzylic carbon (the carbon directly attached to the ring) is the site of oxidation.

Important Points

• - Reaction: Oxidation of alkylbenzene (e.g., toluene) to benzoic acid.
• - Reagents: Hot alkaline KMnO₄, followed by dilute acid.
• - Product: Benzoic acid (C₆H₅COOH).
• - Site of Oxidation: The benzylic carbon (the carbon attached to the benzene ring).
• - Aromatic Ring: Remains intact.
• - General Reaction: Any alkylbenzene can be oxidized to benzoic acid.
• - Equation: C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O.

Remember

• - Benzoic acid from toluene using hot alkaline KMnO₄.
• - Side chain oxidation: CH₃ → COOH.
• - Aromatic ring remains intact.
• - Alkaline conditions followed by acidification.
• - General for alkylbenzenes.
• - Hot alkaline KMnO₄ is the oxidizing agent.
• - Benzylic carbon is oxidized.

Common Mistakes

• - Forgetting the acidification step. The reaction produces the benzoate ion (under alkaline conditions), which must be acidified to obtain the free benzoic acid.
• - Thinking that the aromatic ring is oxidized. Under these conditions, the ring is resistant to oxidation; only the side chain is oxidized.
• - Using cold or neutral KMnO₄. The reaction requires hot alkaline conditions to fully oxidize the side chain.
• - Assuming that only methyl groups can be oxidized. Any alkyl group with at least one benzylic hydrogen can be oxidized to a carboxyl group.
• - Not recognizing the role of the benzylic carbon. The carbon directly attached to the ring is the one that gets oxidized.

Quick Revision

Benzoic acid can be prepared by the oxidation of an alkylbenzene, such as toluene, using hot alkaline potassium permanganate (KMnO₄). The alkyl side chain (e.g., the methyl group) is oxidized to a carboxyl group (-COOH), while the aromatic ring remains intact. The reaction is carried out under alkaline conditions, producing the benzoate ion, which is then acidified with dilute acid to yield benzoic acid. This method is general for alkylbenzenes and is a useful way to synthesize aromatic carboxylic acids.

60-Second Revision

Benzoic acid (C₆H₅COOH) is prepared by the oxidation of toluene with hot alkaline KMnO₄. The methyl group is oxidized to a carboxyl group, while the benzene ring remains intact. The reaction is followed by acidification to obtain the free acid. This method is general for alkylbenzenes.

Frequently Asked Questions (FAQs)

Q1. How is benzoic acid prepared from toluene? Answer: Benzoic acid is prepared by the oxidation of toluene using hot alkaline potassium permanganate (KMnO₄). The methyl group of toluene is oxidized to a carboxyl group, and the aromatic ring remains intact. The reaction mixture is then acidified with dilute acid to yield benzoic acid: C₆H₅CH₃ + 3[O] Alkaline KMnO₄→ C₆H₅COOH + H₂O

Q2. Why is the reaction carried out under alkaline conditions? Answer: The reaction is carried out under alkaline conditions because KMnO₄ is a stronger oxidant in alkaline solution. The alkaline conditions also help to dissolve the organic compounds and facilitate the oxidation of the side chain. The product is initially formed as the benzoate ion, which is then acidified to obtain benzoic acid.

Q3. What is the role of the acidification step? Answer: The acidification step (adding dilute acid) is necessary to convert the benzoate ion (formed under alkaline conditions) into the free benzoic acid. The acid protonates the carboxylate group: C₆H₅COO^- + H⁺ → C₆H₅COOH

Q4. Can other alkylbenzenes be oxidized to benzoic acid? Answer: Yes, the reaction is general for alkylbenzenes. Any alkyl side chain with at least one benzylic hydrogen can be oxidized to a carboxyl group. For example, ethylbenzene can be oxidized to benzoic acid: C₆H₅CH₂CH₃ Alkaline KMnO₄→ C₆H₅COOH

Q5. Why is the aromatic ring resistant to oxidation? Answer: The aromatic ring is resistant to oxidation because of its high resonance energy (aromatic stability). The delocalized pi electron system makes the ring thermodynamically stable and kinetically inert to oxidation under these conditions. Only the alkyl side chain is oxidized.

Q6. What is the site of oxidation in the alkylbenzene? Answer: The site of oxidation is the benzylic carbon—the carbon atom directly attached to the benzene ring. This carbon is more reactive because the intermediate radical or cation formed during oxidation is stabilized by resonance with the aromatic ring.

Q7. What is the oxidizing agent used in the preparation of benzoic acid? Answer: The oxidizing agent is hot alkaline potassium permanganate (KMnO₄). It is a strong oxidant that can convert the alkyl side chain to a carboxyl group.

• Q8. Write the chemical equation for the oxidation of toluene to benzoic acid.
• Answer: The overall equation is:
• C₆H₅CH₃ + 3[O] Alkaline KMnO₄→ C₆H₅COOH + H₂O

• Q9. What is the product of the oxidation of ethylbenzene?
• Answer: The product is benzoic acid (C₆H₅COOH). The ethyl group is oxidized to a carboxyl group.

Multiple Choice Questions (MCQs)

• Q1. Which reagent is used to convert toluene to benzoic acid?
• A. KMnO₄ and dilute acid
• B. Fehling's solution
• C. PCl₅
• D. SOCl₂
• Correct Answer: A
• Explanation: The reagent is hot alkaline KMnO₄ followed by dilute acid.

• Q2. What is the main product of the reaction between toluene and hot alkaline KMnO₄?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q3. What is the role of the acidification step in the preparation of benzoic acid?
• A. To oxidize the side chain.
• B. To convert the benzoate ion into benzoic acid.
• C. To reduce the benzoic acid.
• D. To neutralize the solution.
• Correct Answer: B
• Explanation: The acidification step converts the benzoate ion into benzoic acid.

• Q4. Which of the following is the correct equation for the oxidation of toluene to benzoic acid?
• A. C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O
• B. C₆H₅CH₃ + O₂ → C₆H₅COOH
• C. C₆H₅CH₃ + H₂O → C₆H₅COOH + H₂
• D. C₆H₅CH₃ + CO₂ → C₆H₅COOH
• Correct Answer: A
• Explanation: The correct equation is C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O.

• Q5. What is the site of oxidation in toluene when it is converted to benzoic acid?
• A. The aromatic ring
• B. The methyl carbon
• C. The benzylic carbon
• D. The hydrogen atoms
• Correct Answer: C
• Explanation: The benzylic carbon (the carbon attached to the ring) is the site of oxidation.

• Q6. Why is the aromatic ring resistant to oxidation by KMnO₄?
• A. Because it is electron-rich.
• B. Because it has high resonance energy.
• C. Because it is non-polar.
• D. Because it is a solid.
• Correct Answer: B
• Explanation: The aromatic ring is resistant due to its high resonance energy.

• Q7. Which of the following alkylbenzenes can be oxidized to benzoic acid?
• A. Toluene
• B. Ethylbenzene
• C. Propylbenzene
• D. All of the above
• Correct Answer: D
• Explanation: All alkylbenzenes with at least one benzylic hydrogen can be oxidized to benzoic acid.

• Q8. What is the product of the oxidation of ethylbenzene?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q9. What is the role of KMnO₄ in the reaction?
• A. It acts as a reducing agent.
• B. It acts as an oxidizing agent.
• C. It acts as a catalyst.
• D. It acts as a solvent.
• Correct Answer: B
• Explanation: KMnO₄ is the oxidizing agent.

• Q10. What is the formula of benzoic acid?
• A. C₆H₅OH
• B. C₆H₅COOH
• C. C₆H₅CHO
• D. C₆H₅CH₃
• Correct Answer: B
• Explanation: The formula of benzoic acid is C₆H₅COOH.

Multiple Choice Questions (MCQs)

• Q1. Which reagent is used to convert toluene to benzoic acid?
• A. KMnO₄ and dilute acid
• B. Fehling's solution
• C. PCl₅
• D. SOCl₂
• Correct Answer: A
• Explanation: Hot alkaline KMnO₄ followed by dilute acid is used.

• Q2. What is the main product of the reaction between toluene and hot alkaline KMnO₄?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q3. What is the role of the acidification step in the preparation of benzoic acid?
• A. To oxidize the side chain.
• B. To convert the benzoate ion into benzoic acid.
• C. To reduce the benzoic acid.
• D. To neutralize the solution.
• Correct Answer: B
• Explanation: The acidification step converts the benzoate ion into benzoic acid.

• Q4. Which of the following is the correct equation for the oxidation of toluene to benzoic acid?
• A. C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O
• B. C₆H₅CH₃ + O₂ → C₆H₅COOH
• C. C₆H₅CH₃ + H₂O → C₆H₅COOH + H₂
• D. C₆H₅CH₃ + CO₂ → C₆H₅COOH
• Correct Answer: A
• Explanation: The correct equation is C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O.

• Q5. What is the site of oxidation in toluene when it is converted to benzoic acid?
• A. The aromatic ring
• B. The methyl carbon
• C. The benzylic carbon
• D. The hydrogen atoms
• Correct Answer: C
• Explanation: The benzylic carbon is the site of oxidation.

• Q6. Why is the aromatic ring resistant to oxidation by KMnO₄?
• A. Because it is electron-rich.
• B. Because it has high resonance energy.
• C. Because it is non-polar.
• D. Because it is a solid.
• Correct Answer: B
• Explanation: The aromatic ring is resistant due to its high resonance energy.

• Q7. Which of the following alkylbenzenes can be oxidized to benzoic acid?
• A. Toluene
• B. Ethylbenzene
• C. Propylbenzene
• D. All of the above
• Correct Answer: D
• Explanation: All alkylbenzenes can be oxidized to benzoic acid.

• Q8. What is the product of the oxidation of ethylbenzene?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q9. What is the role of KMnO₄ in the reaction?
• A. It acts as a reducing agent.
• B. It acts as an oxidizing agent.
• C. It acts as a catalyst.
• D. It acts as a solvent.
• Correct Answer: B
• Explanation: KMnO₄ is the oxidizing agent.

• Q10. What is the formula of benzoic acid?
• A. C₆H₅OH
• B. C₆H₅COOH
• C. C₆H₅CHO
• D. C₆H₅CH₃
• Correct Answer: B
• Explanation: The formula of benzoic acid is C₆H₅COOH.

Multiple Choice Questions (MCQs)

• Q1. What is the starting material for the preparation of benzoic acid from alkylbenzene?
• A. Toluene
• B. Benzene
• C. Phenol
• D. Aniline
• Correct Answer: A
• Explanation: Toluene is the starting material.

• Q2. What is the oxidizing agent used in the preparation of benzoic acid?
• A. KMnO₄
• B. K₂Cr₂O₇
• C. H₂SO₄
• D. NaOH
• Correct Answer: A
• Explanation: KMnO₄ is the oxidizing agent.

• Q3. What is the condition required for the oxidation of toluene to benzoic acid?
• A. Cold and acidic
• B. Hot and alkaline
• C. Cold and neutral
• D. Hot and acidic
• Correct Answer: B
• Explanation: Hot alkaline conditions are required.

• Q4. What is the product of the oxidation of toluene after acidification?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q5. Which part of the toluene molecule is oxidized?
• A. The aromatic ring
• B. The methyl group
• C. The hydrogen atoms
• D. The carbon-carbon bonds
• Correct Answer: B
• Explanation: The methyl group is oxidized.

• Q6. What is the intermediate formed during the oxidation of toluene under alkaline conditions?
• A. Benzoate ion
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The intermediate is the benzoate ion.

• Q7. What is the role of the acidification step?
• A. To convert the benzoate ion to benzoic acid.
• B. To oxidize the methyl group.
• C. To reduce the benzoic acid.
• D. To neutralize the solution.
• Correct Answer: A
• Explanation: The acidification step converts the benzoate ion to benzoic acid.

• Q8. Which of the following is an alkylbenzene?
• A. Toluene
• B. Phenol
• C. Aniline
• D. Benzoic acid
• Correct Answer: A
• Explanation: Toluene is an alkylbenzene.

• Q9. What is the formula of benzoic acid?
• A. C₆H₅OH
• B. C₆H₅COOH
• C. C₆H₅CHO
• D. C₆H₅CH₃
• Correct Answer: B
• Explanation: The formula is C₆H₅COOH.

• Q10. What is the product of the oxidation of ethylbenzene?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

Multiple Choice Questions (MCQs)

• Q1. What is the starting material for the preparation of benzoic acid from alkylbenzene?
• A. Toluene
• B. Benzene
• C. Phenol
• D. Aniline
• Correct Answer: A
• Explanation: Toluene is the starting material.

• Q2. What is the oxidizing agent used in the preparation of benzoic acid?
• A. KMnO₄
• B. K₂Cr₂O₇
• C. H₂SO₄
• D. NaOH
• Correct Answer: A
• Explanation: KMnO₄ is the oxidizing agent.

• Q3. What is the condition required for the oxidation of toluene to benzoic acid?
• A. Cold and acidic
• B. Hot and alkaline
• C. Cold and neutral
• D. Hot and acidic
• Correct Answer: B
• Explanation: Hot alkaline conditions are required.

• Q4. What is the product of the oxidation of toluene after acidification?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q5. Which part of the toluene molecule is oxidized?
• A. The aromatic ring
• B. The methyl group
• C. The hydrogen atoms
• D. The carbon-carbon bonds
• Correct Answer: B
• Explanation: The methyl group is oxidized.

• Q6. What is the intermediate formed during the oxidation of toluene under alkaline conditions?
• A. Benzoate ion
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The intermediate is the benzoate ion.

• Q7. What is the role of the acidification step?
• A. To convert the benzoate ion to benzoic acid.
• B. To oxidize the methyl group.
• C. To reduce the benzoic acid.
• D. To neutralize the solution.
• Correct Answer: A
• Explanation: The acidification step converts the benzoate ion to benzoic acid.

• Q8. Which of the following is an alkylbenzene?
• A. Toluene
• B. Phenol
• C. Aniline
• D. Benzoic acid
• Correct Answer: A
• Explanation: Toluene is an alkylbenzene.

• Q9. What is the formula of benzoic acid?
• A. C₆H₅OH
• B. C₆H₅COOH
• C. C₆H₅CHO
• D. C₆H₅CH₃
• Correct Answer: B
• Explanation: The formula is C₆H₅COOH.

• Q10. What is the product of the oxidation of ethylbenzene?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

Multiple Choice Questions (MCQs)

• Q1. What is the starting material for the preparation of benzoic acid from alkylbenzene?
• A. Toluene
• B. Benzene
• C. Phenol
• D. Aniline
• Correct Answer: A
• Explanation: Toluene is the starting material.

• Q2. What is the oxidizing agent used in the preparation of benzoic acid?
• A. KMnO₄
• B. K₂Cr₂O₇
• C. H₂SO₄
• D. NaOH
• Correct Answer: A
• Explanation: KMnO₄ is the oxidizing agent.

• Q3. What is the condition required for the oxidation of toluene to benzoic acid?
• A. Cold and acidic
• B. Hot and alkaline
• C. Cold and neutral
• D. Hot and acidic
• Correct Answer: B
• Explanation: Hot alkaline conditions are required.

• Q4. What is the product of the oxidation of toluene after acidification?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.

• Q5. Which part of the toluene molecule is oxidized?
• A. The aromatic ring
• B. The methyl group
• C. The hydrogen atoms
• D. The carbon-carbon bonds
• Correct Answer: B
• Explanation: The methyl group is oxidized.

• Q6. What is the intermediate formed during the oxidation of toluene under alkaline conditions?
• A. Benzoate ion
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The intermediate is the benzoate ion.

• Q7. What is the role of the acidification step?
• A. To convert the benzoate ion to benzoic acid.
• B. To oxidize the methyl group.
• C. To reduce the benzoic acid.
• D. To neutralize the solution.
• Correct Answer: A
• Explanation: The acidification step converts the benzoate ion to benzoic acid.

• Q8. Which of the following is an alkylbenzene?
• A. Toluene
• B. Phenol
• C. Aniline
• D. Benzoic acid
• Correct Answer: A
• Explanation: Toluene is an alkylbenzene.

• Q9. What is the formula of benzoic acid?
• A. C₆H₅OH
• B. C₆H₅COOH
• C. C₆H₅CHO
• D. C₆H₅CH₃
• Correct Answer: B
• Explanation: The formula is C₆H₅COOH.

• Q10. What is the product of the oxidation of ethylbenzene?
• A. Benzoic acid
• B. Benzyl alcohol
• C. Benzaldehyde
• D. Phenol
• Correct Answer: A
• Explanation: The product is benzoic acid.