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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4:34 minutes

Problem 43c

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.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Verified step by step guidance

Step 1: Understand the structure of alanine. Alanine is an amino acid with a central alpha carbon bonded to a carboxylic acid group (-COOH), an amine group (-NH2), a hydrogen atom, and a methyl group (-CH3). At physiological pH (7.4), the carboxylic acid group and the amine group will exist in their ionized forms.

Step 2: Analyze the pKa values provided. The pKa of the carboxylic acid group is 2.32, meaning it will lose a proton and exist as a carboxylate ion (-COO⁻) at pH 7.4, which is above its pKa. The pKa of the amine group is 9.62, meaning it will remain protonated as -NH3⁺ at pH 7.4, which is below its pKa.

Step 3: Determine the ionization state of alanine at physiological pH. Since the pH is higher than the pKa of the carboxylic acid group, it will be deprotonated (-COO⁻). Since the pH is lower than the pKa of the amine group, it will remain protonated (-NH3⁺).

Step 4: Draw the structure of alanine at physiological pH. The alpha carbon will be bonded to the following groups: a carboxylate ion (-COO⁻), a protonated amine group (-NH3⁺), a hydrogen atom, and a methyl group (-CH3).

Step 5: Verify the zwitterionic nature of alanine at physiological pH. Alanine exists as a zwitterion, meaning it has both a positive charge (on the -NH3⁺ group) and a negative charge (on the -COO⁻ group) at pH 7.4.

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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). The properties of the side chain determine the characteristics of the amino acid, influencing protein structure and function.

pKa and Acid-Base Chemistry

pKa is a measure of the strength of an acid in solution, indicating the pH at which half of the acid is dissociated. In the context of amino acids, the pKa values of the carboxylic acid and the protonated amine groups help determine their ionization states at a given pH. At physiological pH (7.4), the carboxylic acid group is typically deprotonated, while the amine group remains protonated, influencing the overall charge of the amino acid.

Zwitterion Formation

A zwitterion is a molecule that has both positive and negative charges but is overall neutral. In amino acids like alanine at physiological pH, the carboxylic acid group loses a proton (–COO⁻), while the amino group retains a proton (–NH₃⁺). This results in a zwitterionic form, which is crucial for the solubility and reactivity of amino acids in biological systems.

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Related Practice

Textbook Question

Calculate the equilibrium constant for the acid–base reaction between the reactants in each of the following pairs:

c. CH₃NH₂+ H₂O

d. CH₃N⁺H₃ + H2O

Textbook Question

Which reaction in Problem 23 has the smallest equilibrium constant?

Textbook Question

Given that pH + pOH = 14 and that the concentration of water in a solution of water is 55.5 M, show that the pK_a of water is 15.7. (Hint: pOH=−log [HO−])

Textbook Question

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

b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

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:

a. CH₃COOH(pK_a= 4.76)

b. CH₃CH₂N⁺H₃ (pK_a= 11.0)

Textbook Question

At what pH is the concentration of a compound, with a pK_a = 8.4, 100 times greater in its acidic form than in its basic form? At what pH is 50% of a compound, with a pK_a = 7.3, in its basic form?

Textbook Question

If an acid with a pK_a of 5.3 is in an aqueous solution of pH 5.7, what percentage of the acid is present in its acidic form?

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:

i. HON⁺H₃ (pK_a = 6.0)

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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 pKa values of the two groups are shown.c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Key Concepts

Amino Acids

pKa and Acid-Base Chemistry

Zwitterion Formation

Watch next

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.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).