교과서 질문
Rank imidazole, pyrrole, and benzene from most reactive to least reactive toward electrophilic aromatic substitution.
797
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Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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TOC.tableOfContents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding31
- 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
19장, 문제 17
Why is imidazole a stronger acid (pKa ~ 14.4) than pyrrole (pKa ~ 17)?
검증된 단계별 안내1
Step 1: Begin by analyzing the structures of imidazole and pyrrole. Imidazole contains two nitrogen atoms in a five-membered aromatic ring, one of which is sp2-hybridized and part of the aromatic system, while the other is sp2-hybridized and has a lone pair that can act as a proton acceptor. Pyrrole, on the other hand, has one nitrogen atom in a five-membered aromatic ring, where the lone pair on nitrogen contributes to the aromaticity of the ring.
Step 2: Consider the acidity of the compounds. Acidity is determined by the stability of the conjugate base formed after deprotonation. For imidazole, deprotonation occurs at the sp2-hybridized nitrogen, resulting in a conjugate base where the negative charge is delocalized over the aromatic ring, increasing stability. In pyrrole, deprotonation disrupts the aromaticity of the ring, making the conjugate base less stable.
Step 3: Evaluate the role of resonance stabilization. Imidazole's conjugate base benefits from resonance stabilization, as the negative charge can be delocalized across the ring and shared between the two nitrogen atoms. Pyrrole's conjugate base lacks this extensive resonance stabilization because the lone pair on nitrogen is integral to maintaining aromaticity.
Step 4: Examine the hybridization of the nitrogen atoms. In imidazole, the sp2-hybridized nitrogen is more electronegative than the sp3-hybridized nitrogen in pyrrole, making it more likely to donate a proton. This contributes to imidazole's stronger acidity compared to pyrrole.
Step 5: Conclude that the combination of resonance stabilization, preservation of aromaticity, and the electronegativity of the sp2-hybridized nitrogen in imidazole makes it a stronger acid (lower pKa) than pyrrole.
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Acidity and pKa
Acidity refers to the ability of a compound to donate a proton (H+). The strength of an acid is quantitatively expressed by its pKa value; lower pKa values indicate stronger acids. Imidazole, with a pKa of 14.4, is a stronger acid than pyrrole, which has a pKa around 17, meaning imidazole more readily donates a proton.
추천 영상:
07:45
Identifying pKa values
Resonance Stabilization
Resonance stabilization occurs when a molecule can distribute its electron density across multiple structures, enhancing stability. In imidazole, the conjugate base formed after deprotonation is resonance-stabilized by the adjacent nitrogen atoms, making it more stable and favoring proton loss compared to pyrrole, which has less effective resonance stabilization.
추천 영상:
03:43The radical stability trend.
Electronegativity of Nitrogen
Electronegativity is the tendency of an atom to attract electrons. In imidazole, the presence of two nitrogen atoms in the ring increases the overall electronegativity, which stabilizes the negative charge on the conjugate base after deprotonation. In contrast, pyrrole has only one nitrogen, leading to less stabilization of the negative charge and a weaker acidic character.
추천 영상:
1:47Electronegativity
관련 실천
교과서 질문
Imidazole boils at 257 °C, whereas N-methylimidazole boils at 199 °C. Explain the difference in boiling points.
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교과서 질문
What are the products of the following reactions?
a.
836
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교과서 질문
Why is protonated pyrimidine (pKa = 1.0) more acidic than protonated pyridine (pKa = 5.2)?
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교과서 질문
Rank the following compounds from easiest to hardest at removing a proton from its methyl substituent:
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교과서 질문
What percent of imidazole is protonated at physiological pH (7.4)?
1428
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