Textbook Question
Rank the following alcohols in order of descending pKa value. Explain your ranking.
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Ch. 4 - Acids and Bases: Electron Flow
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
Chapter 3, Problem 50
If reacted with a strong base, which of the labeled protons would you expect to be removed first?
Verified step by step guidance1
Step 1: Identify the labeled protons (Ha, Hb, Hc) in the molecule and their chemical environments. Ha is attached to a sp2-hybridized carbon in the cyclohexene ring, Hb is attached to a sp-hybridized carbon in the alkyne group, and Hc is attached to a sp3-hybridized carbon in the cyclohexane ring.
Step 2: Recall that the acidity of a proton is influenced by the hybridization of the carbon it is attached to. Protons attached to sp-hybridized carbons are generally more acidic than those attached to sp2 or sp3 carbons due to the higher electronegativity of sp-hybridized carbons.
Step 3: Consider the role of the strong base (:NH2−). A strong base will preferentially remove the most acidic proton, which corresponds to the proton with the lowest pKa value.
Step 4: Compare the acidity of the labeled protons. Hb, attached to the sp-hybridized carbon, is the most acidic due to the high s-character of the sp orbital. Ha, attached to the sp2-hybridized carbon, is less acidic, and Hc, attached to the sp3-hybridized carbon, is the least acidic.
Step 5: Conclude that the strong base (:NH2−) will first remove Hb, the proton attached to the sp-hybridized carbon in the alkyne group, because it is the most acidic.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity and Basicity
Acidity refers to the tendency of a compound to donate protons (H+ ions), while basicity is the ability to accept protons. In organic chemistry, the strength of an acid is often determined by the stability of its conjugate base. Stronger acids have more stable conjugate bases, making them more likely to lose protons when reacted with a strong base.
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06:21
Understanding the difference between basicity and nucleophilicity.
Proton Removal and pKa
The pKa value is a quantitative measure of the acidity of a proton in a molecule. Lower pKa values indicate stronger acids, meaning the corresponding protons are more easily removed. When considering which proton will be removed first by a strong base, one should look for the proton with the lowest pKa, as it is the most acidic and thus the most reactive towards the base.
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07:45Identifying pKa values
Resonance Stabilization
Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. This delocalization can stabilize a conjugate base formed after proton removal. Protons attached to atoms that can participate in resonance are typically more acidic, as the resulting negative charge can be spread over multiple atoms, enhancing stability.
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03:43The radical stability trend.
Related Practice
Textbook Question
Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.
(a)
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Textbook Question
Rationalize the rather large difference in pKa values for the two carboxylic acids shown.
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Textbook Question
Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.
(e)
1625
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Textbook Question
Rank the following amines in order of their basicity (strongest base = 1 ; weakest base = 6).
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Textbook Question
Identify the most stable conjugate base in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer.
(c)
1160
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