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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6:05 minutes

Problem 43e,f

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.

e. Is there a pH at which alanine is uncharged (that is, neither group has a charge)?
f. At what pH does alanine have no net charge (that is, the amount of negative charge is the same as the amount of positive charge)?

Verified step by step guidance

Step 1: Understand the structure of alanine. Alanine is an amino acid with two functional groups: a carboxylic acid group (-COOH) and a protonated amine group (-NH3+). The pKa values provided are 2.34 for the carboxylic acid group and 9.69 for the protonated amine group.

Step 2: Analyze the question about the pH at which alanine is uncharged. For alanine to be uncharged, neither the carboxylic acid group nor the amine group can carry a charge. This would require the carboxylic acid group to remain protonated (-COOH) and the amine group to remain neutral (-NH2). However, this situation is not possible under normal physiological pH ranges because the carboxylic acid group will deprotonate at pH > 2.34, and the amine group will protonate at pH < 9.69.

Step 3: Address the question about the pH at which alanine has no net charge. Alanine will have no net charge when the positive charge from the protonated amine group (-NH3+) is balanced by the negative charge from the deprotonated carboxylic acid group (-COO−). This occurs at the isoelectric point (pI), which is calculated as the average of the two pKa values.

Step 4: Calculate the isoelectric point (pI). The formula for the isoelectric point is:

pI = \frac{{pK}_{a1} + {pK}_{a2}}{2}

, where pKa1 is the pKa of the carboxylic acid group (2.34) and pKa2 is the pKa of the protonated amine group (9.69).

Step 5: Interpret the result. The calculated pI represents the pH at which alanine exists predominantly in its zwitterionic form, with no net charge. At this pH, the carboxylic acid group is deprotonated (-COO−), and the amine group is protonated (-NH3+), balancing the charges.

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Key Concepts

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

Amino Acid Structure

Amino acids are organic compounds that contain both an amine group (-NH2) and a carboxylic acid group (-COOH). In the case of alanine, it has a side chain that is a methyl group (-CH3). The presence of these functional groups allows amino acids to act as both acids and bases, which is crucial for understanding their behavior in different pH environments.

pKa Values

The pKa value is a measure of the strength of an acid in solution, indicating the pH at which half of the species are deprotonated. For alanine, the pKa of the carboxylic acid group is 2.34, and the pKa of the protonated amine group is 9.69. These values help determine the charge state of alanine at various pH levels, which is essential for answering questions about its net charge.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In the case of alanine, it exists as a zwitterion at a specific pH, where the carboxylic acid group is deprotonated (negatively charged) and the amine group is protonated (positively charged). Understanding the pH at which alanine exists as a zwitterion is key to determining when it has no net charge.

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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.

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

Textbook Question

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

c. CF₃CH₂OH (pK_a= 12.4)

Textbook Question

Give the structures of their conjugate acids, and estimate their pK_as from similar compounds in Appendix 4.

Textbook Question

Ammonia appears in [TABLE 2-2] as both an acid and a conjugate base.

d. Show how methanol (CH₃OH) can serve as both an acid and a base. Write an equation for the reaction of methanol with sulfuric acid.

Textbook Question

The following compounds can all react as acids.

a. For each compound, show its conjugate base. Show any resonance forms if applicable.

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