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 57b

Chapter 14, Problem 57b

How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.]
(b)

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Identify the molecular structure of the given compound. Understanding the structure is crucial as it helps in determining the number of unique hydrogen environments.

Examine the symmetry of the molecule. Symmetrical molecules often have fewer distinct NMR signals because equivalent hydrogens produce the same signal.

Determine the number of unique hydrogen environments. Look for hydrogens in different chemical environments, such as those attached to different functional groups or in different positions relative to other atoms or groups.

Consider the effect of electronegative atoms or groups nearby. Hydrogens near electronegative atoms may experience different chemical shifts, leading to distinct signals.

Count the number of distinct hydrogen environments identified. Each unique environment corresponds to a separate signal in the ¹H NMR spectrum.

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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. Each distinct hydrogen environment in a molecule produces a unique signal in the NMR spectrum, allowing chemists to deduce the number and types of hydrogen atoms present.

Chemical Equivalence

Chemical equivalence in NMR refers to hydrogen atoms that are in identical environments within a molecule, resulting in them producing the same signal in the NMR spectrum. Identifying chemically equivalent hydrogens is crucial for predicting the number of distinct signals, as equivalent hydrogens do not contribute additional signals.

Symmetry in Molecules

Symmetry in molecules can significantly affect the number of distinct NMR signals. Symmetrical molecules often have fewer distinct hydrogen environments because symmetry can make different hydrogens chemically equivalent. Recognizing symmetry elements, such as planes or axes, helps in determining which hydrogens are equivalent and thus how many unique signals will appear in the spectrum.

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Determining Chirality with Plane of Symmetry

Related Practice

Textbook Question

For the following molecules, give the integration you would expect for the signal associated with the hydrogens at the labeled carbons. [Pay attention to the symmetry, or lack of symmetry, in the molecules.]

(c)

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Textbook Question

How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.]

(c)

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Textbook Question

How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.]

(f)

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Textbook Question

Draw the expected results of a DEPT sequence for the molecules shown.

(b)

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Textbook Question

How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.]

(a)

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Textbook Question

With information from the ¹H NMR and the ¹³C DEPT spectra, structure elucidation becomes even easier. Provide the structure that corresponds to the following data. [The identity of the carbons comes from the DEPT experiment.]

C₆H₁₁BrO₂

IR: 1745 cm ⁻¹

¹H NMR: δ 1.25 (t, 3H), 2.18 (quint, 2H), 2.58 (t, 2H), 3.46 (t, 2H), 4.15 (q, 2H)

¹³C NMR: δ 14.2 (CH₃) , 27.8 (CH₂) , 32.5 (CH₂) , 32.6 (CH₂) , 60.5 (CH₂) , 172.4 (C)

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How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.] (b)

Verified video answer for a similar problem:

Key Concepts

¹H NMR Spectroscopy

Chemical Equivalence

Symmetry in Molecules

How many distinct signals would you expect to see in the ¹H NMR spectrum of the following molecules? [Ignore diastereotopic hydrogens for the sake of this assessment.]
(b)