Textbook Question
Propose chemical structures consistent with the following NMR spectra and molecular formulas. In spectrum (a), explain why the peaks around δ1.65 and δ3.75 are not clean multiplets, but show complex splitting.
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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 24b
Five proton NMR spectra are given here, together with molecular formulas. In each case, propose a structure that is consistent with the spectrum.
(b) <IMAGE>
Verified step by step guidance1
Analyze the molecular formula C8H8O provided in the problem. This indicates the molecule contains 8 carbons, 8 hydrogens, and 1 oxygen. The degree of unsaturation can be calculated using the formula: Degree of Unsaturation = (2C + 2 - H)/2. For C8H8O, the degree of unsaturation is (2(8) + 2 - 8)/2 = 5, suggesting the presence of rings and/or double bonds.
Examine the proton NMR spectrum. The peaks near δ = 7-8 ppm suggest the presence of aromatic protons, which are characteristic of benzene rings. This aligns with the degree of unsaturation calculation, indicating an aromatic ring.
The peak near δ = 2.8-3.1 ppm is a multiplet, which suggests the presence of protons adjacent to a functional group, such as a carbonyl group or an aromatic ring. This region is typical for benzylic protons (protons attached to a carbon adjacent to an aromatic ring).
The peak near δ = 9-10 ppm is a singlet, which is characteristic of an aldehyde proton (-CHO group). This strongly suggests the molecule contains an aldehyde functional group.
Combine the observations: The molecule likely contains a benzene ring (aromatic protons), a benzylic group (multiplet near δ = 2.8-3.1 ppm), and an aldehyde group (singlet near δ = 9-10 ppm). Based on the molecular formula and NMR data, propose a structure consistent with these features, such as benzaldehyde (C6H5CHO).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Proton NMR Spectroscopy
Proton Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It provides information about the number of hydrogen atoms in a molecule and their environment, allowing chemists to infer connectivity and functional groups. The chemical shifts, splitting patterns, and integration of peaks in the spectrum are key to interpreting the molecular structure.
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General NMR Features
Chemical Shift
Chemical shift refers to the resonance frequency of a nucleus relative to a standard in a magnetic field, typically measured in parts per million (ppm). In proton NMR, different chemical environments of hydrogen atoms lead to distinct chemical shifts, which help identify functional groups and the electronic environment surrounding the protons. Understanding chemical shifts is crucial for deducing the structure of the compound from the NMR spectrum.
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Spin-Spin Coupling
Spin-spin coupling, or J-coupling, occurs when protons that are close to each other influence each other's magnetic environments, resulting in splitting of NMR signals. This phenomenon provides information about the number of neighboring protons and their arrangement, which is essential for constructing the molecular structure. Analyzing the splitting patterns allows chemists to deduce connectivity and confirm the proposed structure.
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Related Practice
Textbook Question
The standard 13C NMR spectrum of phenyl propanoate is shown here. Predict the appearance of the DEPT-90 and DEPT-135 spectra.
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Textbook Question
Give the spectral assignments for the protons in isobutyl alcohol (Solved Problem 13-4). For example, Ha is a singlet, area = 1, at δ2.4.
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Textbook Question
Draw the expected NMR spectrum of methyl propionate, and point out how it differs from the spectrum of ethyl acetate.
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Textbook Question
(b) Draw the proton NMR spectrum you would expect for butan-2-one. How well do the proton chemical shifts predict the carbon chemical shifts using the "15 to 20 times as large" rule of thumb?
Textbook Question
(a) Show which carbon atoms correspond with which peaks in the 13C NMR spectrum of butan-2-one (Figure 13-45).
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