Textbook Question
For the hydrogen(s) screened in blue, draw the signal you would expect to see in a ¹H NMR spectrum. At which chemical shift would the signal appear?
(e)
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15. Analytical Techniques:IR, NMR, Mass Spect
1H NMR:Spin-Splitting Simple Tree Diagrams
Problem 41
Textbook Question
Though it wasn't discussed, what coupling constant would you expect for Hₐ and H꜀ in the spectrum of trans-but-2-enoic acid in Figure 15.50? Justify your answer.
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Verified step by step guidance1
First, understand the concept of coupling constants in NMR spectroscopy. The coupling constant, denoted as J, is a measure of the interaction between nuclear spins and is expressed in Hertz (Hz). It provides information about the spatial relationship between coupled nuclei.
Identify the structure of trans-but-2-enoic acid. The trans configuration indicates that the substituents on the double bond are on opposite sides. This affects the coupling constant between the hydrogen atoms Hₐ and H꜀.
Consider the geometry of the molecule. In trans-but-2-enoic acid, Hₐ and H꜀ are on opposite sides of the double bond, which typically results in a larger coupling constant compared to a cis configuration due to the dihedral angle between the protons.
Recall typical values for coupling constants in alkenes. For trans alkenes, the coupling constant is usually in the range of 12-18 Hz. This is due to the larger dihedral angle compared to cis alkenes, which have smaller coupling constants (6-12 Hz).
Justify the expected coupling constant for Hₐ and H꜀ in trans-but-2-enoic acid based on the trans configuration and typical values for coupling constants in similar structures. The trans configuration leads to a larger coupling constant due to the spatial arrangement of the protons.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Coupling Constant
The coupling constant, denoted as J, is a measure of the interaction between nuclear spins in NMR spectroscopy. It is expressed in hertz (Hz) and provides information about the spatial relationship between coupled nuclei. The value of J depends on the number of bonds separating the nuclei and the geometry of the molecule, with larger values typically indicating closer or more direct interactions.
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Trans Configuration
In organic chemistry, a trans configuration refers to the arrangement of substituents across a double bond where they are on opposite sides. This configuration affects the coupling constant, as trans protons typically exhibit larger coupling constants compared to cis protons due to the more extended spatial arrangement, which influences the magnetic interaction between them.
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NMR Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy is a technique used to determine the structure of organic compounds by observing the magnetic properties of atomic nuclei. In NMR, the chemical environment of hydrogen atoms (protons) is analyzed, providing insights into molecular structure, including the number of hydrogen atoms, their connectivity, and interactions, such as coupling constants, which help deduce spatial arrangements.
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