Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy

Problem 77

Chapter 14, Problem 77

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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Identify the functional groups present in the reactant and predict how they might change during the reaction. Consider any new functional groups that might form in the product.

Determine the number of unique hydrogen environments in the product. Each unique environment will correspond to a distinct signal in the ¹H NMR spectrum.

Predict the chemical shift for each type of hydrogen in the product. Use known chemical shift ranges for different functional groups to estimate where each signal might appear.

Consider the splitting patterns for each signal. The splitting pattern is determined by the number of neighboring hydrogens (n+1 rule), where n is the number of adjacent hydrogens.

Estimate the integration of each signal, which corresponds to the relative number of hydrogens in each environment. This will help verify the structure of the product by matching the expected hydrogen count.

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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 spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic properties of hydrogen nuclei. It provides information about the number of hydrogen atoms, their environment, and how they are connected within a molecule. Peaks in the spectrum correspond to different hydrogen environments, helping to identify functional groups and structural changes after a reaction.

Chemical Shift

Chemical shift in ¹H NMR refers to the position of the NMR signal relative to a standard reference, typically tetramethylsilane (TMS). It indicates the electronic environment of hydrogen atoms, influenced by nearby electronegative atoms or functional groups. Understanding chemical shifts is crucial for predicting and interpreting the NMR spectrum of a product, as shifts can reveal changes in molecular structure post-reaction.

Spin-Spin Coupling

Spin-spin coupling in ¹H NMR results in the splitting of NMR signals into multiplets, providing insights into the number of neighboring hydrogen atoms. This phenomenon occurs due to interactions between magnetic fields of adjacent hydrogen nuclei. Analyzing coupling patterns helps deduce connectivity and proximity of hydrogen atoms in a molecule, essential for verifying structural changes in reaction products.

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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

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

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

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

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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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.<IMAGE>

Verified video answer for a similar problem:

Key Concepts

¹H NMR Spectroscopy

Chemical Shift

Spin-Spin Coupling

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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