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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5:47 minutes

Problem 64

Textbook Question

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

Verified step by step guidance

Step 1: Analyze the functional groups in each compound. Protonation typically occurs at atoms with lone pairs of electrons, such as oxygen or nitrogen, because they can accept a proton (H⁺).

Step 2: For compound (a), identify the functional groups: it contains an -OH group (hydroxyl) and an -NH₂ group (amine). Both oxygen and nitrogen have lone pairs, but nitrogen is more basic and more likely to be protonated.

Step 3: For compound (b), identify the functional groups: it contains an -OH group attached to a tertiary carbon and an -NH₂ group. The nitrogen atom in the -NH₂ group is more basic than the oxygen atom in the -OH group, so nitrogen is the most likely site for protonation.

Step 4: For compound (c), identify the functional groups: it contains an -NH₂ group and a -CH₂OH group. The nitrogen atom in the -NH₂ group is more basic than the oxygen atom in the -CH₂OH group, making nitrogen the most likely site for protonation.

Step 5: Summarize the findings: In all three compounds, the nitrogen atom in the -NH₂ group is the most apt to be protonated due to its higher basicity compared to the oxygen atoms in the hydroxyl groups.

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Key Concepts

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

Protonation

Protonation is the addition of a proton (H⁺) to a molecule, which can significantly alter its chemical properties. In organic chemistry, the site of protonation is often determined by the basicity of the atoms involved. Atoms with lone pairs of electrons, such as nitrogen or oxygen, are typically more likely to be protonated due to their ability to stabilize the positive charge that results from the addition of a proton.

Acidity and Basicity

Acidity and basicity are fundamental concepts in organic chemistry that describe the tendency of a substance to donate or accept protons, respectively. The strength of an acid is determined by its ability to donate protons, while a base is characterized by its ability to accept protons. Understanding the relative acidity and basicity of functional groups in a compound is crucial for predicting which atom is most likely to be protonated.

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons across multiple atoms in a molecule, which can enhance stability. When considering protonation, atoms that can participate in resonance often become more favorable sites for protonation because the resulting positive charge can be distributed over several atoms, reducing the energy of the protonated species. This concept is essential for predicting the most likely site of protonation in complex organic molecules.

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Related Practice

Textbook Question

For each indicated proton, suggest an approximate pK_a value from Table 4.5. Rationalize your choice.

(b)

Textbook Question

For the bases shown, draw the conjugate acid and identify a pK_a value from Table 4.5 that would help you accurately estimate its stability.

(d)

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

Rank the following from strongest base to weakest base:

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.

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.

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