Textbook Question
At what pH is the concentration of a compound, with a pKa = 8.4, 100 times greater in its acidic form than in its basic form? At what pH is 50% of a compound, with a pKa = 7.3, in its basic form?
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Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
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Bruice 8th EditionCh. 2 - Acids and Bases: Central to Understanding Organic ChemistryProblem 43e,f
Bruice 8th EditionCh. 2 - Acids and Bases: Central to Understanding Organic ChemistryProblem 43e,fChapter 3, Problem 43e,f
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.
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 guidance1
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: , 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.
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11:14
Charged Amino Acids
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.
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07:45Identifying pKa values
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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04:41Why Amino Acids Exist as Zwitterions
Related Practice
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 (pKa = 4.9)
2. CH3N+H3 (pKa = 10.7)
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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 pKa values of the two groups are shown.
d. Draw the structure of alanine in a solution at pH = 12.
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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 pKa values of the two groups are shown.
c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).
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Textbook Question
Indicate whether an alcohol (ROH) with a pKa 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
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Textbook Question
At what pH is the concentration of a compound, with a pKa = 4.6, 10 times greater in its basic form than in its acidic form?
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