Textbook Question
Which acid in each pair would you expect to more readily donate a proton to a basic compound?
(b)
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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 48
Rank the following alcohols in order of descending pKa value. Explain your ranking.
Verified step by step guidance1
Step 1: Understand the concept of pKa. The pKa value of an alcohol is a measure of its acidity. Lower pKa values indicate stronger acids, while higher pKa values indicate weaker acids. The acidity of alcohols is influenced by the stability of the conjugate base formed after deprotonation.
Step 2: Analyze the substituents attached to the alcohols. Electronegative substituents, such as halogens (F, Cl, I), stabilize the conjugate base through inductive effects, making the alcohol more acidic (lower pKa). The closer the electronegative group is to the hydroxyl group, the stronger the inductive effect.
Step 3: Compare the alcohols in the image. For example: (1) The alcohol with iodine directly attached to the carbon bearing the hydroxyl group will have a weaker inductive effect due to iodine's lower electronegativity compared to fluorine or chlorine. (2) Alcohols with fluorine substituents will exhibit the strongest inductive effects due to fluorine's high electronegativity. (3) Alcohols with chlorine substituents will have intermediate inductive effects.
Step 4: Consider the number and position of electronegative substituents. Alcohols with multiple fluorine atoms (e.g., trifluoromethyl alcohol) will have the lowest pKa due to the cumulative inductive effect. Alcohols with halogens farther from the hydroxyl group will have less impact on acidity.
Step 5: Rank the alcohols based on their substituents and inductive effects. Alcohols with fluorine substituents closest to the hydroxyl group will have the lowest pKa, followed by alcohols with chlorine substituents, and then alcohols with iodine substituents. Alcohols without halogens will have the highest pKa.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity of Alcohols
The acidity of alcohols is determined by the stability of their conjugate bases after deprotonation. A more stable conjugate base corresponds to a stronger acid, which is reflected in a lower pKa value. Factors influencing acidity include the electronegativity of substituents and their ability to stabilize negative charge through inductive effects or resonance.
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Forming alcohols through Acid-Catalyzed Hydration.
Inductive Effect
The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on a molecule. Electronegative atoms, such as fluorine and chlorine, can stabilize the negative charge on the conjugate base of an alcohol, thereby increasing its acidity. The strength of this effect diminishes with distance from the functional group, making proximity crucial in determining acidity.
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Comparative Analysis of Substituents
When ranking the acidity of alcohols, it is essential to compare the effects of different substituents on the carbon chain. For instance, halogens like fluorine and chlorine are strong electron-withdrawing groups, enhancing acidity, while alkyl groups are electron-donating and decrease acidity. The overall ranking of pKa values will depend on the combination and position of these substituents.
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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
Which acid in each pair would you expect to more readily donate a proton to a basic compound?
(a)
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Textbook Question
If reacted with a strong base, which of the labeled protons would you expect to be removed first?
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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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