11.4.4 Comparing Strengths of Carboxylic acids, Phenols and Alcohols

The relative strengths of weak acids are compared by their pK_a values. The smaller the pK_a value, the stronger the acid. The pK_a value of an acid depends on the stability of its conjugate base it forms after donating a proton.

CH₃COOH ⇌ CH₃COO^- + H⁺ pK_a = 4.76 \] \[ C₆H₅OH ⇌ C₆H₅O^- + H⁺ pK_a = 10 \] \[ CH₃CH₂OH ⇌ CH₃CH₂O^- + H⁺ pK_a ≈ 16

• Ethanoate ion is stabilized due to resonance. It spreads over the whole of the COO group between the two electronegative atoms.
• Phenoxide ion is also stabilized due to resonance. A negative charge spreads over the ring but between less electronegative atoms. So, it is less stable than ethanoate ion. Phenols are, therefore, less acidic than carboxylic acids.
• Ethoxide ion is not resonance stabilized. Therefore, ethanol is the weakest acid.

Intermediate Level

Carboxylic acids undergo three main types of reactions based on the carboxyl group. The acidic proton can be removed by bases to form salts. The -OH group can be replaced by nucleophiles to form derivatives: acyl chlorides (using SOCl₂, PCl₃, or PCl₅), esters (with alcohols), and amides (with ammonia). The entire carboxyl group can be modified by reduction (to primary alcohols) or decarboxylation. The relative acidities of carboxylic acids, phenols, and alcohols can be explained by the stability of their conjugate bases. The carboxylate ion is resonance-stabilized (charge delocalized over two oxygen atoms), making carboxylic acids the strongest. The phenoxide ion is also resonance-stabilized but less so (charge delocalized over carbon atoms), making phenols weaker acids. The ethoxide ion is not resonance-stabilized, making ethanol the weakest acid.

Important Points

• - Acidic Reactions: Loss of H⁺ from -OH to form salts.
• - Substitution Reactions: Replacement of -OH to form derivatives.
• - Acyl chlorides: SOCl₂, PCl₃, PCl₅.
• - Esters: Alcohols + acid catalyst.
• - Amides: Ammonia or amines.
• - Oxidation Reactions: Methanoic acid and ethanedioic acid can be oxidized.
• - Acidity Order: Carboxylic acids > Phenols > Alcohols.
• - Reason: Stability of the conjugate base (resonance).
• - pK_a Values: Carboxylic acids (~5) < Phenols (~10) < Alcohols (~16).

Remember

• - Carboxylic acids are the strongest of the three (carboxylic acids, phenols, alcohols).
• - pK_a order: Carboxylic acid (~5) < Phenol (~10) < Alcohol (~16).
• - Resonance stabilization of the conjugate base determines acidity.
• - Substitution reactions form acyl chlorides, esters, and amides.
• - Oxidation of methanoic acid gives CO₂ and H₂O.
• - Oxidation of ethanedioic acid gives CO₂.

Common Mistakes

• - Thinking that all carboxylic acids are equally acidic. The acidity depends on the substituents (electron-withdrawing groups increase acidity).
• - Forgetting that the -OH group is a poor leaving group. It must be converted to a better leaving group (e.g., -Cl) for substitution to occur.
• - Confusing the oxidation of methanoic acid and ethanedioic acid. Methanoic acid is oxidized to CO₂ and H₂O; ethanedioic acid is oxidized to CO₂.
• - Assuming that carboxylic acids can be oxidized easily. Only methanoic acid and ethanedioic acid are readily oxidized; most carboxylic acids are resistant.
• - Not recognizing the difference between acidic reactions and substitution reactions. Acidic reactions involve the H of -OH; substitution reactions involve replacing the -OH.

Quick Revision

Carboxylic acids undergo three main types of reactions: acidic reactions (loss of H⁺ from -OH), substitution reactions (replacement of -OH to form derivatives such as acyl chlorides, esters, and amides), and whole-group reactions (oxidation, reduction, decarboxylation). The relative acidities of carboxylic acids, phenols, and alcohols can be explained by the stability of their conjugate bases. Carboxylic acids are the strongest because the carboxylate ion is resonance-stabilized by two electronegative oxygen atoms. Phenols are weaker because the phenoxide ion is less stabilized by resonance. Alcohols are the weakest because the alkoxide ion is not resonance-stabilized. The pK_a values reflect this order: carboxylic acids (~5) < phenols (~10) < alcohols (~16).

60-Second Revision

Carboxylic acids undergo three main reactions: acidic reactions (loss of H⁺), substitution reactions (replacement of -OH to form derivatives), and whole-group reactions (oxidation, reduction). The acidity order is Carboxylic acids > Phenols > Alcohols, due to the resonance stabilization of the carboxylate ion. The pK_a values are: carboxylic acids (~5), phenols (~10), alcohols (~16).

Frequently Asked Questions (FAQs)

Q1. Why are carboxylic acids more acidic than phenols and alcohols? Answer: Carboxylic acids are more acidic than phenols and alcohols because the carboxylate ion (the conjugate base) is more stable. The negative charge on the carboxylate ion is delocalized over two electronegative oxygen atoms through resonance, making it more stable. The phenoxide ion is also resonance-stabilized, but the negative charge is delocalized over less electronegative carbon atoms, so it is less stable. The alkoxide ion (from alcohols) is not resonance-stabilized, so it is the least stable. The greater the stability of the conjugate base, the stronger the acid.

Q2. What are the products of the reaction of a carboxylic acid with SOCl₂? Answer: The reaction of a carboxylic acid with thionyl chloride (SOCl₂) produces an acyl chloride, sulfur dioxide (SO₂), and hydrogen chloride (HCl): R-COOH + SOCl₂ → R-COCl + SO₂ + HCl

• Q3. How is methanoic acid oxidized by Tollens' reagent?
• Answer: Methanoic acid is oxidized by Tollens' reagent to carbon dioxide and water, with the formation of a silver mirror:
• HCOOH + 2OH^- + 2Ag(NH₃)₂⁺ → CO₂ + 2H₂O + 2Ag↓ + 4NH₃

• Q4. What is the product of the oxidation of ethanedioic acid with warm acidified KMnO₄?
• Answer: Ethanedioic acid is oxidized to carbon dioxide by warm acidified potassium manganate(VII):
• (COOH)₂ + 2KMnO₄ + 3H₂SO₄ → 10CO₂ + 2MnSO₄ + K₂SO₄ + 8H₂O
• The purple colour of KMnO₄ changes to a light pink due to the formation of Mn²⁺.

Q5. Compare the strengths of carboxylic acids, phenols, and alcohols. Answer: The strengths of the acids follow the order: Carboxylic acids > Phenols > Alcohols. This is due to the stability of their conjugate bases: the carboxylate ion is resonance-stabilized over two oxygen atoms, the phenoxide ion is resonance-stabilized over the ring but less effectively, and the alkoxide ion is not resonance-stabilized. The pK_a values are approximately: carboxylic acids ~5, phenols ~10, alcohols ~16.

• Q6. What is the product of the reaction of a carboxylic acid with PCl₅?
• Answer: The reaction produces an acyl chloride, phosphorus oxychloride (POCl₃), and hydrogen chloride (HCl):
• R-COOH + PCl₅ → R-COCl + POCl₃ + HCl

• Q7. How does methanoic acid react with Fehling's solution?
• Answer: Methanoic acid is oxidized by Fehling's solution to carbon dioxide and water, with the formation of a red precipitate of copper(I) oxide:
• HCOOH + 4OH^- + 2Cu²⁺ → CO₂ + 3H₂O + Cu₂O↓

Q8. Why is the -OH group of a carboxylic acid a poor leaving group? Answer: The -OH group is a poor leaving group because the hydroxide ion (OH^-) is a strong base and is not stable on its own. In substitution reactions, the -OH group must be protonated (to form -OH₂⁺) or converted into a better leaving group (e.g., -Cl in acyl chlorides) before it can be displaced by a nucleophile.

• Acidic reactions: Involving the H atom of the -OH group (salt formation).
• Substitution reactions: Involving the replacement of the -OH group (formation of acyl chlorides, esters, amides).
• Whole-group reactions: Involving the entire carboxyl group (reduction, decarboxylation, oxidation).

Multiple Choice Questions (MCQs)

• Q1. Which of the following is the strongest acid?
• A. Ethanoic acid
• B. Phenol
• C. Ethanol
• D. Water
• Correct Answer: A
• Explanation: Carboxylic acids are the strongest acids among the given options due to the resonance stabilization of the carboxylate ion.

• Q2. What is the product of the reaction of a carboxylic acid with SOCl₂?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q3. What is the product of the oxidation of methanoic acid with Tollens' reagent?
• A. Carbon dioxide and water
• B. Carbon monoxide and water
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Methanoic acid is oxidized to carbon dioxide and water.

• Q4. What is the product of the oxidation of ethanedioic acid with warm acidified KMnO₄?
• A. Carbon dioxide
• B. Carbon monoxide
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Ethanedioic acid is oxidized to carbon dioxide.

• Q5. Which of the following is the correct order of acid strength?
• A. Alcohols > Phenols > Carboxylic acids
• B. Carboxylic acids > Phenols > Alcohols
• C. Phenols > Carboxylic acids > Alcohols
• D. Carboxylic acids > Alcohols > Phenols
• Correct Answer: B
• Explanation: The correct order is Carboxylic acids > Phenols > Alcohols.

• Q6. What is the product of the reaction of a carboxylic acid with PCl₅?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q7. What is the role of the -OH group in a carboxylic acid?
• A. It is a good leaving group.
• B. It is a poor leaving group.
• C. It is a strong base.
• D. It is a strong acid.
• Correct Answer: B
• Explanation: The -OH group is a poor leaving group.

• Q8. Which of the following reactions is an oxidation of a carboxylic acid?
• A. Reaction with SOCl₂
• B. Reaction with PCl₅
• C. Reaction with Tollens' reagent
• D. Reaction with NaOH
• Correct Answer: C
• Explanation: The reaction with Tollens' reagent is an oxidation.

• Q9. What is the product of the reaction of methanoic acid with Fehling's solution?
• A. Carbon dioxide and water
• B. Carbon monoxide and water
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Methanoic acid is oxidized to carbon dioxide and water.

• Q10. Why is ethanol the weakest acid among carboxylic acids, phenols, and alcohols?
• A. Because it has a higher molecular weight.
• B. Because the ethoxide ion is not resonance-stabilized.
• C. Because it has a lower boiling point.
• D. Because it is a liquid at room temperature.
• Correct Answer: B
• Explanation: The ethoxide ion is not resonance-stabilized, making ethanol the weakest acid.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is the strongest acid?
• A. Ethanoic acid
• B. Phenol
• C. Ethanol
• D. Water
• Correct Answer: A
• Explanation: Carboxylic acids are the strongest acids among the given options.

• Q2. What is the product of the reaction of a carboxylic acid with SOCl₂?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q3. What is the product of the oxidation of methanoic acid with Tollens' reagent?
• A. Carbon dioxide and water
• B. Carbon monoxide and water
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Methanoic acid is oxidized to carbon dioxide and water.

• Q4. What is the product of the oxidation of ethanedioic acid with warm acidified KMnO₄?
• A. Carbon dioxide
• B. Carbon monoxide
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Ethanedioic acid is oxidized to carbon dioxide.

• Q5. Which of the following is the correct order of acid strength?
• A. Alcohols > Phenols > Carboxylic acids
• B. Carboxylic acids > Phenols > Alcohols
• C. Phenols > Carboxylic acids > Alcohols
• D. Carboxylic acids > Alcohols > Phenols
• Correct Answer: B
• Explanation: The correct order is Carboxylic acids > Phenols > Alcohols.

• Q6. What is the product of the reaction of a carboxylic acid with PCl₅?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q7. What is the role of the -OH group in a carboxylic acid?
• A. It is a good leaving group.
• B. It is a poor leaving group.
• C. It is a strong base.
• D. It is a strong acid.
• Correct Answer: B
• Explanation: The -OH group is a poor leaving group.

• Q8. Which of the following reactions is an oxidation of a carboxylic acid?
• A. Reaction with SOCl₂
• B. Reaction with PCl₅
• C. Reaction with Tollens' reagent
• D. Reaction with NaOH
• Correct Answer: C
• Explanation: The reaction with Tollens' reagent is an oxidation.

• Q9. What is the product of the reaction of methanoic acid with Fehling's solution?
• A. Carbon dioxide and water
• B. Carbon monoxide and water
• C. Methanol
• D. Formaldehyde
• Correct Answer: A
• Explanation: Methanoic acid is oxidized to carbon dioxide and water.

• Q10. Why is ethanol the weakest acid among carboxylic acids, phenols, and alcohols?
• A. Because it has a higher molecular weight.
• B. Because the ethoxide ion is not resonance-stabilized.
• C. Because it has a lower boiling point.
• D. Because it is a liquid at room temperature.
• Correct Answer: B
• Explanation: The ethoxide ion is not resonance-stabilized.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is a reaction of carboxylic acids?
• A. Acidic reaction with NaOH
• B. Substitution with SOCl₂
• C. Reduction with LiAlH₄
• D. All of the above
• Correct Answer: D
• Explanation: All are reactions of carboxylic acids.

• Q2. What is the product of the reaction of a carboxylic acid with SOCl₂?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q3. What is the product of the reaction of a carboxylic acid with an alcohol?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: B
• Explanation: The product is an ester.

• Q4. What is the product of the reaction of a carboxylic acid with ammonia?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: C
• Explanation: The product is an amide.

• Q5. What is the product of the reduction of a carboxylic acid with LiAlH₄?
• A. Primary alcohol
• B. Secondary alcohol
• C. Tertiary alcohol
• D. Aldehyde
• Correct Answer: A
• Explanation: The product is a primary alcohol.

• Q6. What is decarboxylation?
• A. Loss of CO₂
• B. Loss of H₂O
• C. Loss of H₂
• D. Loss of O₂
• Correct Answer: A
• Explanation: Decarboxylation is the loss of CO₂.

• Q7. Which of the following is a good leaving group in nucleophilic acyl substitution?
• A. OH^-
• B. Cl^-
• C. NH₂^-
• D. CH₃^-
• Correct Answer: B
• Explanation: Cl^- is a good leaving group.

• Q8. Which of the following is a poor leaving group?
• A. Cl^-
• B. Br^-
• C. OH^-
• D. I^-
• Correct Answer: C
• Explanation: OH^- is a poor leaving group.

• Q9. What is the role of the carbonyl carbon in the reactions of carboxylic acids?
• A. It acts as a nucleophile.
• B. It acts as an electrophile.
• C. It acts as a base.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The carbonyl carbon is electrophilic.

• Q10. What is the product of the reaction of a carboxylic acid with PCl₅?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is a reaction of carboxylic acids?
• A. Acidic reaction with NaOH
• B. Substitution with SOCl₂
• C. Reduction with LiAlH₄
• D. All of the above
• Correct Answer: D
• Explanation: All are reactions of carboxylic acids.

• Q2. What is the product of the reaction of a carboxylic acid with SOCl₂?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q3. What is the product of the reaction of a carboxylic acid with an alcohol?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: B
• Explanation: The product is an ester.

• Q4. What is the product of the reaction of a carboxylic acid with ammonia?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: C
• Explanation: The product is an amide.

• Q5. What is the product of the reduction of a carboxylic acid with LiAlH₄?
• A. Primary alcohol
• B. Secondary alcohol
• C. Tertiary alcohol
• D. Aldehyde
• Correct Answer: A
• Explanation: The product is a primary alcohol.

• Q6. What is decarboxylation?
• A. Loss of CO₂
• B. Loss of H₂O
• C. Loss of H₂
• D. Loss of O₂
• Correct Answer: A
• Explanation: Decarboxylation is the loss of CO₂.

• Q7. Which of the following is a good leaving group in nucleophilic acyl substitution?
• A. OH^-
• B. Cl^-
• C. NH₂^-
• D. CH₃^-
• Correct Answer: B
• Explanation: Cl^- is a good leaving group.

• Q8. Which of the following is a poor leaving group?
• A. Cl^-
• B. Br^-
• C. OH^-
• D. I^-
• Correct Answer: C
• Explanation: OH^- is a poor leaving group.

• Q9. What is the role of the carbonyl carbon in the reactions of carboxylic acids?
• A. It acts as a nucleophile.
• B. It acts as an electrophile.
• C. It acts as a base.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The carbonyl carbon is electrophilic.

• Q10. What is the product of the reaction of a carboxylic acid with PCl₅?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is a reaction of carboxylic acids?
• A. Acidic reaction with NaOH
• B. Substitution with SOCl₂
• C. Reduction with LiAlH₄
• D. All of the above
• Correct Answer: D
• Explanation: All are reactions of carboxylic acids.

• Q2. What is the product of the reaction of a carboxylic acid with SOCl₂?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.

• Q3. What is the product of the reaction of a carboxylic acid with an alcohol?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: B
• Explanation: The product is an ester.

• Q4. What is the product of the reaction of a carboxylic acid with ammonia?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: C
• Explanation: The product is an amide.

• Q5. What is the product of the reduction of a carboxylic acid with LiAlH₄?
• A. Primary alcohol
• B. Secondary alcohol
• C. Tertiary alcohol
• D. Aldehyde
• Correct Answer: A
• Explanation: The product is a primary alcohol.

• Q6. What is decarboxylation?
• A. Loss of CO₂
• B. Loss of H₂O
• C. Loss of H₂
• D. Loss of O₂
• Correct Answer: A
• Explanation: Decarboxylation is the loss of CO₂.

• Q7. Which of the following is a good leaving group in nucleophilic acyl substitution?
• A. OH^-
• B. Cl^-
• C. NH₂^-
• D. CH₃^-
• Correct Answer: B
• Explanation: Cl^- is a good leaving group.

• Q8. Which of the following is a poor leaving group?
• A. Cl^-
• B. Br^-
• C. OH^-
• D. I^-
• Correct Answer: C
• Explanation: OH^- is a poor leaving group.

• Q9. What is the role of the carbonyl carbon in the reactions of carboxylic acids?
• A. It acts as a nucleophile.
• B. It acts as an electrophile.
• C. It acts as a base.
• D. It acts as a catalyst.
• Correct Answer: B
• Explanation: The carbonyl carbon is electrophilic.

• Q10. What is the product of the reaction of a carboxylic acid with PCl₅?
• A. Acyl chloride
• B. Ester
• C. Amide
• D. Alcohol
• Correct Answer: A
• Explanation: The product is an acyl chloride.