Textbook Question
Explain why each compound is aromatic, antiaromatic, or nonaromatic.
(d)
(e)
(f)
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Ch. 16 - Aromatic Compounds
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 16, Problem 17
Show which of the nitrogen atoms in purine are basic, and which one is not basic. For the nonbasic nitrogen, explain why its nonbonding electrons are not easily available to become protonated.
Verified step by step guidance1
Step 1: Begin by identifying the structure of purine. Purine is a heterocyclic aromatic compound with a fused ring system consisting of a pyrimidine ring and an imidazole ring. It contains four nitrogen atoms in total.
Step 2: Analyze the electronic environment of each nitrogen atom. In aromatic systems, the basicity of nitrogen atoms depends on whether their lone pairs are involved in the aromatic π-system or are free to accept a proton.
Step 3: For the nitrogen atoms in purine, determine their roles in the aromatic system. Use resonance structures to see if the lone pairs of electrons on the nitrogen atoms are delocalized into the π-system. Nitrogen atoms with delocalized lone pairs are not basic because their electrons are not readily available for protonation.
Step 4: Identify the basic nitrogen atoms. Nitrogen atoms whose lone pairs are not involved in the aromatic π-system are basic because their electrons are free to accept a proton. These nitrogen atoms can act as proton acceptors.
Step 5: Explain why the nonbasic nitrogen atom is not basic. The lone pair of electrons on this nitrogen is delocalized into the aromatic π-system, contributing to the stability of the aromatic ring. Because these electrons are part of the conjugated π-system, they are not easily available for protonation, making the nitrogen atom nonbasic.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Basicity of Nitrogen Atoms
Basicity in organic chemistry refers to the ability of a nitrogen atom to accept a proton (H+). Nitrogen atoms in purine can vary in their basicity depending on their electronic environment and hybridization. Typically, nitrogen atoms with a lone pair of electrons in a suitable orbital can act as bases, while those involved in aromatic systems or with electron-withdrawing groups may be less basic.
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Structure of Purine
Purine is a bicyclic compound consisting of a fused imidazole and pyrimidine ring. It contains four nitrogen atoms, each positioned differently within the rings. Understanding the spatial arrangement and hybridization of these nitrogen atoms is crucial for determining which are basic and which are not, as their electronic properties are influenced by their connectivity and the overall molecular structure.
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Nonbonding Electrons and Protonation
Nonbonding electrons are lone pairs that are not involved in covalent bonding. For a nitrogen atom to be basic, its nonbonding electrons must be available to accept a proton. In the case of the nonbasic nitrogen in purine, steric hindrance or resonance stabilization can delocalize these electrons, making them less available for protonation, thus reducing the atom's basicity.
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Related Practice
Textbook Question
Explain why each compound is aromatic, antiaromatic, or nonaromatic.
(a)
(b)
(c)
Textbook Question
a. Explain how pyrrole is isoelectronic with the cyclopentadienyl anion.
b. Specifically, what is the difference between the cyclopentadienyl anion and pyrrole?
Textbook Question
The polarization of a carbonyl group can be represented by a pair of resonance structures:
Cyclopropenone and cycloheptatrienone are more stable than anticipated. Cyclopentadienone, however, is relatively unstable and rapidly undergoes a Diels–Alder dimerization. Explain.
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Textbook Question
Explain why each compound is aromatic, antiaromatic, or nonaromatic.
(g)
(h)
Textbook Question
Draw resonance forms to show the charge distribution on the pyrrole structure.
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