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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 67a
Chapter 14, Problem 67a

Draw the ¹³C NMR spectrum you would expect to see for each of the molecules shown.
(a) Chemical structure of methyl propanoate with a terminal alkyne group, showing bonds and functional groups.

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1
Identify the unique carbon environments in the molecule. Each unique carbon environment will correspond to a distinct signal in the ¹³C NMR spectrum.
Consider the symmetry of the molecule. Symmetrical molecules may have fewer unique carbon environments due to equivalent positions.
Determine the chemical shift range for each type of carbon. For example, sp³ hybridized carbons typically appear between 0-50 ppm, sp² hybridized carbons between 100-150 ppm, and carbonyl carbons around 160-220 ppm.
Consider the effects of electronegative atoms or groups attached to the carbons. Electronegative atoms can deshield the carbon, causing a downfield shift (higher ppm value).
Sketch the ¹³C NMR spectrum, placing each signal at the appropriate chemical shift based on the analysis of the carbon environments and their electronic surroundings.

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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 study the carbon atoms in organic molecules. It provides information about the number of unique carbon environments in a molecule by detecting the magnetic environment of carbon-13 isotopes. Each distinct carbon environment appears as a separate signal in the spectrum, allowing chemists to deduce structural information about the molecule.
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Chemical Shift

Chemical shift in NMR spectroscopy refers to the position of an NMR signal relative to a standard reference point, typically measured in parts per million (ppm). It provides insight into the electronic environment surrounding a nucleus, with shifts influenced by factors such as electronegativity, hybridization, and nearby functional groups. Understanding chemical shifts helps in identifying the types of carbon atoms present in a molecule.
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Symmetry and Equivalent Carbons

In ¹³C NMR, symmetry in a molecule can lead to equivalent carbon atoms, which produce identical signals. Identifying symmetry elements, such as planes or axes, helps determine the number of unique carbon signals expected in the spectrum. Recognizing equivalent carbons is crucial for predicting the correct number of peaks and understanding the molecular structure.
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Related Practice
Textbook Question

Draw the 13C NMR spectrum you would expect to see for each of the molecules shown.

(b)

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Textbook Question

Draw the ¹H NMR spectrum you would expect to see for each of the molecules in Assessment 15.63.

(d)

2
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Textbook Question

For the molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.].

(a)

4
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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 molecules in Assessment 15.58, give an approximate chemical shift for each indicated carbon. [The range of correct answers is large here.].

(c)

Textbook Question

Draw the 13C NMR spectrum you would expect to see for each of the molecules shown.

(c)

1
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