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

Ranking Acidity

Organic Chemistry

3. Acids and Bases

Ranking Acidity

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Problem 52b

Textbook Question

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]
(b)

Verified step by step guidance

Step 1: Begin by drawing the conjugate base for each molecule. For the thiol (CH3CH2SH), remove the proton from the sulfur atom to form CH3CH2S⁻. For the amide (CH3CONH2), remove the proton from the nitrogen atom to form CH3CONH⁻.

Step 2: Analyze the stability of each conjugate base. The stability of a conjugate base is a key factor in determining the acidity of the proton. A more stable conjugate base corresponds to a more acidic proton.

Step 3: Consider electronegativity. Sulfur is less electronegative than nitrogen, meaning the negative charge on CH3CH2S⁻ is less stabilized compared to CH3CONH⁻. This suggests that the proton on the thiol group is less acidic than the proton on the amide group.

Step 4: Evaluate resonance effects. The conjugate base of the amide (CH3CONH⁻) benefits from resonance stabilization, as the negative charge can delocalize onto the oxygen atom in the carbonyl group. This resonance stabilization makes the proton on the amide more acidic.

Step 5: Consider inductive effects. The carbonyl group in the amide exerts an electron-withdrawing effect, further stabilizing the conjugate base (CH3CONH⁻). This additional stabilization reinforces the conclusion that the proton on the amide is more acidic than the proton on the thiol.

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Key Concepts

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

Acidity and pKa

Acidity in organic chemistry refers to the tendency of a compound to donate a proton (H+). The strength of an acid is often measured by its pKa value; lower pKa values indicate stronger acids. Understanding the relationship between acidity and pKa is crucial for identifying the most acidic proton in a given structure.

Conjugate Bases

When an acid donates a proton, it forms a conjugate base. The stability of this conjugate base significantly influences the acidity of the original compound. Analyzing the conjugate base helps determine which proton is more acidic, as a more stable conjugate base corresponds to a stronger acid.

Resonance and Inductive Effects

Resonance refers to the delocalization of electrons across multiple atoms, which can stabilize a conjugate base and enhance acidity. Inductive effects involve the influence of electronegative atoms or groups that can withdraw electron density, further stabilizing the conjugate base. Both effects are essential in evaluating the acidity of protons in organic molecules.

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Related Practice

Textbook Question

Without using pK_a values, pick out the least reactive (most stable) base in each pair. Explain your answer.

(b)

Textbook Question

Using qualitative reasoning for the acid–base reactions shown,

(i) which is stronger, the acid or the conjugate acid?

(ii) Which side of the reaction is favored?

(iii) Would you expect a K_eqgreater than, equal to, or less than 1?

(b)

Textbook Question

Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.

(a)

Textbook Question

Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.

(f)

Textbook Question

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]

(d)

Textbook Question

Which acid in each pair would you expect to more readily donate a proton to a basic compound?

(a)

Textbook Question

Why is the pK_a value of protonated hydroxylamine (6.0) so much lower than the value of a protonated primary amine such as protonated methylamine (10.7)?

Textbook Question

In light of your answers to Assessments 24.54 and 24.55, rank the following based on the rate of protonation of the alkene (1 = most basic, 6 = least basic). [Ignore the fact that the alkene may not be the most basic site in the molecule.]

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Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.](b)

Key Concepts

Acidity and pKa

Conjugate Bases

Resonance and Inductive Effects

Watch next

Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]
(b)