Textbook Question
Predict the chemical shifts of the protons in the following compounds.
(a)
(b)
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Ch. 13 - Nuclear Magnetic Resonance Spectroscopy
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 13, Problem 4
The NMR spectrum of toluene (methylbenzene) was shown in Figure 13-11.
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(a) How many different kinds of protons are there in toluene?
(b) Explain why the aromatic region around d7.2 is broad, with more than one sharp absorption.
Verified step by step guidance1
Step 1: Identify the structure of toluene (methylbenzene). Toluene consists of a benzene ring with a methyl group (-CH₃) attached to it. This structure will help us determine the different types of protons present.
Step 2: Analyze the types of protons in toluene. There are two main types of protons: (1) the protons in the methyl group (-CH₃), which are equivalent due to free rotation, and (2) the aromatic protons on the benzene ring. The aromatic protons are not equivalent because the methyl group creates an asymmetry in the ring.
Step 3: Explain the broad aromatic region around δ7.2. The aromatic protons in toluene are split into multiple signals due to the influence of the methyl group. The methyl group is an electron-donating group, which affects the electronic environment of the aromatic protons, causing them to appear at slightly different chemical shifts.
Step 4: Discuss why the aromatic region is broad. The aromatic protons experience spin-spin coupling with each other, leading to a complex splitting pattern. This results in multiple overlapping peaks in the aromatic region, making it appear broad.
Step 5: Summarize the findings. Toluene has two distinct types of protons: the methyl protons and the aromatic protons. The broad aromatic region around δ7.2 is due to the complex splitting and overlapping of signals caused by the asymmetry introduced by the methyl group and the spin-spin coupling between aromatic protons.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Chemical Environment of Protons
In organic chemistry, the chemical environment of protons refers to the unique surroundings that influence the magnetic field experienced by each proton in a molecule. In toluene, the protons on the methyl group and the aromatic ring are in different environments, leading to distinct signals in the NMR spectrum. This concept is crucial for determining the number of different kinds of protons present.
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Aromaticity and Proton Signals
Aromatic compounds, like toluene, exhibit unique NMR characteristics due to their cyclic, planar structure and delocalized π-electrons. The protons in the aromatic region typically resonate at lower fields (around d7.2) and can show broad signals due to interactions between protons on the ring. Understanding aromaticity helps explain the complexity of the signals observed in the NMR spectrum.
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Spin-Spin Coupling
Spin-spin coupling occurs when protons in close proximity influence each other's magnetic environments, leading to splitting of NMR signals. In toluene, the presence of multiple protons on the aromatic ring can cause broadening and splitting of signals in the aromatic region. This phenomenon is essential for interpreting the NMR spectrum and understanding the interactions between protons.
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Related Practice
Textbook Question
Determine the number of different kinds of protons in each compound.
(a) 1-chloropropane
(b) 2-chloropropane
(c) 2,2-dimethylbutane
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Textbook Question
In a 300-MHz spectrometer, the protons in iodomethane absorb at a position 650 Hz downfield from TMS.
(a) What is the chemical shift of these protons?
(b) What is the chemical shift of the iodomethane protons in a 60-MHz spectrometer?
(c) How many hertz downfield from TMS would they absorb at 60 MHz?
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Textbook Question
An unknown compound (C3H2NCl) shows moderately strong IR absorptions around 1650 cm–1 and 2200 cm–1. Its NMR spectrum consists of two doublets (J = 14 Hz) at δ5.9 and δ7.1. Propose a structure consistent with these data.
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Textbook Question
Draw the NMR spectra you expect for the following compounds.
(a)
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Textbook Question
Draw the NMR spectra you expect for the following compounds.
(b)
1
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