Textbook Question
Draw the NMR spectrum expected from ethanol that has been shaken with a drop of D2O.
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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 18a,b
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
Verified step by step guidance1
Step 1: Analyze the molecular structure of 2-bromobutane. Identify the unique hydrogen environments by considering symmetry and connectivity. Note that the presence of the bromine atom can influence the chemical shifts due to its electronegativity.
Step 2: Determine the theoretical number of NMR signals for 2-bromobutane. Look for equivalent hydrogens (hydrogens in the same chemical environment) and consider diastereotopic relationships, which arise when hydrogens are in non-equivalent environments due to chirality or asymmetry.
Step 3: Approximate the chemical shifts for each unique hydrogen environment in 2-bromobutane. For example, hydrogens near the bromine atom will experience deshielding and appear downfield (higher ppm), while hydrogens further away will appear upfield (lower ppm).
Step 4: Analyze the molecular structure of cyclopentanol. Identify the unique hydrogen environments, considering the hydroxyl group (-OH) and the ring structure. Note that the hydroxyl group can cause hydrogen bonding, which affects chemical shifts.
Step 5: Determine the theoretical number of NMR signals for cyclopentanol and approximate the chemical shifts. Consider the influence of the hydroxyl group and the ring strain on the chemical environment of the hydrogens. Point out any diastereotopic relationships, such as hydrogens on the same carbon but in different spatial environments due to the ring structure.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
NMR Signals
Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to observe the local magnetic fields around atomic nuclei. The number of distinct NMR signals corresponds to the number of unique hydrogen environments in a molecule. Each unique environment produces a separate signal, allowing chemists to deduce structural information about the compound.
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General Assumption for 1H NMR Signals
Chemical Shifts
Chemical shifts in NMR spectroscopy refer to the resonant frequency of a nucleus relative to a standard in a magnetic field, typically measured in parts per million (ppm). These shifts provide insight into the electronic environment surrounding the nuclei, influenced by factors such as electronegativity and hybridization. Understanding typical chemical shift ranges for different functional groups is essential for predicting the signals in a spectrum.
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Diastereotopic Relationships
Diastereotopic protons are non-equivalent protons in a molecule that are not related by symmetry and can give rise to different NMR signals. Identifying diastereotopic relationships is crucial for understanding the stereochemistry of a compound, as these relationships can affect the chemical shifts and splitting patterns observed in the NMR spectrum. Recognizing these distinctions helps in accurately interpreting the NMR data.
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Related Practice
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
The spectrum of trans-hex-2-enoic acid follows.
(a) Assign peaks to show which protons give rise to which peaks in the spectrum.
(b) Draw a tree to show the complex splitting of the vinyl proton centered around 7 ppm. Estimate the values of the coupling constants.
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Textbook Question
If the imaginary replacement of either of two protons forms enantiomers, then those protons are said to be enantiotopic. The NMR is not a chiral probe, and it cannot distinguish between enantiotopic protons. They are seen to be “equivalent by NMR.”
(a) Use the imaginary replacement technique to show that the two allylic protons (those on C3) of allyl bromide are enantiotopic.
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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
Draw a splitting tree, similar to Figures 13-32 and 13-33, for proton Hc in styrene. What is the chemical shift of proton Hc?
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