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 (pKa = 6.0)
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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 42b(1,2,3)
Bruice 8th EditionCh. 2 - Acids and Bases: Central to Understanding Organic ChemistryProblem 42b(1,2,3)Chapter 3, Problem 42b(1,2,3)
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.
1. pH = 1
2. pH = 3
3. pH = 5
Verified step by step guidance1
Step 1: Recall the relationship between pKa, pH, and the ratio of charged to neutral species. The Henderson-Hasselbalch equation is used to determine the ratio of the conjugate base (charged form, RO⁻) to the neutral form (ROH): \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{RO}^-]}{[\text{ROH}]} \right) \).
Step 2: Rearrange the Henderson-Hasselbalch equation to solve for the ratio \( \frac{[\text{RO}^-]}{[\text{ROH}]} \): \( \frac{[\text{RO}^-]}{[\text{ROH}]} = 10^{\text{pH} - \text{pKa}} \).
Step 3: For each pH value (12, 33, and 5), calculate \( \text{pH} - \text{pKa} \) to determine whether the solution favors the charged form (RO⁻) or the neutral form (ROH). If \( \text{pH} > \text{pKa} \), the charged form (RO⁻) predominates. If \( \text{pH} < \text{pKa} \), the neutral form (ROH) predominates.
Step 4: For pH = 12, calculate \( \text{pH} - \text{pKa} = 12 - 15 = -3 \). Since \( \text{pH} < \text{pKa} \), the neutral form (ROH) predominates in this case.
Step 5: Repeat the process for pH = 33 and pH = 5. For pH = 33, \( \text{pH} - \text{pKa} = 33 - 15 = 18 \), so the charged form (RO⁻) predominates. For pH = 5, \( \text{pH} - \text{pKa} = 5 - 15 = -10 \), so the neutral form (ROH) predominates.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
pKa and pH Relationship
The pKa value of a compound indicates the acidity of that compound, with lower pKa values corresponding to stronger acids. In a solution, the pH reflects the concentration of hydrogen ions (H+). When the pH is lower than the pKa, the compound is more likely to be protonated (charged), while at a higher pH, it tends to be deprotonated (neutral). Understanding this relationship is crucial for predicting the charge state of alcohols in different pH environments.
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Identifying pKa values
Protonation and Deprotonation
Protonation refers to the addition of a proton (H+) to a molecule, which can change its charge state. For alcohols, the hydroxyl group (–OH) can be protonated to form an oxonium ion (–OH2+), making it positively charged. Conversely, deprotonation occurs when a proton is removed, resulting in a neutral molecule. The balance between these processes is influenced by the solution's pH relative to the compound's pKa.
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Charge Distribution in Solutions
In a solution, the distribution of charged versus neutral molecules depends on the pH relative to the pKa of the solute. For an alcohol with a pKa of 15, at very low pH (like 1 or 3), the majority of molecules will be protonated and thus charged. As the pH increases towards 5, more molecules will be deprotonated, leading to a higher proportion of neutral molecules. This concept is essential for predicting the behavior of alcohols in various pH conditions.
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Related Practice
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).
Textbook Question
As long as the pH is not less than ___________, at least 50% of a protonated amine with a pKa value of 10.4 will be in its neutral, nonprotonated form.
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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
Indicate whether a protonated amine (RN+H3) with a pKa value of 9 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
4. pH = 7
5. pH = 10
6. pH = 13
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