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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 78b
Chapter 14, Problem 78b

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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Start by analyzing the molecular formula provided. Determine the degree of unsaturation using the formula: \( \text{Degree of Unsaturation} = rac{2C + 2 + N - H - X}{2} \), where \( C \) is the number of carbons, \( N \) is the number of nitrogens, \( H \) is the number of hydrogens, and \( X \) is the number of halogens.
Examine the IR spectrum data. The presence of two \( \text{C=O} \) bonds suggests the molecule may contain functional groups such as ketones, aldehydes, or esters. Consider how these groups might fit into the molecular structure.
Analyze the \( ^1\text{H} \) NMR spectrum. Identify the number of unique hydrogen environments, their chemical shifts, and splitting patterns. This will help you deduce the connectivity and environment of hydrogen atoms in the molecule.
Consider the integration values from the \( ^1\text{H} \) NMR spectrum, which indicate the relative number of protons in each environment. Use this information to match the hydrogen environments to the molecular formula.
Combine all the information: degree of unsaturation, IR data, and \( ^1\text{H} \) NMR analysis to propose a plausible structure. Ensure that the proposed structure accounts for all spectral data and matches the molecular formula.

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Key Concepts

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

¹H NMR Spectroscopy

¹H NMR (Proton Nuclear Magnetic Resonance) spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic environment of hydrogen atoms. It provides information about the number of hydrogen atoms, their chemical environment, and how they are connected within a molecule. Key features include chemical shifts, multiplicity, and integration, which help deduce the molecular structure.
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Molecular Formula Analysis

The molecular formula of a compound indicates the types and numbers of atoms present in a molecule. It is crucial for determining possible structures by providing the elemental composition. When combined with spectroscopic data, the molecular formula helps narrow down structural possibilities by ensuring that the proposed structure matches the given atomic composition and degree of unsaturation.
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IR Spectroscopy and Functional Groups

Infrared (IR) spectroscopy is used to identify functional groups within a molecule by measuring the absorption of infrared light, which causes molecular vibrations. The presence of specific absorption bands, such as those for carbonyl (C=O) groups, can confirm the existence of certain functional groups. In this context, the IR spectrum indicating two C=O bonds suggests the presence of functional groups like ketones, aldehydes, or carboxylic acids, guiding the structural determination.
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Related Practice
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

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.

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Textbook Question

(a) Using the ¹H NMR spectra, identify the reactants and product for this reaction.

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Textbook Question

The spectral data below are presented in a manner similar to what you would find in a chemistry research journal. Identify the structure for each set of data.

(c) C₇H₁₄O₂ : ¹H NMR: δ 0.91 (3H, t, J = 7.0 Hz), 1.11 (6H, d, J = 7.0 Hz), 1.82 (2H, sextet), 2.40 (1H, sept, J = 7.0 Hz), 4.14 (2H, t, J = 7.1 Hz); IR (cm ⁻¹) : 1745, 1200

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Textbook Question

(b) Provide an arrow-pushing mechanism for the reaction.

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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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