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

Equilibrium Constant

Organic Chemistry

3. Acids and Bases

Equilibrium Constant

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4:48 minutes

Problem 42b(1,2,3)

Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
1. pH = 1
2. pH = 3
3. pH = 5

Verified step by step guidance

Step 1: Recall the relationship between pKa, pH, and the ratio of charged to neutral species. The Henderson-Hasselbalch equation is used to determine the ratio of the conjugate base (charged form, RO⁻) to the neutral form (ROH): \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{RO}^-]}{[\text{ROH}]} \right) \).

Step 2: Rearrange the Henderson-Hasselbalch equation to solve for the ratio \( \frac{[\text{RO}^-]}{[\text{ROH}]} \): \( \frac{[\text{RO}^-]}{[\text{ROH}]} = 10^{\text{pH} - \text{pKa}} \).

Step 3: For each pH value (12, 33, and 5), calculate \( \text{pH} - \text{pKa} \) to determine whether the solution favors the charged form (RO⁻) or the neutral form (ROH). If \( \text{pH} > \text{pKa} \), the charged form (RO⁻) predominates. If \( \text{pH} < \text{pKa} \), the neutral form (ROH) predominates.

Step 4: For pH = 12, calculate \( \text{pH} - \text{pKa} = 12 - 15 = -3 \). Since \( \text{pH} < \text{pKa} \), the neutral form (ROH) predominates in this case.

Step 5: Repeat the process for pH = 33 and pH = 5. For pH = 33, \( \text{pH} - \text{pKa} = 33 - 15 = 18 \), so the charged form (RO⁻) predominates. For pH = 5, \( \text{pH} - \text{pKa} = 5 - 15 = -10 \), so the neutral form (ROH) predominates.

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Key Concepts

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

pKa and pH Relationship

The pKa value of a compound indicates the acidity of that compound, with lower pKa values corresponding to stronger acids. In a solution, the pH reflects the concentration of hydrogen ions (H+). When the pH is lower than the pKa, the compound is more likely to be protonated (charged), while at a higher pH, it tends to be deprotonated (neutral). Understanding this relationship is crucial for predicting the charge state of alcohols in different pH environments.

Protonation and Deprotonation

Protonation refers to the addition of a proton (H+) to a molecule, which can change its charge state. For alcohols, the hydroxyl group (–OH) can be protonated to form an oxonium ion (–OH2+), making it positively charged. Conversely, deprotonation occurs when a proton is removed, resulting in a neutral molecule. The balance between these processes is influenced by the solution's pH relative to the compound's pKa.

Charge Distribution in Solutions

In a solution, the distribution of charged versus neutral molecules depends on the pH relative to the pKa of the solute. For an alcohol with a pKa of 15, at very low pH (like 1 or 3), the majority of molecules will be protonated and thus charged. As the pH increases towards 5, more molecules will be deprotonated, leading to a higher proportion of neutral molecules. This concept is essential for predicting the behavior of alcohols in various pH conditions.

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Related Practice

Textbook Question

Indicate whether a protonated amine (RN⁺H₃) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

4. pH = 7

5. pH = 10

6. pH = 13

Textbook Question

For each of the following compounds, indicate the pH at which

a. 50% of the compound is in a form that possesses a charge.

1. CH₃CH₂COOH (pK_a= 4.9)

2. CH₃N⁺H₃ (pK_a= 10.7)

Textbook Question

For each of the following compounds, draw the form that predominates at pH = 3, pH = 6, pH = 10, and pH = 14:

a. CH₃COOH (pK_a= 4.8)

Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

e. ⁺NH4 (pK_a= 9.4)

f. HC≡N (pK_a= 9.1)

Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

3. pH = 7

4. pH = 10

5. pH = 13

Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

d. Draw the structure of alanine in a solution at pH = 12.

Textbook Question

For the following acid–base reaction, (c) calculate the ratio of butan-2-ol to 2-butoxide.

Textbook Question

What percent of imidazole is protonated at physiological pH (7.4)?

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