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Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 15 - Structural Identification II: Nuclear Magnetic Resonance SpectroscopyProblem 57c
Chapter 14, Problem 57c

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) Chemical structure of a cyclopentane ring with a propyl group attached at one carbon.

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1
Identify the unique hydrogen environments in the molecule. Each unique environment will correspond to a distinct signal in the ¹H NMR spectrum.
Consider the symmetry of the molecule. Symmetrical molecules often have fewer unique hydrogen environments because equivalent hydrogens will produce the same signal.
Look for different types of hydrogens such as those attached to sp3 hybridized carbons, sp2 hybridized carbons, and any hydrogens involved in functional groups like alcohols or amines.
Assess the connectivity of the molecule. Hydrogens on carbons that are connected to different groups or atoms will often be in different environments.
Count the number of distinct hydrogen environments you have identified. This count will give you the number of distinct signals expected 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 observing the magnetic environment of hydrogen atoms. Each distinct hydrogen environment in a molecule produces a separate signal in the NMR spectrum, allowing chemists to infer the number and types of hydrogen atoms present.
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Chemical Equivalence

Chemical equivalence in NMR refers to hydrogen atoms that are in identical environments and thus produce the same signal. Hydrogens are considered equivalent if they can be interchanged by a symmetry operation, such as rotation or reflection, without altering the molecule's structure. Identifying equivalent hydrogens is crucial for predicting the number of NMR signals.
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Symmetry in Molecules

Symmetry in molecules plays a key role in determining the number of distinct NMR signals. Symmetrical molecules often have fewer unique hydrogen environments, leading to fewer signals. By analyzing the symmetry elements, such as planes of symmetry or axes of rotation, one can predict which hydrogens are equivalent and thus how many signals will appear in the NMR spectrum.
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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

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.]

(e)

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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

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

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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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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