Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

3. Acids and Bases

pKa

Organic Chemistry

3. Acids and Bases

pKa

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4:22 minutes

Problem 72b

Textbook Question

Citrus fruits are rich in citric acid, a compound with three COOH groups. Explain the following:
b. The third pK_a is greater than the pKa of acetic acid.

Verified step by step guidance

Step 1: Understand the structure of citric acid. Citric acid is a tricarboxylic acid with three carboxylic acid (-COOH) groups. Each -COOH group has a different pKa value due to the influence of the molecule's structure and the interactions between the groups.

Step 2: Compare the third pKa of citric acid (5.8) with the pKa of acetic acid (4.76). The pKa value indicates the strength of an acid; a higher pKa means the acid is weaker. The third -COOH group in citric acid is less acidic than acetic acid.

Step 3: Consider the electronic effects within citric acid. When the first and second -COOH groups lose their protons, the molecule becomes negatively charged. This negative charge creates electrostatic repulsion, making it harder for the third -COOH group to lose its proton, thus increasing its pKa value.

Step 4: Compare acetic acid's structure. Acetic acid has only one -COOH group, so there are no additional negative charges or intramolecular interactions to stabilize the conjugate base. This makes acetic acid more acidic than the third -COOH group in citric acid.

Step 5: Summarize the reasoning. The third pKa of citric acid is greater than the pKa of acetic acid because the negative charges on the conjugate base of citric acid after losing two protons create repulsion, making it harder for the third proton to dissociate. This effect is absent in acetic acid, which has only one -COOH group.

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Key Concepts

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

pKa and Acid Strength

pKa is a measure of the acidity of a compound, indicating the strength of an acid in solution. A lower pKa value corresponds to a stronger acid, as it reflects a greater tendency to donate protons (H+ ions). Understanding pKa is essential for comparing the acidity of different compounds, such as citric acid and acetic acid.

Citric Acid Structure

Citric acid is a tricarboxylic acid, meaning it contains three carboxyl (COOH) groups. The presence of multiple acidic protons allows for multiple dissociation steps, each with its own pKa value. The structure of citric acid influences its acidity, as the electron-withdrawing effects of the other carboxyl groups can stabilize the negative charge on the conjugate base formed after deprotonation.

Comparative Acidity of Acids

When comparing the acidity of citric acid and acetic acid, it is important to consider the number of acidic protons and the stability of their conjugate bases. The third pKa of citric acid being greater than that of acetic acid suggests that the third proton is less easily lost, indicating that the corresponding conjugate base is less stable compared to that of acetic acid, which has only one acidic proton.

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Related Practice

Textbook Question

a. Which is a stronger base: CH₃COO⁻ or HCOO⁻? (The pK_a of CH₃COOH is 4.8; the pK_a of HCOOH is 3.8.)

b. Which is a stronger base: HO⁻ or ^-NH₂? (The pK_a of H2O is 15.7; the pK_a of NH3 is 36.)

c. Which is a stronger base: H₂O or CH₃OH? (The pK_a of H3O+ is −1.7; the pK_a of CH₃O⁺H₂ is −2.5.)

Textbook Question

A medicinal chemist needed to deprotonate acetylene ( HC≡CH ) for use in a coupling reaction. Among the options given, which base(s) could be used for this process?

Textbook Question

For each compound, indicate the atom that is most apt to be protonated.

Textbook Question

Rank the following from strongest base to weakest base:

Textbook Question

Given the structure of ascorbic acid (vitamin C):

(a) Is ascorbic acid a carboxylic acid?

(b) Compare the acid strength of ascorbic acid (pK_a = 4.71) with acetic acid.

Textbook Question

Which is a stronger base: ethoxide ion or acetate ion? Give pK_b values (without looking them up) to support your choice.

Textbook Question

Given the structure of ascorbic acid (vitamin C):