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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 50b
Chapter 14, Problem 50b

Assign each signal in the ¹³C NMR spectra to the molecule shown.
(b) <IMAGE>

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Begin by examining the molecular structure provided. Identify the different types of carbon environments present in the molecule. Each unique carbon environment will correspond to a distinct signal in the ¹³C NMR spectrum.
Consider the hybridization of each carbon atom. Carbons can be sp³, sp², or sp hybridized, and this affects their chemical shift in the NMR spectrum. Typically, sp³ hybridized carbons appear at lower chemical shifts, while sp² and sp hybridized carbons appear at higher chemical shifts.
Look for symmetry in the molecule. Symmetrical molecules may have equivalent carbon atoms that produce the same signal in the NMR spectrum. Identify any such equivalent carbons.
Consider the electronegativity of atoms or groups attached to each carbon. Electronegative atoms or groups can deshield the carbon, causing its signal to appear at a higher chemical shift. Analyze the proximity of each carbon to electronegative atoms or groups.
Finally, match each carbon environment to the corresponding signal in the ¹³C NMR spectrum based on the chemical shift values. Use known chemical shift ranges for different types of carbon environments to make these assignments.

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Key Concepts

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

¹³C NMR Spectroscopy

¹³C NMR spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic environment of carbon atoms. Each carbon atom in a molecule can produce a distinct signal in the NMR spectrum, depending on its electronic environment. This method is crucial for identifying the number and types of carbon environments in a compound.
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Chemical Shift

Chemical shift in NMR spectroscopy refers to the resonant frequency of a nucleus relative to a standard in a magnetic field. It provides information about the electronic environment surrounding a nucleus. In ¹³C NMR, chemical shifts are influenced by factors such as electronegativity of nearby atoms and hybridization, helping to distinguish between different carbon environments in a molecule.
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Signal Assignment

Signal assignment in NMR involves correlating each peak in the spectrum to a specific atom or group of atoms in the molecule. This process requires understanding the chemical shift values and the molecular structure. By analyzing the number of signals and their chemical shifts, one can deduce which carbon atoms correspond to which signals, aiding in the structural elucidation of the compound.
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Related Practice
Textbook Question

Assign each signal in the ¹³C NMR spectra to the molecule shown.

(a) <IMAGE>

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Textbook Question

Draw the 13C NMR spectrum of each molecule in Assessment 15.48.

(c)

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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

Draw the 13C NMR spectrum of each molecule in Assessment 15.48.

(b)

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Textbook Question

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

(a)

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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