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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5:06 minutes

Problem 43d

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.

Verified step by step guidance

Step 1: Understand the structure of alanine. Alanine is an amino acid with 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 pH of the solution. At pH = 12, the solution is basic, which means it is above the pKa values of both functional groups. This will affect the protonation state of the groups.

Step 3: Determine the state of the carboxylic acid group. Since the pH (12) is much higher than the pKa of the carboxylic acid group (2.34), the carboxylic acid group will lose its proton and exist as a carboxylate ion (-COO⁻).

Step 4: Determine the state of the amine group. Since the pH (12) is higher than the pKa of the protonated amine group (9.69), the amine group will lose its proton and exist as a neutral amine (-NH2).

Step 5: Draw the structure of alanine at pH = 12. The structure will have a carboxylate group (-COO⁻) and a neutral amine group (-NH2), along with the methyl group (-CH3) attached to the alpha carbon.

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Key Concepts

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

Amino Acids

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains a central carbon atom bonded to an amino group (–NH2), a carboxylic acid group (–COOH), a hydrogen atom, and a variable side chain (R group) that determines its unique properties. Understanding the structure and function of amino acids is essential for grasping their behavior in different pH environments.

pKa and Ionization

pKa is a measure of the acidity of a proton in a molecule, indicating the pH at which half of the species are deprotonated. In the context of amino acids, the pKa values of the carboxylic acid and the amine groups determine their ionization state at a given pH. At pH 12, which is above the pKa of the carboxylic acid, this group will be deprotonated, while the amine group will remain protonated if its pKa is higher than 12.

Zwitterion Form

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In amino acids, at physiological pH, they often exist as zwitterions, where the amino group is protonated (–NH3+) and the carboxylic acid group is deprotonated (–COO–). At pH 12, alanine will predominantly exist in a form where the carboxylic acid is deprotonated, leading to a net negative charge on the molecule.

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Related Practice

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.

1. pH = 1

2. pH = 3

3. pH = 5

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

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)?

Textbook Question

For the following acid–base pairs, (iii) predict the favored side of equilibrium; (iv) calculate K_eq;

(h)

Multiple Choice

Which of the following is a correct statement about the equilibrium constant (K) in a chemical reaction?