Organic Chemistry

Janice Gorzynski Smith

Chapter 14

Nuclear Magnetic Resonance Spectroscopy

Educators


Problem 1

The $^1H$ NMR spectrum of $CH_3OH$ recorded on a 500 MHz NMR spectrometer consists of two
signals, one due to the $CH_3$ protons at 1715 Hz and one due to the OH proton at 1830 Hz, both
measured downfield from TMS. (a) Calculate the chemical shift of each absorption. (b) Do the $CH_3$ protons absorb upfield or downfield from the OH proton?

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Problem 2

The $^1H$ NMR spectrum of 1,2-dimethoxyethane $(CH_3OCH_2CH_2OCH_3)$ recorded on a 300 MHz NMR spectrometer consists of signals at 1017 Hz and 1065 Hz downfield from TMS. (a) Calculate the chemical shift of each absorption. (b) At what frequency would each absorption occur if the spectrum were recorded on a 500 MHz NMR spectrometer?

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Problem 3

How many $^1H$ NMR signals does each compound show?
a. $CH_3CH_3$ b. $CH_3CH_2CH_3$ c. $CH_3CH_2CH_2CH_3$ d. $(CH_3)_2CHCH(CH_3)_2$ e. $CH_3CH_2CO_2CH_2CH_3$ f. $CH_3OCH_2CH(CH_3)_2$ g. $CH_3(CH_2)_7CI$ h. $CH_3CH_2CH_2OH$

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Problem 4

How many $^1H$ NMR signals does each dimethylcyclopropane show?

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Problem 5

Label the protons in each indicated $CH_2$ group as enantiotopic, diastereotopic, or neither.

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Problem 6

How many $^1H$ NMR signals would you expect for each compound: (a) $CH_3CH(Cl)CH_2CH_3$;
(b) $CICH_2CH(CH_3)OCH_3$; (c) $CH_3CH(Br)CH_2CH_2CH_3$?

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Problem 7

For each compound, which of the underlined protons absorbs farther downfield:
(a) $FC \underline{H}_2CH_2C \underline{H}_2CI$; (b) $CH_3C \underline{H}_2CH_2C \underline{H}_2OCH_3$; (c) $C \underline{H}_3OC(C \underline{H}_3)_3$?

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Problem 8

For each compound, first label each different type of proton and then rank the protons in order of increasing chemical shift.

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Problem 9

Rank each group of protons in order of increasing chemical shift.

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Problem 10

Which compounds give a $^1H$ NMR spectrum with two signals in a ratio of 2:3?
a. $CH_3CH_2CI$ b. $CH_3CH_2CH_3$ c. $CH_3CH_20CH_2CH_3$
d. $CH_30CH_2CH_20CH_3$
Knowing the molecular formula of a compound and integration values from its $^1H$ NMR spectrum
gives the actual number of protons responsible for a particular signal.

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Problem 11

A compound of molecular formula $C_8H_{14}O_2$ gives three NMR signals having the indicated
integration values: signal [A] 14 units, signal [Bl 12 units, and signal [C] 44 units. How many protons
give rise to each signal?

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Problem 12

Compound $\textbf{A}$ exhibits two signals in its $^1H$ NMR spectrum at 2.64 and 3.69 ppm and the ratio of the absorbing signals is 2:3. Compound $\textbf{B}$ exhibits two signals in its $^1H$ NMR spectrum at 2.09 and 4.27 ppm and the ratio of the absorbing signals is 3:2. Which compound corresponds to $CH_3O_2CCH_2CH_2CO_2CH_3$ (dimethyl succinate) and which compound corresponds to $CH_3CO_2CH_2CH_2O_2CCH_3$ (ethylene diacetate)?

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Problem 13

Into how many peaks will each indicated proton be split?

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Problem 14

For each compound give the number of $^1H$ NMR signals, and then determine how many peaks are
present for each NMR signal.

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Problem 15

Sketch the NMR spectrum of $CH_3CH_2CI$, giving the approximate location of each NMR signal.

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Problem 16

How many peaks are present in the NMR signal of each indicated proton?

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Problem 17

Describe the $^1H$ NMR spectrum of each compound. State how many NMR signals are present, the
splitting pattern for each signal, and the approximate chemical shift.

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Problem 18

Draw a splitting diagram for $H_b$ in $trans$-1,3-dichloropropene, given that $J_{ab}= 13.1$ Hz and $J_{bc} = 7.2$ Hz.

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Problem 19

Identify $\textbf{A}$ and $\textbf{B}$, isomers of molecular formula $C_3H_4Cl_2$, from the given $^1H$ NMR data: Compound $\textbf{A}$ exhibits signals at 1.75 (doublet, 3 H, $J = 6.9$ Hz) and 5.89 (quartet, 1 H, $J = 6.9$ Hz) ppm. Compound $\textbf{B}$ exhibits signals at 4.16 (singlet, 2 H), 5.42 (doublet, 1 H, $J = 1.9$ Hz), and 5.59 (doublet, 1 H, $J = 1.9$ Hz) ppm.

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Problem 20

How many signals are present in the $^1H$ NMR spectrum for each molecule? What splitting is
observed in each signal: (a) $(CH_3)_3CCH_2OH$; (b) $CH_3CH_2CH_2OH$; (c) $(CH_3)_2CHNH_2$?

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Problem 21

What protons in alcohol $\textbf{A}$ give rise to each signal in its $^1H$ NMR spectrum? Explain all splitting patterns observed for absorptions between 0-7 ppm.

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Problem 22

How many peaks are observed in the $^1H$ NMR signal for each proton shown in red in palau 'amine, the complex chapter-opening molecule?

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Problem 23

Propose a structure for a compound of molecular formula $C_7H_{14}0_2$ with an IR absorption at
1740 $cm^{-1}$ and the following $^1H$ NMR data:

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Problem 24

Propose a structure for a compound of molecular formula $C_3H_80$ with an IR absorption at
3600-3200 $cm^{-1}$ and the following NMR spectrum:

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Problem 25

Identify products $\textbf{A}$ and $\textbf{B}$ from the given $^1H$ NMR data.
a. Treatment of $CH_2 = CHCOCH_3$ with one equivalent of HCI forms compound $\textbf{A}$. $\textbf{A}$ exhibits the following absorptions in its $^1H$ NMR spectrum: 2.2 (singlet, 3 H), 3.05 (triplet, 2 H), and 3.6 (triplet, 2 H) ppm. What is the structure of $\textbf{A}$?
b. Treatment of acetone $[(CH_3)_2C = O]$ with dilute aqueous base forms $\textbf{B}$. Compound $\textbf{B}$ exhibits four singlets in its $^1H$ NMR spectrum at 1.3 (6 H), 2.2 (3 H), 2.5 (2 H), and 3.8 (1 H) ppm. What is the structure of $\textbf{B}$?

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Problem 26

How many lines are observed in the $^{13}C$ NMR spectrum of each compound?

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Problem 27

Draw all constitutional isomers of molecular formula $C_3H_6Cl_2$.
a. How many signals does each isomer exhibit in its $^1H$ NMR spectrum?
b. How many lines does each isomer exhibit in its $^{13}C$ NMR spectrum?
c. When only the number of signals in both $^1H$ and $^{13}C$ NMR spectroscopy is considered, is it possible to distinguish all of these constitutional isomers?

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Problem 28

Esters of chrysanthemic acid are naturally occurring insecticides. How many lines are present in the $^{13}C$ NMR spectrum of chrysanthemic acid?

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Problem 29

Which of the indicated carbon atoms in each molecule absorbs farther downfield?

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Problem 30

Identify the carbon atoms that give rise to each NMR signal.

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Problem 31

A compound of molecular formula $C_4H_8O_2$ shows no IR peaks at 3600-3200 or 1700 $cm^{-1}$. It exhibits one singlet in its $^1H$ NMR spectrum at 3.69 ppm, and one line in its $^{13}C$ NMR spectrum at 67 ppm. What is the structure of this unknown?

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Problem 32

Draw the structure of a compound of molecular formula $C_4H_8O$ that has a signal in its $^{13}C$ NMR spectrum at > 160 ppm. Then draw the structure of an isomer of molecular formula $C_4H_8O$ that has all of its $^{13}C$ NMR signals at < 160 ppm.

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Problem 33

(a) How many $^1H$ NMR signals does each of the following compounds exhibit? (b) How many $^{13}C$ NMR signals does each compound exhibit?

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Problem 34

(a) How many $^1H$ NMR signals does each compound show? (b) Into how many peaks is each signal split?

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Problem 35

How many different types of protons are present in each compound?

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Problem 36

How many $^1H$ NMR signals does each compound give?

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Problem 37

How many $^1H$ NMR signals does each natural product exhibit?

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Problem 38

Using a 300 MHz NMR instrument:
a. How many Hz downfield from TMS is a signal at 2.5 ppm?
b. If a signal comes at 1200 Hz downfield from TMS, at what ppm does it occur?
c. If two peaks are separated by 2 ppm, how many Hz does this correspond to?

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Problem 39

Acetone exhibits a singlet in its $^1H$ NMR spectrum at 2.16 ppm. If $CH_2Cl_2$ exhibits a singlet 1570 Hz downfield from acetone on a 500 MHz NMR spectrometer, what is the chemical shift of the singlet due to $CH_2Cl_2$?

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Problem 40

Which of the indicated protons in each pair absorbs farther downfield?

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Problem 41

A compound of molecular formula $C_6H_{10}$ gives three signals in its $^1H$ NMR spectrum with the following integration units: 13, 33, 73 units. How many protons are responsible for each signal?

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Problem 42

How could you use chemical shift and integration data in $^1H$ NMR spectroscopy to distinguish between each pair of compounds? The $^1H$ NMR spectrum of each compound contains only singlets.

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Problem 43

Which compounds give one singlet in the $^1H$ NMR spectrum?

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Problem 44

Into how many peaks will the signal for each of the indicated protons be split?

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Problem 45

How can you use $^1H$ NMR spectroscopy to distinguish between $CH_2 = C(Br)C0_2CH_3$ and methyl (2E)-3-bromo-2-propenoate, $BrCH = CHC0_2CH_3$?

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Problem 46

Label the signals due to $H_a$, $H_b$, and $H_c$ in the $^1H$ NMR spectrum of acrylonitrile $(CH_2= CHCN)$. Draw a splitting diagram for the absorption due to the $H_a$ proton.

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Problem 47

Draw the four constitutional isomers having molecular formula $C_4H_9Br$ and indicate how many different kinds of carbon atoms each has.

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Problem 48

Which compounds in Problem 14.43 give one signal in their $^{13}C$ NMR spectra?

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Problem 49

Explain why the carbonyl carbon of an aldehyde or ketone absorbs farther downfield than the carbonyl carbon of an ester in a $^{13}C$ NMR spectrum.

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Problem 50

How many $^{13}C$ NMR signals does each compound exhibit?

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Problem 51

Rank the indicated carbon atoms in each compound in order of increasing chemical shift.

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Problem 52

Identify the carbon atoms that give rise to the signals in the $^{13}C$ NMR spectrum of each compound.
a, $CH_3CH_2CH_2CH_2OH$; $^{13}C$ NMR: 14, 19, 35, and 62 ppm
b. $(CH_3)_2CHCHO$; $^{13}C$ NMR: 16, 41, and 205 ppm
c. $CH_2 =CHCH(OH)CH_3$; $^{13}C$ NMR: 23, 69, 113, and 143 ppm

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Problem 53

a. How many signals does dimethyl fumarate $(CH_3O_2CCH = CHCO_2CH_3$, with a trans C = C) exhibit in its $^{13}C$ NMR spectrum?
b, Draw the structure of an isomer of dimethyl fumarate that has each of the following number of signals in its $^{13}C$ NMR spectrum: [1] three; [2] four; [5] five.

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Problem 54

Propose a structure consistent with each set of spectral data:
a. $C_4H_8Br_2$: IR peak at 3000-2850 $cm^{-1}$; NMR (ppm):
1.87 (singlet, 6 H)
3.86 (singlet, 2 H)
b. $C_3H_6Br_2$: IR peak at 3000-2850 $cm^{-1}$; NMR (ppm):
2.4 (quintet)
3.5 (triplet)
c. $C_5H_{10}O_2$: IR peak at 17 40 $cm^{-1}$; NMR (ppm):
1.15 (triplet, 3 H) 2.30 (quartet, 2 H)
1.25 (triplet, 3 H) 4.72 (quartet, 2 H)
d. $C_6H_{14}O$: IR peak at 3600-3200 $cm^{-1}$; NMR (ppm):
0.8 (triplet, 6 H) 1.5 (quartet, 4 H)
1.0 (singlet, 3 H) 1.6 (singlet, 1 H)
e. $C_6H_{14}O$: IR peak at 3000-2850 $cm^{-1}$; NMR (ppm):
1.10 (doublet, 30 units)
3.60 (septet, 5 units)
f. $C_3H_6O$: IR peak at 1730 $cm^{-1}$; NMR (ppm):
1.11 (triplet)
2.46 (multiplet)
9.79 (triplet)

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Problem 55

Identify the structures of isomers $\textbf{A}$ and $\textbf{B}$ (molecular formula $C_9H_{10}O$).
Compound $\textbf{A}$: IR peak at 1742 $cm^{-1}$; $^1H$ NMR data (ppm) at 2.15 (singlet, 3 H), 3.70 (singlet, 2 H), and 7.20 (broad singlet, 5 H).
Compound $\textbf{B}$: IR peak at 1688 $cm^{-1}$; $^1H$ NMR data (ppm) at 1.22 (triplet, 3 H), 2.98 (quartet, 2 H), and 7.28-7.95 (multiplet, 5 H).

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Problem 56

Reaction of $C_6H_5CH_2CH_2OH$ with $CH_3COCI$ affords compound $\textbf{W}$, which has molecular formula $C_{10}H_{12}O_2$. $\textbf{W}$ shows prominent IR absorptions at 3088-2897, 1740, and 1606 $cm^{-1}$. $\textbf{W}$ exhibits the following signals in its $^1H$ NMR spectrum: 2.02 (singlet), 2.91 (triplet), 4.25 (triplet), and 7.20-7.35 (multiplet) ppm. What is the structure of $\textbf{W}$? We will learn about this reaction in Chapter 22.

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Problem 57

Treatment of 2-methylpropanenitrile $[(CH_3)_2CHCN]$ with $CH_3CH_2CH_2MgBr$, followed by aqueous acid, affords compound $\textbf{V}$, which has molecular formula $C_7H_{14}O$. $\textbf{V}$ has a strong absorption in its IR spectrum at 1713 $cm^{-1}$, and gives the following $^1H$ NMR data: 0.91 (triplet, 3 H), 1.09 (doublet, 6 H), 1.6 (multiplet, 2 H), 2.43 (triplet, 2 H), and 2.60 (septet, 1 H) ppm. What is the structure of $\textbf{V}$? We will learn about this reaction in Chapter 22.

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Problem 58

Compound $\textbf{C}$ has a molecular ion in its mass spectrum at 146 and a prominent absorption in its IR spectrum at 1762 $cm^{-1}$. C shows the following $^1H$ NMR spectral data: 1.47 (doublet, 3 H), 2.07 (singlet, 6 H), and 6.84 (quartet, 1 H) ppm. What is the structure of $\textbf{C}$?

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Problem 59

As we will learn in Chapter 20, reaction of $(CH_3)CO$ with $LiC \equiv CH$ followed by $H_2O$ affords compound $\textbf{D}$, which has a molecular ion in its mass spectrum at 84 and prominent absorptions in its IR spectrum at 3600-3200, 3303, 2938, and 2120 $cm^{-1}$. $\textbf{D}$ shows the following $^1H$ NMR spectral data: 1.53 (singlet, 6 H), 2.37 (singlet, 1 H), and 2.43 (singlet, 1 H) ppm. What is the structure of $\textbf{D}$?

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Problem 60

Identify the structures of isomers $\textbf{E}$ and $\textbf{F}$ (molecular formula $C_4H_80_2$).

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Problem 61

Identify the structures of isomers $\textbf{H}$ and $\textbf{I}$ (molecular formula $C_8H_{11} N$).

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Problem 62

Propose a structure consistent with each set of data.

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Problem 63

Reaction of $(CH_3)_3CCHO$ with $(C_6H_5)_3P = C(CH_3)OCH_3$, followed by treatment with aqueous acid, affords $\textbf{R}$ $(C_7H_{14}O$). $\textbf{R}$ has a strong absorption in its IR spectrum at 1717 $cm^{-1}$ and three singlets in its $^1H$ NMR spectrum at 1.02 (9 H), 2.13 (3 H), and 2.33 (2 H) ppm. What is the structure of $\textbf{R}$? We will learn about this reaction in Chapter 21.

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Problem 64

Reaction of aldehyde $\textbf{D}$ with amino alcohol $\textbf{E}$ in the presence of NaH forms $\textbf{F}$ (molecular formula $C_{11}H_{15}NO_2$). F absorbs at 1730 $cm^{-1}$ in its IR spectrum.Falso shows eight lines in its $^{13}C$ NMR spectrum, and gives the following $^1H$ NMR spectrum: 2.32 (singlet, 6 H), 3.05 (triplet, 2 H), 4.20 (triplet, 2 H), 6.97 (doublet, 2 H), 7.82 (doublet, 2 H), and 9.97 (singlet, 1 H) ppm. Propose a structure for $\textbf{F}$. We will learn about this reaction in Chapter 18.

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Problem 65

Propose a structure consistent with each set of data.
a. Compound $\textbf{J}$: molecular ion at 72; IR peak at 1710 $cm^{-1}$; $^1H$ NMR data (ppm) at 1.0 (triplet, 3 H), 2.1 (singlet, 3 H), and 2.4 (quartet, 2 H)
b. Compound $\textbf{K}$: molecular ion at 88; IR peak at 3600-3200 $cm^{-1}$; $^1H$ NMR data (ppm) at 0.9 (triplet, 3 H), 1.2 (singlet, 6 H), 1.5 (quartet, 2 H), and 1.6 (singlet, 1 H)

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Problem 66

In the presence of a small amount of acid, a solution of acetaldehyde $(CH_3CHO)$ in methanol $(CH_3CHO)$ was allowed to stand and a new compound $\textbf{L}$ was formed. $\textbf{L}$ has a molecular ion in its mass spectrum at 90 and IR absorptions at 2992 and 2941 $cm^{-1}$. $\textbf{L}$ shows three signals in its $^{13}C$ NMR at 19, 52, and 101 ppm. The $^1H$ NMR spectrum of Lis given below. What is the structure of $\textbf{L}$?

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Problem 67

Treatment of $(CH_3)_2CHCH(OH)CH_2CH_3$ with TsOH affords two products ($\textbf{M}$ and $\textbf{N}$) with molecular formula $C_6H_{12}$ . The $^1H$ NMR spectra of $\textbf{M}$ and $\textbf{N}$ are given below. Propose structures for $\textbf{M}$ and $\textbf{N}$ and draw a mechanism to explain their formation.

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Problem 68

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Problem 69

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Problem 70

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Problem 71

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Problem 72

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Problem 73

Propose a structure consistent with each set of data.
a. A compound $\textbf{X}$ (molecular formula $C_6H_{12}0_2$) gives a strong peak in its IR spectrum at 17 40 $cm^{-1}$. The $^1H$ NMR spectrum of $\textbf{X}$ shows only two singlets, including one at 3.5 ppm. The $^{13}C$ NMR spectrum is given below. Propose a structure for $\textbf{X}$.
b. A compound $\textbf{Y}$ (molecular formula $C_6H_{10}$) gives four lines in its $^{13}C$ NMR spectrum (27, 30, 67, and 93 ppm), and the IR spectrum given here. Propose a structure for $\textbf{Y}$.

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Problem 74

Reaction of unknown $\textbf{A}$ with HCI forms chlorohydrin $\textbf{B}$ as the major product. $\textbf{A}$ shows no absorptions in its IR spectrum at 1700 $cm^{-1}$ or 3600-3200 $cm^{-1}$, and gives the following $^1H$ NMR data: 1.4 (doublet, 3 H), 3.0 (quartet of doublets, 1 H), 3.5 (doublet, 1 H), 3.8 (singlet, 3 H), 6.9 (doublet, 2 H), and 7.2 (doublet, 2 H) ppm. (a) Propose a structure for $\textbf{A}$, including stereochemistry. (b) Explain why $\textbf{B}$ is the major product in this reaction.

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Problem 75

The $^1H$ NMR spectrum of $N,N$-dimethylformamide shows three singlets at 2.9, 3.0, and 8.0 ppm. Explain why the two $CH_3$ groups are not equivalent to each other, thus giving rise to two NMR signals.

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Problem 76

18-Annulene shows two signals in its $^1H$ NMR spectrum, one at 8.9 (12 H) and one at -1.8 (6 H) ppm. Using a similar argument to that offered for the chemical shift of benzene protons, explain why both shielded and deshielded values are observed for 18-annulene.

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Problem 77

Explain why the $^{13}C$ NMR spectrum of 3-methyl-2-butanol shows five signals.

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Problem 78

Since $^{31}P$ has an odd mass number, $^{31}P$ nuclei absorb in the NMR and, in many ways, these nuclei behave similarly to protons in NMR spectroscopy. With this in mind, explain why the $^1H$ NMR spectrum of methyl dimethylphosphonate, $CH_3PO(OCH_3)_2$ consists of two doublets at 1 .5 and 3. 7 ppm.

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Problem 79

2-Cyclohexenone has two protons on its carbon-carbon double bond (labeled $H_a$ and $H_b$) and two protons on the carbon adjacent to the double bond (labeled $H_c$). (a) If $J_{ab}$ = 11 Hz and $J_{bc}$ = 4 Hz, sketch the splitting pattern observed for each proton on the $sp^2$ hybridized carbons. (b) Despite the fact that Ha is located adjacent to an electron-withdrawing C= 0, its absorption occurs upfield from the signal due to $H_b$ (6.0 vs. 7.0 ppm). Offer an explanation.

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