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Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 15 - Structural Identification II: Nuclear Magnetic Resonance SpectroscopyProblem 76
Chapter 14, Problem 76

For the compound shown, produce a table of the shift, integration, and multiplicity of each peak you would expect to see in a ¹H NMR spectrum.
Chemical structure of 4-methylpent-3-en-2-one, showing a benzene ring, ketone group, and alkene side chain.

Verified step by step guidance
1
Identify the different types of hydrogen environments in the compound. Look for unique groups or atoms that might affect the chemical shift, such as electronegative atoms or pi bonds.
Estimate the chemical shift for each type of hydrogen. Use typical chemical shift ranges for common functional groups and environments. For example, alkane hydrogens typically appear between 0-2 ppm, while hydrogens on a carbon adjacent to an electronegative atom might appear between 3-4 ppm.
Determine the integration for each peak. This corresponds to the number of hydrogens in each environment. Count the hydrogens in each distinct environment to determine the relative area under each peak.
Analyze the multiplicity of each peak. Consider the number of neighboring hydrogens (n) and apply the n+1 rule to determine the splitting pattern. For example, a hydrogen with two neighboring hydrogens will appear as a triplet.
Create a table summarizing the chemical shift, integration, and multiplicity for each type of hydrogen in the compound. Ensure that each entry in the table corresponds to a distinct hydrogen environment identified in the first step.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Chemical Shift in ¹H NMR

Chemical shift refers to the position of an NMR signal relative to a standard reference, typically tetramethylsilane (TMS). It indicates the electronic environment of hydrogen atoms in a molecule, affected by nearby electronegative atoms or functional groups. Understanding chemical shifts helps predict where peaks will appear in the spectrum.
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Integration in ¹H NMR

Integration in ¹H NMR quantifies the area under each peak, corresponding to the number of hydrogen atoms contributing to that signal. It provides insight into the relative number of protons in different environments within the molecule, aiding in determining the molecular structure.
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Multiplicity in ¹H NMR

Multiplicity describes the splitting pattern of NMR signals, resulting from spin-spin coupling between neighboring hydrogen atoms. The number of peaks in a multiplet follows the n+1 rule, where n is the number of adjacent protons. Multiplicity reveals information about the connectivity and proximity of hydrogen atoms in the molecule.
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Related Practice
Textbook Question

Given the ¹H NMR spectrum and the molecular formula, suggest a structure for each molecule. [The IR spectrum suggests the presence of two C=O bonds.]

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Textbook Question

In the lab, ¹H NMR is often used to verify that a reaction has worked as expected by comparing the product spectrum with what is expected. Given the ¹H NMR of the reactant shown, draw the spectrum you'd expect to see of the product that results.

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Textbook Question

A graduate student ran a reaction that produced a mixture of the following two compounds. After painstaking purification, she is able to separate the two compounds. Using ¹H NMR, how can she determine which diastereomer is in which separated sample?

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Textbook Question

Assign the peaks in the ¹H NMR spectrum for the molecule shown.

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Textbook Question

Draw the 13C NMR spectrum you would expect to see for each of the molecules shown.

(c)

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Textbook Question

Given the ¹H NMR spectrum and the molecular formula, suggest a structure for each molecule. [The IR spectrum suggests the presence of two C=O bonds.]

(a) Important IR bands (cm ⁻ ¹) : 1728, 1708 [Note: The pKₐ of this molecule is around 10.]

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