Textbook Question
Peroxyacetic acid (pKa = 8.2) is a much weaker acid than acetic acid (pKa = 4.74). Explain why peroxyacetic acid is a weaker acid than acetic acid.
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Ch. 2 - Acids and Bases; Functional Groups
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 2, Problem 42a
Hydrogen peroxide (HOOH) has a pKa of 11.6, making it roughly 10,000 times as strong an acid as water (pKa = 15.7). Explain why H2O2 is a stronger acid than H2O.
Verified step by step guidance1
Understand the concept of pKa: The pKa value is a measure of the strength of an acid. A lower pKa value indicates a stronger acid because it implies the acid more readily donates protons (H⁺ ions) in solution.
Compare the pKa values: Hydrogen peroxide (HOOH) has a pKa of 11.6, while water (H₂O) has a pKa of 15.7. Since 11.6 is lower than 15.7, HOOH is a stronger acid than H₂O.
Analyze the molecular structure: The O-O bond in hydrogen peroxide is weaker and more polar than the O-H bond in water. This makes it easier for HOOH to donate a proton, contributing to its stronger acidic nature.
Consider the stability of the conjugate base: When HOOH donates a proton, it forms the conjugate base HO₂⁻. The presence of an additional oxygen atom in HO₂⁻ compared to OH⁻ (the conjugate base of water) can stabilize the negative charge through resonance and inductive effects, making the conjugate base more stable.
Relate acidity to conjugate base stability: The greater stability of the conjugate base (HO₂⁻) compared to OH⁻ is a key reason why HOOH is a stronger acid than H₂O. A more stable conjugate base corresponds to a stronger acid.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid Strength and pKa
The strength of an acid is determined by its ability to donate protons (H+ ions) in solution. The pKa value is a logarithmic measure of this ability; lower pKa values indicate stronger acids. Hydrogen peroxide (H2O2) has a pKa of 11.6, which is lower than water's pKa of 15.7, indicating that H2O2 is a stronger acid than H2O.
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07:45
Identifying pKa values
Molecular Structure and Stability
The molecular structure of a compound affects its acidity. In hydrogen peroxide, the presence of the O-O bond and the additional oxygen atom can stabilize the negative charge on the conjugate base (HO2-) after losing a proton, more effectively than in water (OH-). This increased stability of the conjugate base contributes to H2O2's stronger acidity.
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03:43The radical stability trend.
Resonance and Inductive Effects
Resonance and inductive effects can influence acid strength. In H2O2, the electronegative oxygen atoms can delocalize the negative charge through resonance, stabilizing the conjugate base. Additionally, the inductive effect from the extra oxygen atom in H2O2 can pull electron density away, making it easier for the molecule to release a proton, thus enhancing its acidity compared to water.
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01:47Understanding the Inductive Effect.
Related Practice
Textbook Question
The following compound can become protonated on any of the three nitrogen atoms. One of these nitrogens is much more basic than the others, however.
b. Determine which nitrogen atom is the most basic.
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Textbook Question
The following compounds are listed in increasing order of acidity. In each case, the most acidic proton is shown in red.
b. Explain why X is a stronger acid than W.
c. Explain why Y is a stronger acid than X.
d. Explain why Z is a stronger acid than Y.
Textbook Question
Predict the products of the following acid–base reactions.
(a) H2SO4 + CH3COO– ⇌
(b) CH3COOH + (CH3)3N: ⇌
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Textbook Question
The following compound can become protonated on any of the three nitrogen atoms. One of these nitrogens is much more basic than the others, however.
a. Draw the important resonance forms of the products of protonation on each of the three nitrogen atoms.
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Textbook Question
Predict the products of the following acid–base reactions.
(c)
(d)
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