ChemistryChapter 11: Carbonyl Compounds

11.1 Acidity

- Carboxylic Acid:

In this chapter on carbonyl compounds, we begin by exploring the acidity of carboxylic acids. Carboxylic acids are a class of carbonyl compounds that contain a carboxyl group (-COOH). The acidity of a carboxylic acid is determined by its ability to donate a proton (H⁺) from the -OH part of the carboxyl group. Understanding why carboxylic acids are acidic, and how structural features affect their acidity, is fundamental to organic chemistry.

Definition

Carboxylic Acid: An organic compound containing a carboxyl group (-COOH). The general formula is R-COOH.
Acidity: The tendency of a substance to donate a proton (H⁺).
K_a (Acid Dissociation Constant): A measure of acid strength. A larger K_a means a stronger acid.
pK_a: The negative logarithm of K_a. A smaller pK_a means a stronger acid.

Simple Explanation: Carboxylic acids are weak acids. They are more acidic than alcohols and phenols but less acidic than mineral acids like HCl or H₂SO₄. The acidity comes from the ability of the carboxyl group to lose a proton and form a stable carboxylate ion (the conjugate base).

Main Idea

The main idea is that carboxylic acids are acidic because the carboxylate ion (the conjugate base formed after proton loss) is stabilized by resonance. The negative charge on the carboxylate ion is delocalized over two electronegative oxygen atoms, making it more stable than the alkoxide ion (formed from alcohols) or the phenoxide ion (formed from phenols). The acidity of a carboxylic acid can be further increased by electron-withdrawing groups attached to the carbon chain, which further stabilize the carboxylate ion by inductive effect.

Core Concept

The core concept is conjugate base stability. The more stable the conjugate base, the stronger the acid. In a carboxylic acid, the conjugate base is the carboxylate ion (RCOO^-). The resonance stabilization of this ion (the negative charge is shared between two oxygen atoms) is the primary reason for the acidity of carboxylic acids. Electron-withdrawing groups (like chlorine) pull electron density away from the carboxylate ion, further stabilizing it and increasing acidity. Electron-donating groups (like alkyl groups) push electron density towards the carboxylate ion, destabilizing it and decreasing acidity.

Detailed Explanation

The textbook states that carboxylic acids are acidic simple organic compounds (pK_a ≈ 5). But they are weak acids compared to mineral acids like HCl or H₂SO₄. Remember: the lower the pK_a, the stronger the acid.

The Acid Dissociation Constant (K_a) and pK_a

The strength of a carboxylic acid is expressed by its acid dissociation constant, K_a: RCOOH ⇌ RCOO^- + H⁺ K_a = [RCOO^-][H⁺][RCOOH] A larger K_a indicates a stronger acid. For convenience, pK_a is used: pK_a = -log K_a A smaller pK_a indicates a stronger acid.

Effect of Electron-Withdrawing Groups on Acidity

The textbook provides a table (Table 11.1) of pK_a values for various organic acids to illustrate the effect of electron-withdrawing groups.

Table 11.1: pK_a values of organic acids

Name of Acid	Structure	pK_a
Ethanoic acid	CH₃CO₂H	4.7
Propanoic acid	CH₃CH₂CO₂H	4.9
Fluoroethanoic acid	CH₂FCO₂H	2.6
Chloroethanoic acid	CH₂ClCO₂H	2.9
Dichloroethanoic acid	CHCl₂CO₂H	1.3
Trichloroethanoic acid	CCl₃CO₂H	0.9
Nitroethanoic acid	O₂NCH₂CO₂H	1.7

Ethanoic acid (pK_a = 4.7) is a weak acid.
Propanoic acid (pK_a = 4.9) is slightly weaker than ethanoic acid because the ethyl group is electron-donating (destabilizes the carboxylate ion).
Chloroethanoic acid (pK_a = 2.9) is stronger than ethanoic acid because the chlorine atom is electron-withdrawing (stabilizes the carboxylate ion).
Dichloroethanoic acid (pK_a = 1.3) is stronger than chloroethanoic acid because two chlorine atoms withdraw more electron density.
Trichloroethanoic acid (pK_a = 0.9) is the strongest acid in the table because three chlorine atoms withdraw even more electron density.
Fluoroethanoic acid (pK_a = 2.6) is stronger than chloroethanoic acid because fluorine is more electronegative than chlorine.
Nitroethanoic acid (pK_a = 1.7) is also a strong acid due to the electron-withdrawing nitro group.

The Acidity Order of Chlorine-Substituted Acetic Acids: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH

The electron-withdrawing group bonded to the carbon atom next to the carboxyl group (-COOH) makes the acid stronger. Due to the electron-withdrawing group, electrons will move away from the oxygen. Hence, it decreases the charge density on the oxygen atom of the carboxylate ion, which stabilizes the group and makes it less likely to bond with an H⁺ ion. For example, chloroacetic acid is 100 times stronger acid than acetic acid.
The electron-donating group (alkyl) strengthens the O-H bond in the acid's -COOH group. It donates a negative charge towards the group of the carboxylate ion, making it more likely to accept an H⁺ ion, hence acidic strength decreases. For example, formic acid is more acidic than acetic acid.

Resonance Stabilization of the Carboxylate Ion

The carboxylate ion is stabilized by resonance. The negative charge is delocalized over the two electronegative oxygen atoms: R-C(=O)O^- ↔ R-C(O^-)=O This delocalization makes the carboxylate ion more stable than the alkoxide ion (from alcohols) or the phenoxide ion (from phenols), which is why carboxylic acids are more acidic than alcohols and phenols.

Intermediate Level

The acidity of carboxylic acids is determined by the stability of the carboxylate ion (the conjugate base). The carboxylate ion is resonance-stabilized, with the negative charge delocalized over two oxygen atoms. This makes carboxylic acids more acidic than alcohols and phenols. The acidity can be increased by electron-withdrawing groups (e.g., -Cl, -F, -NO₂), which stabilize the carboxylate ion through the inductive effect. The more electron-withdrawing groups, the stronger the acid. For example, trichloroacetic acid (pK_a = 0.9) is much stronger than acetic acid (pK_a = 4.7). Conversely, electron-donating groups (e.g., alkyl groups) destabilize the carboxylate ion and decrease acidity. The pK_a value is used to compare acid strengths: a smaller pK_a means a stronger acid.

Important Points

- Carboxylic acids: Weak acids, stronger than alcohols and phenols.
- pK_a range: Typically around 5 (e.g., ethanoic acid pK_a = 4.7).
- Resonance stabilization: The carboxylate ion is stabilized by delocalization of the negative charge over two oxygen atoms.
- Electron-withdrawing groups: Increase acidity (e.g., -Cl, -F, -NO₂).
- Electron-donating groups: Decrease acidity (e.g., alkyl groups).
- Acidity order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
- pK_a and acidity: Smaller pK_a = stronger acid.

Remember

- Carboxylic acids are acidic due to resonance stabilization of the carboxylate ion.
- pK_a is a measure of acid strength. Smaller pK_a = stronger acid.
- Electron-withdrawing groups increase acidity.
- Electron-donating groups decrease acidity.
- Chlorine substituents increase acidity (CCl₃COOH is strongest).
- Acidity order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
- Resonance delocalizes the negative charge over two oxygen atoms.

Common Mistakes

- Thinking that all carboxylic acids have the same strength. The strength depends on the substituents attached to the carboxyl group.
- Forgetting that electron-withdrawing groups increase acidity. This is because they stabilize the carboxylate ion.
- Assuming that electron-donating groups increase acidity. They actually decrease acidity by destabilizing the carboxylate ion.
- Confusing K_a and pK_a. A larger K_a means a stronger acid; a smaller pK_a means a stronger acid.
- Not recognizing the role of resonance in stabilizing the carboxylate ion. This is the primary reason for the acidity of carboxylic acids.

Quick Revision

Carboxylic acids are acidic because the carboxylate ion (the conjugate base) is stabilized by resonance, with the negative charge delocalized over two oxygen atoms. The acidity is quantified by K_a or pK_a; a smaller pK_a indicates a stronger acid. The acidity is increased by electron-withdrawing groups (e.g., -Cl, -F, -NO₂) through the inductive effect, which further stabilizes the carboxylate ion. For example, trichloroacetic acid (pK_a = 0.9) is much stronger than acetic acid (pK_a = 4.7). Electron-donating groups (e.g., alkyl groups) decrease acidity by destabilizing the carboxylate ion. The order of acidity for chlorine-substituted acetic acids is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

60-Second Revision

Carboxylic acids are acidic due to the resonance stabilization of the carboxylate ion. pK_a is a measure of acid strength (smaller pK_a = stronger acid). Electron-withdrawing groups (like -Cl, -F, -NO₂) increase acidity by stabilizing the carboxylate ion. Electron-donating groups (like alkyl) decrease acidity. The order of acidity for chlorinated acetic acids is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH. Ethanoic acid has a pK_a of 4.7, while trichloroethanoic acid has a pK_a of 0.9.

Frequently Asked Questions (FAQs)

Q1. Why are carboxylic acids acidic? Answer: Carboxylic acids are acidic because the carboxylate ion (the conjugate base formed after losing a proton) is stabilized by resonance. The negative charge on the carboxylate ion is delocalized over two electronegative oxygen atoms, making the ion more stable. This stabilization makes it easier for the carboxylic acid to donate a proton.

Q2. How does the presence of chlorine substituents affect the acidity of carboxylic acids? Answer: Chlorine is an electron-withdrawing group. It pulls electron density away from the carboxylate ion, reducing the charge density on the oxygen atoms and stabilizing the ion. This increases the acidity of the carboxylic acid. The more chlorine atoms, the stronger the acid. For example, trichloroacetic acid (pK_a = 0.9) is much stronger than acetic acid (pK_a = 4.7).

Q3. What is the relationship between K_a and pK_a?
Answer: The relationship is:
pK_a = -log K_a
A larger K_a (stronger acid) corresponds to a smaller pK_a. A smaller K_a (weaker acid) corresponds to a larger pK_a.

Q4. Which is the strongest acid: trichloroacetic acid, dichloroacetic acid, or monochloroacetic acid? Answer: Trichloroacetic acid is the strongest acid among the three. This is because it has three electron-withdrawing chlorine atoms, which stabilize the carboxylate ion the most. The pK_a values are: trichloroacetic acid (0.9) < dichloroacetic acid (1.3) < monochloroacetic acid (2.9). A smaller pK_a means a stronger acid.

Q5. Why is formic acid more acidic than acetic acid? Answer: Formic acid (HCOOH) is more acidic than acetic acid (CH₃COOH) because the methyl group in acetic acid is electron-donating. This electron-donating group destabilizes the carboxylate ion, making acetic acid less acidic. Formic acid has no such electron-donating group, so its carboxylate ion is more stable.

Q6. Explain the resonance stabilization of the carboxylate ion. Answer: The carboxylate ion has two equivalent resonance structures, where the negative charge is shared between the two oxygen atoms. The delocalization of the negative charge stabilizes the ion, making the corresponding carboxylic acid more acidic than alcohols or phenols.

Q7. What is the effect of an electron-donating group on the acidity of a carboxylic acid? Answer: An electron-donating group (e.g., an alkyl group) pushes electron density towards the carboxylate ion, intensifying the negative charge on the oxygen atoms. This destabilizes the carboxylate ion, making it less stable. Therefore, the acid is weaker (lower acidity).

Q8. How is the acidity of a carboxylic acid related to the stability of its conjugate base? Answer: The acidity of a carboxylic acid is directly related to the stability of its conjugate base (the carboxylate ion). The more stable the conjugate base, the more willing the acid is to donate a proton, and the stronger the acid. Factors that stabilize the carboxylate ion (like resonance and electron-withdrawing groups) increase acidity.

Q9. What is the pK_a of ethanoic acid?
Answer: The pK_a of ethanoic acid (acetic acid) is approximately 4.7.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid (pK_a = 4.7)
B. Chloroethanoic acid (pK_a = 2.9)
C. Dichloroethanoic acid (pK_a = 1.3)
D. Trichloroethanoic acid (pK_a = 0.9)
Correct Answer: D
Explanation: A smaller pK_a means a stronger acid. Trichloroethanoic acid has the lowest pK_a (0.9), so it is the strongest acid.

Q2. Why is trichloroethanoic acid stronger than ethanoic acid?
A. Because it has more hydrogen atoms.
B. Because chlorine is electron-withdrawing and stabilizes the carboxylate ion.
C. Because chlorine is electron-donating.
D. Because it has a higher molecular weight.
Correct Answer: B
Explanation: Chlorine is an electron-withdrawing group that stabilizes the carboxylate ion by pulling electron density away, increasing acidity.

Q3. What is the relationship between K_a and acid strength?
A. Larger K_a means weaker acid.
B. Larger K_a means stronger acid.
C. K_a has no relation to acid strength.
D. K_a is always 1.
Correct Answer: B
Explanation: A larger K_a indicates a stronger acid.

Q4. Which of the following substituents increases the acidity of a carboxylic acid?
A. Alkyl group
B. Chlorine atom
C. Hydrogen atom
D. Ethyl group
Correct Answer: B
Explanation: Chlorine is an electron-withdrawing group that increases acidity.

Q5. What is the pK_a of ethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: D
Explanation: The pK_a of ethanoic acid is 4.7.

Q6. Which of the following is the weakest acid?
A. Trichloroethanoic acid
B. Dichloroethanoic acid
C. Chloroethanoic acid
D. Ethanoic acid
Correct Answer: D
Explanation: Ethanoic acid has the highest pK_a (4.7), so it is the weakest acid among the given options.

Q7. What is the effect of an electron-donating group on the acidity of a carboxylic acid?
A. It increases acidity.
B. It decreases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Electron-donating groups destabilize the carboxylate ion, decreasing acidity.

Q8. Which of the following is the correct order of acidity?
A. CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH
B. CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
C. CHCl₂COOH > CCl₃COOH > CH₃COOH > CH₂ClCOOH
D. CH₂ClCOOH > CH₃COOH > CCl₃COOH > CHCl₂COOH
Correct Answer: B
Explanation: The correct order is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: The resonance stabilization of the carboxylate ion is the primary reason for the acidity of carboxylic acids.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid (pK_a = 4.7)
B. Chloroethanoic acid (pK_a = 2.9)
C. Dichloroethanoic acid (pK_a = 1.3)
D. Trichloroethanoic acid (pK_a = 0.9)
Correct Answer: D
Explanation: Trichloroethanoic acid has the lowest pK_a, so it is the strongest acid.

Q2. Why is trichloroethanoic acid stronger than ethanoic acid?
A. Because it has more hydrogen atoms.
B. Because chlorine is electron-withdrawing and stabilizes the carboxylate ion.
C. Because chlorine is electron-donating.
D. Because it has a higher molecular weight.
Correct Answer: B
Explanation: Chlorine stabilizes the carboxylate ion.

Q3. What is the relationship between K_a and acid strength?
A. Larger K_a means weaker acid.
B. Larger K_a means stronger acid.
C. K_a has no relation to acid strength.
D. K_a is always 1.
Correct Answer: B
Explanation: Larger K_a means stronger acid.

Q4. Which of the following substituents increases the acidity of a carboxylic acid?
A. Alkyl group
B. Chlorine atom
C. Hydrogen atom
D. Ethyl group
Correct Answer: B
Explanation: Chlorine increases acidity.

Q5. What is the pK_a of ethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: D
Explanation: The pK_a of ethanoic acid is 4.7.

Q6. Which of the following is the weakest acid?
A. Trichloroethanoic acid
B. Dichloroethanoic acid
C. Chloroethanoic acid
D. Ethanoic acid
Correct Answer: D
Explanation: Ethanoic acid is the weakest acid.

Q7. What is the effect of an electron-donating group on the acidity of a carboxylic acid?
A. It increases acidity.
B. It decreases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Electron-donating groups decrease acidity.

Q8. Which of the following is the correct order of acidity?
A. CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH
B. CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
C. CHCl₂COOH > CCl₃COOH > CH₃COOH > CH₂ClCOOH
D. CH₂ClCOOH > CH₃COOH > CCl₃COOH > CHCl₂COOH
Correct Answer: B
Explanation: The correct order is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: D
Explanation: Trichloroethanoic acid is the strongest acid.

Q2. Which of the following is the weakest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: A
Explanation: Ethanoic acid is the weakest acid.

Q3. What is the effect of adding more chlorine atoms to acetic acid?
A. It decreases acidity.
B. It increases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Adding more chlorine atoms increases acidity.

Q4. What is the pK_a of trichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: A
Explanation: The pK_a of trichloroethanoic acid is 0.9.

Q5. What is the pK_a of chloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: C
Explanation: The pK_a of chloroethanoic acid is 2.9.

Q6. What is the pK_a of dichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: B
Explanation: The pK_a of dichloroethanoic acid is 1.3.

Q7. Which of the following is an electron-withdrawing group?
A. -CH₃
B. -Cl
C. -CH₂CH₃
D. -H
Correct Answer: B
Explanation: -Cl is an electron-withdrawing group.

Q8. Which of the following is an electron-donating group?
A. -Cl
B. -F
C. -CH₃
D. -NO₂
Correct Answer: C
Explanation: -CH₃ is an electron-donating group.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: D
Explanation: Trichloroethanoic acid is the strongest acid.

Q2. Which of the following is the weakest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: A
Explanation: Ethanoic acid is the weakest acid.

Q3. What is the effect of adding more chlorine atoms to acetic acid?
A. It decreases acidity.
B. It increases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Adding more chlorine atoms increases acidity.

Q4. What is the pK_a of trichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: A
Explanation: The pK_a of trichloroethanoic acid is 0.9.

Q5. What is the pK_a of chloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: C
Explanation: The pK_a of chloroethanoic acid is 2.9.

Q6. What is the pK_a of dichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: B
Explanation: The pK_a of dichloroethanoic acid is 1.3.

Q7. Which of the following is an electron-withdrawing group?
A. -CH₃
B. -Cl
C. -CH₂CH₃
D. -H
Correct Answer: B
Explanation: -Cl is an electron-withdrawing group.

Q8. Which of the following is an electron-donating group?
A. -Cl
B. -F
C. -CH₃
D. -NO₂
Correct Answer: C
Explanation: -CH₃ is an electron-donating group.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Did you understand this topic?

Your answer is saved with your learning record. It does not mark the topic as mastered.

Quick revision

Revision Zone

Review the key facts, formulas and checks before you move on.

11.1 Acidity

Sticky Notes · Quick Revision

Key Point 1

Carboxylic acids: RCOOH.

Key Point 2

pK_a: Smaller = stronger acid.

Key Point 3

Resonance stabilizes carboxylate ion.

Key Point 4

Electron-withdrawing groups: Increase acidity.

Key Point 5

Electron-donating groups: Decrease acidity.

Key Point 6

CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Key Point 7

Ethanoic acid: pK_a = 4.7.

Key Point 8

Trichloroethanoic acid: pK_a = 0.9 (strongest in the table).

Ask Tutor about this Open in Topics

Same supplied content as this Notes page (shared canonical source).

ChemistryChapter 11: Carbonyl Compounds

11.1 Acidity

- Carboxylic Acid:

Definition

Carboxylic Acid: An organic compound containing a carboxyl group (-COOH). The general formula is R-COOH.
Acidity: The tendency of a substance to donate a proton (H⁺).
K_a (Acid Dissociation Constant): A measure of acid strength. A larger K_a means a stronger acid.
pK_a: The negative logarithm of K_a. A smaller pK_a means a stronger acid.

Main Idea

Core Concept

Detailed Explanation

The Acid Dissociation Constant (K_a) and pK_a

Effect of Electron-Withdrawing Groups on Acidity

The textbook provides a table (Table 11.1) of pK_a values for various organic acids to illustrate the effect of electron-withdrawing groups.

Table 11.1: pK_a values of organic acids

Name of Acid	Structure	pK_a
Ethanoic acid	CH₃CO₂H	4.7
Propanoic acid	CH₃CH₂CO₂H	4.9
Fluoroethanoic acid	CH₂FCO₂H	2.6
Chloroethanoic acid	CH₂ClCO₂H	2.9
Dichloroethanoic acid	CHCl₂CO₂H	1.3
Trichloroethanoic acid	CCl₃CO₂H	0.9
Nitroethanoic acid	O₂NCH₂CO₂H	1.7

Ethanoic acid (pK_a = 4.7) is a weak acid.
Propanoic acid (pK_a = 4.9) is slightly weaker than ethanoic acid because the ethyl group is electron-donating (destabilizes the carboxylate ion).
Chloroethanoic acid (pK_a = 2.9) is stronger than ethanoic acid because the chlorine atom is electron-withdrawing (stabilizes the carboxylate ion).
Dichloroethanoic acid (pK_a = 1.3) is stronger than chloroethanoic acid because two chlorine atoms withdraw more electron density.
Trichloroethanoic acid (pK_a = 0.9) is the strongest acid in the table because three chlorine atoms withdraw even more electron density.
Fluoroethanoic acid (pK_a = 2.6) is stronger than chloroethanoic acid because fluorine is more electronegative than chlorine.
Nitroethanoic acid (pK_a = 1.7) is also a strong acid due to the electron-withdrawing nitro group.

The Acidity Order of Chlorine-Substituted Acetic Acids: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH

The electron-withdrawing group bonded to the carbon atom next to the carboxyl group (-COOH) makes the acid stronger. Due to the electron-withdrawing group, electrons will move away from the oxygen. Hence, it decreases the charge density on the oxygen atom of the carboxylate ion, which stabilizes the group and makes it less likely to bond with an H⁺ ion. For example, chloroacetic acid is 100 times stronger acid than acetic acid.
The electron-donating group (alkyl) strengthens the O-H bond in the acid's -COOH group. It donates a negative charge towards the group of the carboxylate ion, making it more likely to accept an H⁺ ion, hence acidic strength decreases. For example, formic acid is more acidic than acetic acid.

Resonance Stabilization of the Carboxylate Ion

Intermediate Level

Important Points

- Carboxylic acids: Weak acids, stronger than alcohols and phenols.
- pK_a range: Typically around 5 (e.g., ethanoic acid pK_a = 4.7).
- Resonance stabilization: The carboxylate ion is stabilized by delocalization of the negative charge over two oxygen atoms.
- Electron-withdrawing groups: Increase acidity (e.g., -Cl, -F, -NO₂).
- Electron-donating groups: Decrease acidity (e.g., alkyl groups).
- Acidity order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
- pK_a and acidity: Smaller pK_a = stronger acid.

Remember

- Carboxylic acids are acidic due to resonance stabilization of the carboxylate ion.
- pK_a is a measure of acid strength. Smaller pK_a = stronger acid.
- Electron-withdrawing groups increase acidity.
- Electron-donating groups decrease acidity.
- Chlorine substituents increase acidity (CCl₃COOH is strongest).
- Acidity order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.
- Resonance delocalizes the negative charge over two oxygen atoms.

Common Mistakes

- Thinking that all carboxylic acids have the same strength. The strength depends on the substituents attached to the carboxyl group.
- Forgetting that electron-withdrawing groups increase acidity. This is because they stabilize the carboxylate ion.
- Assuming that electron-donating groups increase acidity. They actually decrease acidity by destabilizing the carboxylate ion.
- Confusing K_a and pK_a. A larger K_a means a stronger acid; a smaller pK_a means a stronger acid.
- Not recognizing the role of resonance in stabilizing the carboxylate ion. This is the primary reason for the acidity of carboxylic acids.

Quick Revision

60-Second Revision

Frequently Asked Questions (FAQs)

Q3. What is the relationship between K_a and pK_a?
Answer: The relationship is:
pK_a = -log K_a
A larger K_a (stronger acid) corresponds to a smaller pK_a. A smaller K_a (weaker acid) corresponds to a larger pK_a.

Q9. What is the pK_a of ethanoic acid?
Answer: The pK_a of ethanoic acid (acetic acid) is approximately 4.7.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid (pK_a = 4.7)
B. Chloroethanoic acid (pK_a = 2.9)
C. Dichloroethanoic acid (pK_a = 1.3)
D. Trichloroethanoic acid (pK_a = 0.9)
Correct Answer: D
Explanation: A smaller pK_a means a stronger acid. Trichloroethanoic acid has the lowest pK_a (0.9), so it is the strongest acid.

Q2. Why is trichloroethanoic acid stronger than ethanoic acid?
A. Because it has more hydrogen atoms.
B. Because chlorine is electron-withdrawing and stabilizes the carboxylate ion.
C. Because chlorine is electron-donating.
D. Because it has a higher molecular weight.
Correct Answer: B
Explanation: Chlorine is an electron-withdrawing group that stabilizes the carboxylate ion by pulling electron density away, increasing acidity.

Q3. What is the relationship between K_a and acid strength?
A. Larger K_a means weaker acid.
B. Larger K_a means stronger acid.
C. K_a has no relation to acid strength.
D. K_a is always 1.
Correct Answer: B
Explanation: A larger K_a indicates a stronger acid.

Q4. Which of the following substituents increases the acidity of a carboxylic acid?
A. Alkyl group
B. Chlorine atom
C. Hydrogen atom
D. Ethyl group
Correct Answer: B
Explanation: Chlorine is an electron-withdrawing group that increases acidity.

Q5. What is the pK_a of ethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: D
Explanation: The pK_a of ethanoic acid is 4.7.

Q6. Which of the following is the weakest acid?
A. Trichloroethanoic acid
B. Dichloroethanoic acid
C. Chloroethanoic acid
D. Ethanoic acid
Correct Answer: D
Explanation: Ethanoic acid has the highest pK_a (4.7), so it is the weakest acid among the given options.

Q7. What is the effect of an electron-donating group on the acidity of a carboxylic acid?
A. It increases acidity.
B. It decreases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Electron-donating groups destabilize the carboxylate ion, decreasing acidity.

Q8. Which of the following is the correct order of acidity?
A. CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH
B. CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
C. CHCl₂COOH > CCl₃COOH > CH₃COOH > CH₂ClCOOH
D. CH₂ClCOOH > CH₃COOH > CCl₃COOH > CHCl₂COOH
Correct Answer: B
Explanation: The correct order is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: The resonance stabilization of the carboxylate ion is the primary reason for the acidity of carboxylic acids.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid (pK_a = 4.7)
B. Chloroethanoic acid (pK_a = 2.9)
C. Dichloroethanoic acid (pK_a = 1.3)
D. Trichloroethanoic acid (pK_a = 0.9)
Correct Answer: D
Explanation: Trichloroethanoic acid has the lowest pK_a, so it is the strongest acid.

Q2. Why is trichloroethanoic acid stronger than ethanoic acid?
A. Because it has more hydrogen atoms.
B. Because chlorine is electron-withdrawing and stabilizes the carboxylate ion.
C. Because chlorine is electron-donating.
D. Because it has a higher molecular weight.
Correct Answer: B
Explanation: Chlorine stabilizes the carboxylate ion.

Q3. What is the relationship between K_a and acid strength?
A. Larger K_a means weaker acid.
B. Larger K_a means stronger acid.
C. K_a has no relation to acid strength.
D. K_a is always 1.
Correct Answer: B
Explanation: Larger K_a means stronger acid.

Q4. Which of the following substituents increases the acidity of a carboxylic acid?
A. Alkyl group
B. Chlorine atom
C. Hydrogen atom
D. Ethyl group
Correct Answer: B
Explanation: Chlorine increases acidity.

Q5. What is the pK_a of ethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: D
Explanation: The pK_a of ethanoic acid is 4.7.

Q6. Which of the following is the weakest acid?
A. Trichloroethanoic acid
B. Dichloroethanoic acid
C. Chloroethanoic acid
D. Ethanoic acid
Correct Answer: D
Explanation: Ethanoic acid is the weakest acid.

Q7. What is the effect of an electron-donating group on the acidity of a carboxylic acid?
A. It increases acidity.
B. It decreases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Electron-donating groups decrease acidity.

Q8. Which of the following is the correct order of acidity?
A. CH₃COOH > CH₂ClCOOH > CHCl₂COOH > CCl₃COOH
B. CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
C. CHCl₂COOH > CCl₃COOH > CH₃COOH > CH₂ClCOOH
D. CH₂ClCOOH > CH₃COOH > CCl₃COOH > CHCl₂COOH
Correct Answer: B
Explanation: The correct order is CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: D
Explanation: Trichloroethanoic acid is the strongest acid.

Q2. Which of the following is the weakest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: A
Explanation: Ethanoic acid is the weakest acid.

Q3. What is the effect of adding more chlorine atoms to acetic acid?
A. It decreases acidity.
B. It increases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Adding more chlorine atoms increases acidity.

Q4. What is the pK_a of trichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: A
Explanation: The pK_a of trichloroethanoic acid is 0.9.

Q5. What is the pK_a of chloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: C
Explanation: The pK_a of chloroethanoic acid is 2.9.

Q6. What is the pK_a of dichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: B
Explanation: The pK_a of dichloroethanoic acid is 1.3.

Q7. Which of the following is an electron-withdrawing group?
A. -CH₃
B. -Cl
C. -CH₂CH₃
D. -H
Correct Answer: B
Explanation: -Cl is an electron-withdrawing group.

Q8. Which of the following is an electron-donating group?
A. -Cl
B. -F
C. -CH₃
D. -NO₂
Correct Answer: C
Explanation: -CH₃ is an electron-donating group.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Multiple Choice Questions (MCQs)

Q1. Which of the following is the strongest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: D
Explanation: Trichloroethanoic acid is the strongest acid.

Q2. Which of the following is the weakest acid?
A. Ethanoic acid
B. Chloroethanoic acid
C. Dichloroethanoic acid
D. Trichloroethanoic acid
Correct Answer: A
Explanation: Ethanoic acid is the weakest acid.

Q3. What is the effect of adding more chlorine atoms to acetic acid?
A. It decreases acidity.
B. It increases acidity.
C. It has no effect.
D. It makes the acid neutral.
Correct Answer: B
Explanation: Adding more chlorine atoms increases acidity.

Q4. What is the pK_a of trichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: A
Explanation: The pK_a of trichloroethanoic acid is 0.9.

Q5. What is the pK_a of chloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: C
Explanation: The pK_a of chloroethanoic acid is 2.9.

Q6. What is the pK_a of dichloroethanoic acid?
A. 0.9
B. 1.3
C. 2.9
D. 4.7
Correct Answer: B
Explanation: The pK_a of dichloroethanoic acid is 1.3.

Q7. Which of the following is an electron-withdrawing group?
A. -CH₃
B. -Cl
C. -CH₂CH₃
D. -H
Correct Answer: B
Explanation: -Cl is an electron-withdrawing group.

Q8. Which of the following is an electron-donating group?
A. -Cl
B. -F
C. -CH₃
D. -NO₂
Correct Answer: C
Explanation: -CH₃ is an electron-donating group.

Q9. What is the primary reason for the acidity of carboxylic acids?
A. Resonance stabilization of the carboxylate ion.
B. The presence of a carbonyl group.
C. The presence of a hydroxyl group.
D. The presence of an alkyl group.
Correct Answer: A
Explanation: Resonance stabilization of the carboxylate ion is the primary reason.

Q10. Which of the following statements is correct about pK_a?
A. A larger pK_a means a stronger acid.
B. A smaller pK_a means a weaker acid.
C. A smaller pK_a means a stronger acid.
D. pK_a is always positive.
Correct Answer: C
Explanation: A smaller pK_a means a stronger acid.

Did you understand this topic?

Your answer is saved with your learning record. It does not mark the topic as mastered.

Quick revision

Revision Zone

Review the key facts, formulas and checks before you move on.

11.1 Acidity

Sticky Notes · Quick Revision

Key Point 1

Carboxylic acids: RCOOH.

Key Point 2

pK_a: Smaller = stronger acid.

Key Point 3

Resonance stabilizes carboxylate ion.

Key Point 4

Electron-withdrawing groups: Increase acidity.

Key Point 5

Electron-donating groups: Decrease acidity.

Key Point 6

CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH.

Key Point 7

Ethanoic acid: pK_a = 4.7.

Key Point 8

Trichloroethanoic acid: pK_a = 0.9 (strongest in the table).

Ask Tutor about this Open in Topics

Same supplied content as this Notes page (shared canonical source).

Definition

Main Idea

Core Concept

Detailed Explanation

The Acid Dissociation Constant (Ka) and pKa

Effect of Electron-Withdrawing Groups on Acidity

Resonance Stabilization of the Carboxylate Ion

Intermediate Level

Important Points

Remember

Common Mistakes

Quick Revision

60-Second Revision

Frequently Asked Questions (FAQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Did you understand this topic?

Definition

Main Idea

Core Concept

Detailed Explanation

The Acid Dissociation Constant (Ka) and pKa

Effect of Electron-Withdrawing Groups on Acidity

Resonance Stabilization of the Carboxylate Ion

Intermediate Level

Important Points

Remember

Common Mistakes

Quick Revision

60-Second Revision

Frequently Asked Questions (FAQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Multiple Choice Questions (MCQs)

Did you understand this topic?

The Acid Dissociation Constant (K_a) and pK_a

The Acid Dissociation Constant (K_a) and pK_a