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

29. Amino Acids

Acid-Base Properties of Amino Acids

Organic Chemistry

29. Amino Acids

Acid-Base Properties of Amino Acids

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4:14 minutes

Problem 5

Textbook Question

Alanine has pK_a values of 2.34 and 9.69. Therefore, alanine exists predominately as a zwitterion in an aqueous solution with pH > and pH <.

Verified step by step guidance

Identify the pKa values of alanine: pKa1 = 2.34 (carboxylic acid group) and pKa2 = 9.69 (amino group). These values indicate the pH at which each functional group is 50% ionized.

Understand the concept of a zwitterion: A zwitterion is a molecule with both positive and negative charges but is overall neutral. For alanine, this occurs when the carboxylic acid group is deprotonated (-COO⁻) and the amino group is protonated (-NH₃⁺).

Determine the pH range where alanine exists predominantly as a zwitterion: This occurs between the pKa values of the carboxylic acid group (2.34) and the amino group (9.69).

Analyze the pH conditions: Alanine will exist predominantly as a zwitterion in an aqueous solution with pH greater than 2.34 (where the carboxylic acid group is deprotonated) and less than 9.69 (where the amino group remains protonated).

Conclude the pH range: Alanine exists predominantly as a zwitterion in the range 2.34 < pH < 9.69. For the given problem, the blanks should be filled with pH > 2.34 and pH < 9.69.

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Key Concepts

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

Zwitterion

A zwitterion is a molecule that has both positive and negative charges but is overall electrically neutral. In the case of amino acids like alanine, the zwitterionic form occurs when the amino group is protonated (positively charged) and the carboxyl group is deprotonated (negatively charged). This form is significant in biological systems and influences the solubility and reactivity of the molecule.

pKa and pH

pKa is a measure of the acidity of a compound, indicating the pH at which half of the species are deprotonated. For alanine, the two pKa values represent the ionization of the carboxyl and amino groups. The pH of a solution relative to these pKa values determines the predominant form of alanine, with pH values below the lower pKa leading to a cationic form and above the higher pKa leading to an anionic form.

Amino Acid Ionization

Amino acids can exist in different ionic forms depending on the pH of the solution. At low pH, amino acids are protonated and exist as cations, while at high pH, they can lose protons and exist as anions. The isoelectric point (pI) is the pH at which the amino acid has no net charge, and for alanine, this occurs between its two pKa values, indicating that it will predominantly exist as a zwitterion in a neutral to slightly basic environment.

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