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 20a
Draw the NMR spectrum expected from ethanol that has been shaken with a drop of D2O.
Verified step by step guidance1
Step 1: Understand the impact of D2O on ethanol. When ethanol is shaken with D2O, the hydroxyl proton (H from -OH group) undergoes exchange with deuterium (D) from D2O. This results in the replacement of the -OH proton with deuterium, which is not detected in proton NMR.
Step 2: Analyze the remaining protons in ethanol. Ethanol (CH3CH2OH) has two types of protons that will appear in the NMR spectrum: the protons in the methyl group (-CH3) and the protons in the methylene group (-CH2). The hydroxyl proton is replaced by deuterium and will not contribute to the spectrum.
Step 3: Predict the splitting patterns. The methyl group (-CH3) protons will experience splitting due to the neighboring methylene (-CH2) group, resulting in a triplet signal. The methylene (-CH2) protons will experience splitting due to the neighboring methyl (-CH3) group, resulting in a quartet signal.
Step 4: Consider the chemical shifts. The methyl group (-CH3) protons will appear upfield (lower chemical shift) due to their electron-rich environment, while the methylene (-CH2) protons will appear downfield (higher chemical shift) due to their proximity to the electronegative oxygen atom.
Step 5: Summarize the expected NMR spectrum. The spectrum will consist of two signals: a triplet for the methyl group (-CH3) and a quartet for the methylene group (-CH2), with the hydroxyl proton signal absent due to deuterium exchange.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Nuclear Magnetic Resonance (NMR) Spectroscopy
NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It relies on the magnetic properties of certain nuclei, such as hydrogen (1H) and carbon (13C), to provide information about the environment surrounding these nuclei. The resulting spectrum displays peaks that correspond to different chemical environments, allowing chemists to infer structural details about the molecule.
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General NMR Features
Deuterium Exchange
Deuterium exchange occurs when a hydrogen atom in a molecule is replaced by a deuterium atom (D), which is an isotope of hydrogen. In the context of ethanol shaken with D2O, the hydroxyl (–OH) hydrogen can exchange with deuterium, leading to the formation of deuterated ethanol. This exchange affects the NMR spectrum by altering the number and position of peaks, particularly in the region corresponding to the hydroxyl group.
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Chemical Shift
Chemical shift refers to the variation in the resonance frequency of a nucleus due to its electronic environment. In NMR spectroscopy, different functional groups and molecular environments cause shifts in the position of peaks on the spectrum. For ethanol, the chemical shifts of the protons in the –OH, –CH2, and –CH3 groups will be distinct, and the exchange with deuterium will lead to a noticeable change in the peak corresponding to the hydroxyl proton.
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Related Practice
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
Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships.
c. Ph—CHBr—CH2Br
d. vinyl chloride
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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
Propose mechanisms to show the interchange of protons between ethanol molecules under
(a) acid catalysis.
(b) base catalysis.
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Textbook Question
Predict the theoretical number of different NMR signals produced by each compound, and give approximate chemical shifts. Point out any diastereotopic relationships.
a. 2-bromobutane
b. cyclopentanol
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