Textbook Question
Which is a stronger base: ethoxide ion or acetate ion? Give pKb values (without looking them up) to support your choice.
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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 16b
Give the structures of their conjugate acids, and estimate their pKas from similar compounds in Appendix 4.
Verified step by step guidance1
Identify the basic sites in each molecule where protonation can occur to form the conjugate acid. In the first structure, the nitrogen atom in the imine group (NCH₃) is the basic site. In the second structure, the nitrogen atom in the nitrile group (C≡N) is the basic site. In the third structure, the nitrogen atom in the amine group (NHCH₃) is the basic site.
Draw the conjugate acid for each structure by adding a proton (H⁺) to the identified basic site. For the first structure, add a proton to the nitrogen atom in the imine group, resulting in a positively charged nitrogen. For the second structure, add a proton to the nitrogen atom in the nitrile group, resulting in a positively charged nitrogen. For the third structure, add a proton to the nitrogen atom in the amine group, resulting in a positively charged nitrogen.
Estimate the pKa values of the conjugate acids by comparing them to similar compounds listed in Appendix 4. Typically, imines have pKa values around 7-8, nitriles have pKa values around 9-10, and amines have pKa values around 10-11. Use these typical values as a reference to estimate the pKa of each conjugate acid.
Consider the structural features that might affect the acidity of the conjugate acids. For example, the presence of electron-withdrawing groups or resonance stabilization can influence the pKa values. Analyze the structures for any such features that might alter the typical pKa values.
Summarize the findings by listing the conjugate acid structures and their estimated pKa values based on the analysis and comparison with similar compounds.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Conjugate Acids
A conjugate acid is formed when a base gains a proton (H+). In the context of organic chemistry, identifying the conjugate acid involves adding a proton to the base, typically at the site of a lone pair or a negatively charged atom. This process is crucial for understanding acid-base reactions and predicting the behavior of molecules in different environments.
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Conjugated states
pKa and Acid Strength
The pKa value is a measure of the strength of an acid, representing the equilibrium constant for its dissociation in water. A lower pKa indicates a stronger acid, which more readily donates protons. Estimating pKa values involves comparing the structure of the acid in question to similar known compounds, considering factors like electronegativity, resonance, and inductive effects that influence acidity.
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Functional Groups and Reactivity
Functional groups are specific groups of atoms within molecules that determine the chemical reactivity and properties of those molecules. In the given structures, the presence of nitrogen-containing groups like imines and amines significantly affects their basicity and the formation of conjugate acids. Understanding the role of these functional groups is essential for predicting how the molecules will interact in acid-base reactions.
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Related Practice
Textbook Question
Consider the type of orbitals involved, and rank the following nitrogen compounds in order of decreasing basicity. 2. Rank the conjugate acids in order of increasing acidity. (Hint: These two orders should be the same!)
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Textbook Question
Like nitrogen and carbon, oxygen also shows this same hybridization effect on acidity. Both of the following compounds can lose a proton from a positively charged oxygen with three bonds to give a conjugate base containing a neutral oxygen with two bonds. One of these structures has pKa = −2.4, while the other has pKa = −8.0.
a. Show the reaction of each compound with water.
b. Match each structure with its pKa, and explain your choice.
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Textbook Question
Rank the following acids in decreasing order of their acid strength. In each case, explain why the previous compound should be a stronger acid than the one that follows it.
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Textbook Question
Acetic acid can also react as a very weak base (pKb = 20). Two different sites on acetic acid might become protonated to give the conjugate acid. Draw both of these possible conjugate acids, and explain (resonance) why the correct one is more stable. Calculate the pKa of this conjugate acid.
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Textbook Question
Consider each pair of bases, and explain which one is more basic. Draw their conjugate acids, and show which one is a stronger acid.
(a)
(b)
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