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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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 17a
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.
Verified step by step guidance1
Identify the allylic protons on C3 of allyl bromide. Allyl bromide has the structure CH2=CH-CH2Br. The two protons on the CH2 group (C3) are the ones in question.
Apply the imaginary replacement technique. Replace one of the protons on C3 with a hypothetical group, such as a deuterium atom (D), and analyze the resulting molecule.
Determine the stereochemical relationship of the resulting molecules. If replacing one proton with D creates a molecule that is the mirror image of the molecule formed by replacing the other proton with D, then the protons are enantiotopic.
Visualize the symmetry of the molecule. Allyl bromide is not a chiral molecule, but replacing one proton on C3 breaks the symmetry and introduces chirality. The two resulting molecules (one with D replacing the first proton and the other with D replacing the second proton) will be non-superimposable mirror images, confirming that the protons are enantiotopic.
Conclude that since the two protons on C3 are enantiotopic, they are equivalent in an achiral environment such as NMR spectroscopy. This is why they appear as a single signal in the NMR spectrum.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enantiotopic Protons
Enantiotopic protons are pairs of protons in a molecule that, when one is replaced by a different atom or group, result in the formation of enantiomers. This means that the two protons are not equivalent in terms of their chemical environment, even though they may appear similar. The concept is crucial for understanding stereochemistry and how molecular symmetry affects reactivity and properties.
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Q-Test (Proton Relationships)
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, to provide information about the environment of atoms in a molecule. However, NMR cannot differentiate between enantiotopic protons, as they are chemically equivalent in the context of the NMR experiment, leading to overlapping signals.
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Imaginary Replacement Technique
The imaginary replacement technique is a theoretical approach used to analyze the stereochemical relationships between protons in a molecule. By conceptually replacing one proton with a different atom or group, chemists can assess whether the resulting structure is chiral or achiral. This method is particularly useful for identifying enantiotopic protons and understanding their role in stereochemistry.
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Related Practice
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
Propose a structure that corresponds to each spectrum.
(a) <IMAGE>
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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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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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