Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy
Organic Chemistry Second Edition David Klein Chapter 16 Nuclear Magnetic Resonance Spectroscopy Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Please note: If your clicker system can only hold 5 multiple choice answers, we have provided ‘alternate answers’ for those questions in which the author originally had more than 5 choices. These answers appear on the slides with a green background. If your clicker system can hold more than 5 multiple choice answers, please delete the slides with the green backgrounds, and use the original answers the author has listed. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

16.1 1. Nuclear spin occurs when there is ______
an even number of protons. an odd number of protons. an equal number of protons and electrons. twice as many neutrons as protons. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: B. an odd number of protons.
A nucleus with an odd number of protons and/or an odd number of neutrons possesses nuclear spin. Hydrogen atom with 1 proton or Carbon-13 with 7 neutrons are important nuclei for organic chemists. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

diamagnetism, resonance
The presence of circulating electron density around a proton will affect the magnetic field the proton experiences. If the electron density is high the proton is considered _______ and if the electron density is relatively low, the proton is considered to be _________ resonance, flipped shielded, deshielded deshielded, shielded diamagnetism, resonance Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: B. shielded, deshielded.
When the electrons are spinning, they are creating a weak magnetic field that will oppose the external magnetic field. The more electrons (higher electron density) the greater the opposition between the weak field and external field, thus the proton is shielded from the external magnetic field. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: 16.1, A _____ proton has more electron density surrounding it and the proton will appear _____ . a. Shielded, upfield b. Shielded, downfield c. Deshielded, upfield d. Deshielded, downfield Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: a. Protons that are shielded by electron density absorb at a higher magnetic field strength and appear upfield in the spectra. Deshielded protons do not have as much electron density surrounding them and will absorb at a lower magnetic field and appear downfield in the spectra. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: Each signal produced in an NMR spectrum has 3 variables. Those variables are ___ . a. Signal strength, signal splitting and signal height b. Signal strength, signal shift and signal height c. Signal shift, signal splitting and signal strength d. Signal shift, signal splitting and signal integration Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: d. Signal shift is shown by the location from 0–10 on the graph. Signal shift tells what functional group is next to the hydrogen. Signal splitting is shown by the number of lines that the signal produces. Signal splitting tells how many hydrogens are next to the hydrogen. Signal integration is shown by the area under the curve. Signal integration shows how many hydrogens each signal represents. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

16. 3 5. There are three main ways an NMR spectrum gives information
There are three main ways an NMR spectrum gives information. The _________indicates the electronic environment, the _________ indicates the number of protons giving rise to the signal, and ________ indicates the number of neighboring protons. location, shape, area shape, area, location location, area, shape area, location, shape Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: C. location, area, shape
The location is the known as chemical shift, the area is the integration and the shape will be the number of peaks. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

For more examples of this type of problem, see Skillbuilder 16.1.
Answer: I: C II: B III: A For more examples of this type of problem, see Skillbuilder 16.1. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Diastereotopic Enantiotopic Homotopic I = A; II = B; III = C
Alternate Answer: Section: Indicate if the two protons in the following are homotopic, enantiotopic or diastereotopic. Diastereotopic Enantiotopic Homotopic I = A; II = B; III = C I = A; II = C; III = B I = B; II = A; III = C I = B; II = C; III = A I = C; II = B; III = A Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Homotropic equivalent Enantiotopic equivalent
The relationship between the protons indicated by the arrows would be considered Homotropic equivalent Enantiotopic equivalent Diastereotopic nonequivalent Not related, so nonequivalent Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: B. Enantiotopic, equivalent
The protons can be interchanged by either rotational symmetry or reflectional symmetry. They are chemically equivalent. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Homotropic equivalent Enantiotopic equivalent
The relationship between the protons indicated by the arrows would be considered Homotropic equivalent Enantiotopic equivalent Diastereotopic nonequivalent Not related, so nonequivalent Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: A. Homotropic, equivalent
Homotropic protons are chemically equivalent. The molecule has an axis of symmetry. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: 16.4 9. Identify the number of signals expected in the 1H NMR spectra of the following
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

shielded, deshielded deshielded, shielded
The chemical shift (d) indicates the relative location of the signal as compared to a reference of TMS. When the chemical shift is upfield, the protons are typically ________, and when the chemical shift is downfield the protons are __________ shielded, deshielded deshielded, shielded Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: A. shielded, deshielded.
Remember, shielded protons experience a smaller difference in energy between spin states because there is a greater local magnetic field opposing the external magnetic field. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: In a proton NMR spectrum, which of the following will appear between 2 and 4 ppm on the spectrum? a. unsubstituted alkane b. alkene c. alkyne d. aromatic hydrogen Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

16.5 12. Predict the expected chemical shift for the indicated protons.
0.9 ppm 1.7 ppm 3.8 ppm 4.2 ppm Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: D. 4.2 ppm A methine (CH) has a signal of 1.7. The effect of the oxygen atom from an alcohol alpha to the methine proton is = 4.2 ppm. See Skillbuilder 16.3 for more examples Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: A compound with the formula C6H14O has the following signal integration, how many protons are represented by each signal. A. Signal at 0.9, integration of 6.3 B. Signal at 1.4, integration of 4.2 C. Signal at 3.5, integration of 4.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

A. Signal at 0.9, integration of 6.3
Alternate Answers: Section: 16.6 13. A compound with the formula C6H14O has the following signal integration, how many protons are represented by each signal. A. Signal at 0.9, integration of 6.3 B. Signal at 1.4, integration of 4.2 C. Signal at 3.5, integration of 4.2 A = 3; B = 4; C = 4 A = 3; B = 2; C = 2 A = 6; B = 2; C = 2 A = 6; B = 4; C = 4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

16.7 15. If a 1H NMR peak shows a sextet, what does this indicate?
There are four neighboring protons. There are six protons in the compound. There are five neighboring protons. There are five equivalent protons. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Answer: C. There are five neighboring protons.
Spin-spin splitting or coupling will determine the shape of the peaks. The number of peaks follow the n+1 rule in which n is the number of neighboring protons, in this case 5. See Skillbuilder 16.5 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: 16.7 16. What would be the splitting of the indicated hydrogen?

Section: 16.8, 16.9, 16.10 20. Which compound corresponds to the 1H NMR spectrum below?

Alternate Answer: Section: 16. 8, 16. 9, 16. 10 20
Alternate Answer: Section: 16.8, 16.9, Which compound corresponds to the 1H NMR spectrum below? A D E F G Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: In a carbon NMR sample, which carbon atoms will appear between 100 and 200 ppm on the spectrum? a. carbon of an alkene b. carbon of an alkyne c. carbon of an aromatic ring d. carbon of a ketone e. a, b and c f. a, c and d g. all of the above h. none of the above Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Alternate Answer: Section: 16. 12 23
Alternate Answer: Section: In a carbon NMR sample, which carbon atoms will appear between 100 and 200 ppm on the spectrum? a. carbon of an alkene b. carbon of an alkyne c. carbon of an aromatic ring d. carbon of a ketone e. a, b and c f. a, c and d g. all of the above h. none of the above B E F G H Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: Predict the number of signals and location of each signal in 13C NMR for the following. A: Number of signals B: Number of signals below 60 ppm C: Number of signals between 60 and 100 ppm D: Number of signals between 100 and 150 ppm E: Number of signals above 150 ppm Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Alternate Answer: Section: Predict the number of signals and location of each signal in 13C NMR for the following. A: Number of signals B: Number of signals below 60 ppm C: Number of signals between 60 and 100 ppm D: Number of signals between 100 and 150 ppm E: Number of signals above 150 ppm A = 7; B = 1; C = 1; D = 3; E = 2 A = 7; B = 2; C = 0; D = 4; E = 1 A = 9; B = 2; C = 0; D = 3; E = 2 A = 9; B = 2; C = 0; D = 4; E = 1 A = 9; B = 1; C = 1; D = 3; E = 2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: Predict the number of signals and location of each signal in 13C NMR for the following. A: Number of signals _________ B: Number of signals below 50 ppm _________ C: Number of signals between 50 and 100 ppm _______ D: Number of signals between 100 and 150 ppm ______ E: Number of signals above 150 ppm _________ Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Alternate Answer: Section: Predict the number of signals and location of each signal in 13C NMR for the following. A: Number of signals _________ B: Number of signals below 50 ppm _________ C: Number of signals between 50 and 100 ppm _______ D: Number of signals between 100 and 150 ppm ______ E: Number of signals above 150 ppm _________ A = 7; B = 3; C = 1; D = 1; E = 3 A = 7; B = 4; C = 0; D = 3; E = 2 A = 7; B = 3; C = 1; D = 2; E = 2 A = 8; B = 4; C = 0; D = 1; E = 3 A = 8; B = 4; C = 1; D = 2; E = 1 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: ___ carbon NMR spectra allow you to determine whether a carbon is a CH, CH2, CH3 or CH. a. broadband proton decoupled b. DEPT c. COSY d. HETCOR Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

Section: 16.13 27. Which compound corresponds to the 13C NMR spectrum below?

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