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

Acids and Bases

Organic Chemistry

3. Acids and Bases

Acids and Bases

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4:56 minutes

Problem 33

Textbook Question

If a base has a conjugate acid with a high pK_a value, is it stable or unstable? How do you know this is true (besides 'I just remember')?

Verified step by step guidance

Understand the relationship between pKₐ and acid strength: A high pKₐ value indicates a weak acid, meaning the acid does not readily donate protons (H⁺). This is because the equilibrium for proton dissociation lies far to the left, favoring the undissociated form of the acid.

Recall the conjugate acid-base relationship: A weak acid has a strong conjugate base. This is because the weak acid does not easily lose its proton, so its conjugate base is more likely to accept a proton, making it a strong base.

Analyze the stability of the base: A strong base is less stable because it has a high tendency to react with protons (H⁺) to form its conjugate acid. Stability is inversely related to reactivity; the more reactive a base, the less stable it is.

Connect the pKₐ value to the base's stability: Since a high pKₐ value corresponds to a weak acid and a strong conjugate base, the base in question is less stable. This is because it is highly reactive and eager to accept protons.

Summarize the reasoning: The stability of a base is determined by the strength of its conjugate acid. A high pKₐ value for the conjugate acid means the base is strong and therefore less stable due to its high reactivity.

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Key Concepts

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

Conjugate Acid-Base Pairs

In acid-base chemistry, a conjugate acid is formed when a base accepts a proton (H⁺). The strength of the acid is often measured by its pKₐ value, which is the negative logarithm of its acid dissociation constant (Kₐ). A high pKₐ indicates a weak acid, meaning it does not dissociate easily in solution, which is a key factor in determining the stability of the conjugate acid.

Stability of Conjugate Acids

The stability of a conjugate acid is influenced by its ability to hold onto the proton it has gained. A conjugate acid with a high pKₐ value is typically more stable because it is less likely to donate the proton back to the solution. This stability can be attributed to factors such as electronegativity, resonance, and the overall structure of the molecule, which affect how well the positive charge is delocalized.

Relationship Between pKₐ and Acid Strength

The pKₐ value is inversely related to the strength of an acid; lower pKₐ values correspond to stronger acids that dissociate more readily. Therefore, when considering a base with a conjugate acid that has a high pKₐ, it indicates that the conjugate acid is weak and stable. This relationship helps predict the behavior of acids and bases in chemical reactions and their relative stabilities.

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Textbook Question

Specify which in each pair is the harder Lewis acid. Justify your choice beyond just looking at Table 20.2.

(b) Li⁺ vs. Na⁺

Textbook Question

Using the convention that the acid and base are on the left side of the chemical equation, label the acid, base, conjugate acid, and conjugate base in the following reactions.

(b)

Textbook Question

In comparison to CH₃^- in Assessment 2.86, the related molecule BH₄^- is itself not a Lewis base at boron. Why?

Textbook Question

Design an acid–base extraction scheme to separate a mixture of the basic amine N,N-dimethylaniline and naphthalene.

Textbook Question

Specify which in each pair is the harder Lewis acid. Justify your choice beyond just looking at Table 20.2.

Textbook Question

In the following reactions,

(i) identify the acid and base,

(ii) identify the most electron-rich atom in the base,

(iii) identify the most acidic hydrogen in the acid,

(iv) provide an arrow-pushing mechanism of the proton transfer that will occur, and

(v) predict the product of the reactions. [You'll need to provide the lone pairs here.]

(d) H₂O + HCl ⇌

Textbook Question

Triosephosphate isomerase (TIM) catalyzes the conversion of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate. The enzyme’s catalytic groups are Glu 165 and His 95. In the first step of the reaction, these catalytic groups function as a general-base and a general-acid catalyst, respectively. Propose a mechanism for the reaction.

Textbook Question

What products are formed when each of the following reacts with HO⁻?

c. CH₃N⁺H₃

d. BF₃

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