Organic Chemistry

John McMurry

Chapter 13

Structure Determination: Nuclear Magnetic Resonance Spectroscopy - all with Video Answers

Educators

Chapter Questions

Problem 1

The amount of energy required to spin-flip a nucleus depends both on the strength of the external magnetic field and on the nucleus. At a field strength of $4.7 \mathrm{T},$ rf energy of 200 MHz is required to bring a 'H nucleus into resonance, but energy of only $187 \mathrm{MHz}$ will bring a 19 F nucleus into resonance. Calculate the amount of energy required to spin-flip a 19 F nucleus. Is this amount greater or less than that required to spin-flip a 'H nucleus?

Lottie Adams

Lottie Adams

Numerade Educator

Problem 2

Calculate the amount of energy required to spin-flip a proton in a spectrometer operating at $300 \mathrm{MHz}$. Does increasing the spectrometer frequency from 200 to $300 \mathrm{MHz}$ increase or decrease the amount of energy necessary for resonance?

Lottie Adams

Numerade Educator

Problem 3

2-Chloropropene shows signals for three kinds of protons in its $^{1} \mathrm{H}$ NMR spectrum. Explain.

Lottie Adams

Numerade Educator

Problem 4

The following $^{1} \mathrm{H}$ NMR peaks were recorded on a spectrometer operating at $200 \mathrm{MHz}$. Convert each into $\delta$ units.
(a) $\mathrm{CHCl}_{3}: 1454 \mathrm{Hz}$
(b) $\mathrm{CH}_{3} \mathrm{Cl} ; 610 \mathrm{Hz}$
(c) $\mathrm{CH}_{3} \mathrm{OH} ; 693 \mathrm{Hz}$
(d) $\mathrm{CH}_{2} \mathrm{Cl}_{2} ; 1060 \mathrm{Hz}$

Lottie Adams

Numerade Educator

Problem 5

When the 'H NMR spectrum of acetone, $\mathrm{CH}_{3} \mathrm{COCH}_{3}$, is recorded on an instrument operating at $200 \mathrm{MHz}$ a single sharp resonance at $2.1 \delta$ is seen.
(a) How many hertz downfield from TMS does the acetone resonance correspond to?
(b) If the 'H NMR spectrum of acetone were recorded at $500 \mathrm{MHz}$, what would the position of the absorption be in $\delta$ units?
(c) How many hertz downfield from TMS does this $500 \mathrm{MHz}$ resonance correspond to?

Lottie Adams

Numerade Educator

Problem 6

Predict the number of carbon resonance lines you would expect in the $^{13} \mathrm{C}$ NMR spectra of the following compounds:
(a) Methylcyclopentane
(b) $1-$ Methylcyclohexene
(c) $1,2-$ Dimethylbenzene
(d) 2 -Methyl-2-butene
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 7

Propose structures for compounds that fit the following descriptions:
(a) A hydrocarbon with seven lines in its $^{13} \mathrm{C}$ NMR spectrum
(b) A six-carbon compound with only five lines in its $^{13} \mathrm{C}$ NMR spectrum
(c) A four-carbon compound with three lines in its $^{13} \mathrm{C}$ NMR spectrum

Lottie Adams

Numerade Educator

Problem 8

Assign the resonances in the $^{13} \mathrm{C}$ NMR spectrum of methyl propanoate, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{3}$

Lottie Adams

Numerade Educator

Problem 9

Assign a chemical shift to each carbon in 6 -methyl- - -hepten-2-ol (Figure 13.10).

Lottie Adams

Numerade Educator

Problem 10

Estimate the chemical shift of each carbon in the following molecule. Predict which carbons will appear in the DEPT-90 spectrum, which will give positive peaks in the DEPT-135 spectrum, and which will give negative peaks in the DEPT-135 spectrum.
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 11

Propose a structure for an aromatic hydrocarbon, $\mathrm{C}_{\mathrm{n} \mathrm{H}_{16}, \text { that has the following }^{13} \mathrm{C} \mathrm{NMR}}$ spectral data:
Broadband decoupled: 29.5,31.8,50.2,125.5,127.5,130.3,139.88
DEPT-90: 125.5, 127.5, 130.3 $\delta$ DEPT-135: positive peaks at $29.5,125.5,127.5,130.3 \delta ;$ negative peak at $50.2 \delta.$

Lottie Adams

Numerade Educator

Problem 12

We saw in Section 9.3 that addition of HBr to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use $^{13} \mathrm{C}$ NMR to identify the product of the addition of 1 equivalent of HBr to 1-hexyne?

Lottie Adams

Numerade Educator

Problem 13

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 14

How many kinds of electronically nonequivalent protons are present in each of the following compounds, and thus how many NMR absorptions might you expect in each?
(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}$
(b) $\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}$
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NO}_{2}$
(d) Methylbenzene
(e) 2 -Methyl-l-butene
(f) cis-3-Hexene

Isaac Huidobro

Numerade Educator

Problem 15

How many absorptions would you expect ( $S$ )-malate, an intermediate in carbohydrate metabolism, to have in its 'H NMR spectrum? Explain.
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 16

Each of the following compounds has a single $^{1} \mathrm{H}$ NMR peak. Approximately where would you expect each compound to absorb?
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 17

Identify the different kinds of nonequivalent protons in the following molecule, and tell where you would expect each to absorb:
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 18

How many peaks would you expect in the 'H NMR spectrum of 1,4-dimethylbenzene (paraxylene, or $p$ -xylene)? What ratio of peak areas would you expect on integration of the spectrum? Refer to Table 13.3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2.4 that aromatic rings have two resonance forms.)
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 19

Predict the splitting patterns you would expect for each proton in the following molecules:
(a) $\mathrm{CHBr}_{2} \mathrm{CH}_{3} \quad$ (b) $\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{2} \mathrm{Br} \quad$ (c) $\mathrm{CICH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 20

Draw structures for compounds that meet the following descriptions:
(a) $\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O} ;$ one singlet
(b) $\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl} ;$ one doublet and one septet
(c) $C_{4} \mathrm{H}_{8} \mathrm{Cl}_{2} \mathrm{O} ;$ two triplets
(d) $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}$ one singlet, one triplet, and one quartet

Lottie Adams

Numerade Educator

Problem 21

The integrated 'H NMR spectrum of a compound of formula $\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}$ is shown in Figure 13.17. Propose a structure.
(TABLE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 22

3.-Bromo-1-phenyl-1-propene shows a complex NMR spectrum in which the vinylic proton at $C 2$ is coupled with both the Cl vinylic proton $(J=16 \mathrm{Hz}$ ) and the C3 methylene protons $(J=8 \mathrm{Hz}) .$ Draw a tree diagram for the $\mathrm{C} 2$ proton signal, and account for the fact that a five-line multiplet is observed.
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 23

How could you use $^{1} \mathrm{H}$ NMR to determine the regiochemistry of electrophilic addition to alkenes? For example, does addition of HCI to 1 methylcyclohexene yield 1-chloro-1-methylcyclohexane or 1 -chloro-2-methylcyclohexane?

Lottie Adams

Numerade Educator

Problem 24

Into how many peaks would you expect the $^{1} \mathrm{H}$ NMR signals of the indicated protons to be split? (Green = Cl.)
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 25

How many absorption's would you expect the following compound to have in its $1 \mathrm{H}$ and $^{13} \mathrm{C}$ NMR spectra?
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 26

Sketch what you might expect the $^{1} \mathrm{H}$ and $^{13} \mathrm{C}$ NMR spectra of the following compound to look like (green = Cl):
(FIGURE CANNOT COPY)

Anish Wadhwa

Numerade Educator

Problem 27

How many electronically nonequivalent kinds of protons and how many kinds of carbons are present in the following compound? Don't forget that cyclohexane rings can ring-flip.
(FIGURE CANNOT COPY)

Anish Wadhwa

Numerade Educator

Problem 28

Identify the indicated protons in the following molecules as unrelated, homotopic, enantiotopic, or diastereotopic.
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 29

Chemical Shifts and NMR Spectroscopy
The following $^{1} \mathrm{H}$ NMR absorptions were obtained on a spectrometer operating at $200 \mathrm{MHz}$ and are given in hertz downfield from the TMS standard. Convert the absorptions to $\delta$ units.
(a) $436 \mathrm{Hz}$
(b) $956 \mathrm{Hz}$
(c) $1504 \mathrm{Hz}$

Lottie Adams

Numerade Educator

Problem 30

The following $^{1}$ H NMR absorptions were obtained on a spectrometer operating at 300 MHz. Convert the chemical shifts from $\delta$ units to hertz downfield from TMS.
(a) $2.1 \delta$
(b) 3.45 .8
(c) 6.30 .8
(d) 7.708

Lottie Adams

Numerade Educator

Problem 31

When measured on a spectrometer operating at $200 \mathrm{MHz},$ chloroform (CHCla) shows a single sharp absorption at 7.3 \delta.
(a) How many parts per million downfield from TMS does chloroform absorb?
(b) How many hertz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at $360 \mathrm{MHz} ?$
(c) What would be the position of the chloroform absorption in $\delta$ units when measured on a $360 \mathrm{MHz}$ spectrometer?

Zubair Abdulla

Numerade Educator

Problem 32

Why do you suppose accidental overlap of signals is much more common in 1 H NMR than in $^{13}$ C NMR?

Zubair Abdulla

Numerade Educator

Problem 33

Is a nucleus that absorbs at 6.50 s more shielded or less shielded than a nucleus that absorbs at $3.20 .8 ?$ Does the nucleus that absorbs at $6.50 \delta$ require a stronger applied field or a weaker applied field to come into resonance than the nucleus that absorbs at $3.208 ?$

Zubair Abdulla

Numerade Educator

Problem 34

$^{1} \mathrm{H}$ NMR Spectroscopy
How many types of nonequivalent protons are present in each of the following molecules?
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 35

The following compounds all show a single line in their $1 \mathrm{H}$ NMR spectra. List them in expected order of increasing chemical shift:
$\mathrm{CH}_{4}, \mathrm{CH}_{2} \mathrm{Cl}_{2},$ cyclohexane, $\mathrm{CH}_{3} \mathrm{COCH}_{3}, \mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2},$ benzene

Lottie Adams

Numerade Educator

Problem 36

How many signals would you expect each of the following molecules to have in its $1 \mathrm{H}$ and $13 \mathrm{C}$ spectra?
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 37

Propose structures for compounds with the following formulas that show only one peak in their $^{1} \mathrm{H}$ NMR spectra:
(a) $\mathrm{C}_{5} \mathrm{H}_{12}$
(b) $\mathrm{C}_{5} \mathrm{H}_{10}$
(c) $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}$

Lottie Adams

Numerade Educator

Problem 38

Predict the splitting pattern for each kind of hydrogen in the following molecules:
(a) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CH}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}_{3}$
(c) trans-2-Butene

Lottie Adams

Numerade Educator

Problem 39

Predict the splitting pattern for each kind of hydrogen in isopropyl propanoate, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}$

Lottie Adams

Numerade Educator

Problem 40

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:

Lottie Adams

Numerade Educator

Problem 41

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic:
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 42

The acid-catalyzed dehydration of 1 -methylcyclohexanol yields a mixture of two alkenes. How could you use $^{1} \mathrm{H}$ NMR to help you decide which was which?

Ian Kaigh

Numerade Educator

Problem 43

$\beta$ -Ocimene is a pleasant-smelling hydrocarbon found in the leaves of certain herbs. It has the molecular formula $\mathrm{C}_{10} \mathrm{H}_{16}$ and a UV absorption maximum at $232 \mathrm{nm}$. On hydrogenation with a palladium catalyst, $2,6-$ dimethyloctane is obtained. Ozonolysis of $\beta$ -ocimene, followed by treatment with zinc and acetic acid, produces the following four fragments:
(FIGURE CANNOT COPY)
(a) Propose two possible structures for A.
(b) Hydrocarbon A reacts with maleic anhydride to yield a Diels-Alder adduct. Which of your structures for $A$ is correct?
(c) Write the reactions, showing starting material and products.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 44

Treatment of 3,4 -dibromohexane with strong base leads to loss of 2 equivalents of HBr and formation of a product with formula $C_{6} \mathrm{H}_{10}$ - Three products are possible. Name each of the three, and tell how you would use $^{1} \mathrm{H}$ and $^{13} \mathrm{C}$ NMR spectroscopy to help identify them. How would you use UV spectroscopy?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 45

Addition of HCI to 1 -methoxycyclohexene yields 1 -chloro- 1 methoxycyclohexane as the sole product. Use resonance structures to explain why none of the other regioisomer is formed.
(FIGURE CANNOT COPY)

Prashant Bana

Numerade Educator

Problem 46

Aldrin, a chlorinated insecticide now banned for use in the United States, can be made by Diels-Alder reaction of hexachloro-1,3-cyclopentadiene with norbornadiene. What is the structure of aldrin?
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 47

Norbornadiene (Problem 14.46 ) can be prepared by reaction of chloroethylene with 1,3 -cyclopentadiene, followed by treatment of the product with sodium ethoxide. Write the overall scheme, and identify the two kinds of reactions.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 48

A We've seen that the Diels-Alder cycloaddition reaction is a one-step, pericyclic process that occurs through a cyclic transition state. Propose a mechanism for the following reaction:
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 49

In light of your answer to Problem $14.48,$ propose a mechanism for the following reaction:
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 50

The triene shown here reacts with 2 equivalents of maleic anhydride to yield
a product with the formula $\mathrm{C}_{17} \mathrm{H}_{16} \mathrm{O}_{6}$. Predict a structure for the product.
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 51

Myrcene, $\mathrm{C}_{10} \mathrm{H}_{16}$, is found in oil of bay leaves and is isomeric with $\beta$ -ocimene (Problem 14.43 ). It has an ultraviolet absorption at $226 \mathrm{nm}$ and can be catalytically hydrogenated to yield $2,6-$ dimethyloctane. On ozonolysis followed by zinc/acetic acid treatment, myrcene yields formaldehyde, acetone, and 2-oxopentanedial:
(FIGURE CANNOT COPY)
Propose a structure for myrcene, and write the reactions, showing starting material and products.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 52

Hydrocarbon $\mathrm{A}, \mathrm{C}_{10} \mathrm{H}_{14},$ has a UV absorption at $\lambda_{\max }=236 \mathrm{nm}$ and gives hydrocarbon $\mathbf{B}, \mathbf{C}_{10} \mathbf{H}_{18},$ on catalytic hydrogenation. Ozonolysis of A followed by zinc/acetic acid treatment yields the following diketo dialdehyde:
(FIGURE CANNOT COPY)
(a) Propose two possible structures for A.
(b) Hydrocarbon A reacts with maleic anhydride to yield a Diels-Alder adduct. Which of your structures for $A$ is correct?
(c) Write the reactions, showing starting material and products.

Raghvendra Singh

Raghvendra Singh

Numerade Educator

Problem 53

Adiponitrile, a starting material used in the manufacture of nylon, can be prepared in three steps from 1,3 -butadiene. How would you carry out this synthesis?
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 54

Ergosterol, a precursor of vitamin D, has $\lambda_{\max }=282 \mathrm{nm}$ and molar absorptiv. ity $\epsilon=11,900 .$ What is the concentration of ergosterol in a solution whose absorbance $A=0.065$ with a sample pathlength $I=1.00 \mathrm{cm} ?$
(FIGURE CANNOT COPY)

Lottie Adams

Numerade Educator

Problem 55

A Dimethyl butynedioate undergoes a Diels-Alder reaction with $(2 E, 4 E)$ hexadiene. Show the structure and stereochemistry of the product.
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 56

Dimethyl butynedioate also undergoes a Diels-Alder reaction with $(2 E, 4 Z)$ hexadiene, but the stereochemistry of the product is different from that of the $(2 E, 4 E)$ isomer (Problem 14.55). Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 57

How would you carry out the following synthesis (more than one step is required)? What stereochemical relationship between the $-\mathrm{CO}_{2} \mathrm{CH}_{3}$ group attached to the cyclohexane ring and the - CHO groups would your synthesis produce?
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 58

The double bond of an enamine (alkene + amine) is much more nucleophilic than a typical alkene double bond. Assuming that the nitrogen atom in an enamine is $s p^{2}$ -hybridized, draw an orbital picture of an enamine, and explain why the double bond is electron-rich.
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 59

Benzene has an ultraviolet absorption at $\lambda_{\max }=204 \mathrm{nm},$ and para-toluidine has $\lambda_{\max }=235 \mathrm{nm} .$ How do you account for this difference?
(FIGURE CANNOT COPY)

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator