Organic Chemistry

Paula Yurkanis Bruice

Chapter 14

NMR Spectroscopy - all with Video Answers

Educators

Chapter Questions

Problem 1

What frequency (in $\mathrm{MHz}$ ) is required to cause a proton to flip its spin when it is exposed to a magnetic field of 1 tesla?

Tyler Walter

Tyler Walter

Numerade Educator

Problem 2

a. Calculate the magnetic field (in tesla) required to flip an $^{1} \mathrm{H}$ nucleus in an $\mathrm{NMR}$ spectrometer that operates at $360 \mathrm{MHz}.$
b. What strength magnetic field is required when a $500-\mathrm{MHz}$ instrument is used?

Tyler Walter

Numerade Educator

Problem 3

How many signals would you expect to see in the $^{1} \mathrm{H}$ NMR spectrum of each of the following compounds?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 4

How could you distinguish the 'H NMR spectra of the following compounds?

Tyler Walter

Numerade Educator

Problem 5

There are three isomeric dichlorocyclopropanes. Their $^{1} \mathrm{H}$ NMR spectra show one signal for isomer $1,$ two signals for isomer $2,$ and three signals for isomer $3 .$ Draw the structures of isomers $1,2,$ and 3.

Tyler Walter

Numerade Educator

Problem 6

A signal has been reported to occur at $600 \mathrm{Hz}$ downfield from TMS in an NMR spectrometer with a 300 -MHz operating frequency.
a. What is the chemical shift of the signal?
b. What would its chemical shift be in an instrument operating at $100 \mathrm{MHz} ?$
c. How many hertz downfield from TMS would the signal be in a 100 -MHz spectrometer?

George Bennett

Numerade Educator

Problem 7

a. If two signals differ by $1.5 \mathrm{ppm}$ in a $300-\mathrm{MHz}$ spectrometer, by how much do they differ in a 100 -MHz spectrometer?
b. If two signals differ by 90 hertz in a $300-\mathrm{MHz}$ spectrometer, by how much do they differ in a 100 -MHz spectrometer?

George Bennett

Numerade Educator

Problem 8

Where would you expect to find the $^{1} \mathrm{H}$ NMR signal of $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Mg}$ relative to the TMS signal? (Hint: See Table 12.3 on p. 467.)

George Bennett

Numerade Educator

Problem 9

a. Which set of protons in each of the following compounds is the least shielded?
b. Which set of protons in each compound is the most shielded?

Tyler Walter

Numerade Educator

Problem 10

One of the spectra in Figure 14.6 is due to 1 -chloropropane, and the other to 1 -iodopropane. Which is which?

Tyler Walter

Numerade Educator

Problem 11

In each of the following compounds, which of the underlined protons has the greater chemical shift (i.e., the farther downfield signal or the higher frequency signal)?

Tyler Walter

Numerade Educator

Problem 12

In each of the following pairs of compounds, which of the underlined protons has the greater chemical shift (i.e., the farther downfield signal or the higher frequency signal)?

Tyler Walter

Numerade Educator

Problem 13

Without referring to Table $14.1,$ label the protons in the following compounds. The proton that gives the signal at the lowest frequency should be labeled $a,$ the next $b,$ etc.

George Bennett

Numerade Educator

Problem 14

How would integration distinguish the $^{1} \mathrm{H}$ NMR spectra of the following compounds?

Tyler Walter

Numerade Educator

Problem 15

a. Calculate the ratios of the different kinds of protons in a compound with an integral ratio of 6: 4: 18.4 (going from left to right across the spectrum).
b. Determine the structure of a compound that would give these relative integrals in the observed order.
a. Divide each by the smallest number:
$$
\frac{6}{4}=1.5 \quad \frac{4}{4}=1 \quad \frac{18.4}{4}=4.6
$$
Multiply by a number that will cause all the numbers to be close to whole numbers:
$$
1.5 \times 2=3 \quad 1 \times 2=2 \quad 4.6 \times 2=9
$$
The ratio 3: 2: 9 gives the relative numbers of the different kinds of protons. The actual ratio could be $6: 4: 18,$ or even some higher multiple, but let's not go there if we don't have to.
b. The "3" suggests a methyl, the "2" a methylene, and the "9" a tert butyl. The methyl is closest to a group causing deshielding, and the tert-butyl group is farthest away from the group causing deshielding. The following compound meets these requirements:

George Bennett

Numerade Educator

Problem 16

The $^{1} \mathrm{H}$ NMR spectrum shown in Figure 14.8 corresponds to one of the following compounds. Which compound is responsible for this spectrum?

Tyler Walter

Numerade Educator

Problem 17

[18]-Annulene shows two signals in its $^{1} \mathrm{H}$ NMR spectrum: one at $9.25 \mathrm{ppm}$ and the other very far upfield (beyond TMS) at -2.88 ppm. What hydrogens are responsible for each of the signals? (Hint: Notice the direction of the induced magnetic field outside and inside the benzene ring in Figure 14.9.)

George Bennett

Numerade Educator

Problem 18

Using a diagram like the one in Figure $14.13,$ predict
a. the relative intensities of the peaks in a triplet
b. the relative intensities of the peaks in a quintet

George Bennett

Numerade Educator

Problem 19

The $^{1} \mathrm{H}$ NMR spectra of two carboxylic acids with molecular formula $\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{O}_{2} \mathrm{Cl}$ are shown in Figure $14.14 .$ Identify the carboxylic acids. (The "offset" notation means that the signal has been moved to the right by the indicated amount).

Natalie Johns

Numerade Educator

Problem 20

Indicate the number of signals and the multiplicity of each signal in the $^{1} \mathrm{H}$ NMR spectrum of each of the following compounds:
a. $\mathrm{ICH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}$
b. $\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$
c. $\mathrm{ICH}_{2} \mathrm{CH}_{2} \mathrm{CHBr}_{2}$

Tyler Walter

Numerade Educator

Problem 21

Explain why the signal for the protons identified as $\mathrm{H}_{a}$ in Figure 14.19 appears at the lowest frequency and the signal for the protons identified as $\mathrm{H}_{c}$ appears at the highest frequency. (Hint: Draw the contributing resonance structures.)

George Bennett

Numerade Educator

Problem 22

How could $^{1} \mathrm{H}$ NMR spectra distinguish the following compounds?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 23

How would the $^{1} \mathrm{H}$ NMR spectra for the four compounds with molecular formula $\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{Br}_{2}$ differ?

Tyler Walter

Numerade Educator

Problem 24

Identify each compound from its molecular formula and its $^{1} \mathrm{H}$ NMR spectrum:

George Bennett

Numerade Educator

Problem 25

Predict the splitting patterns for the signals given by each of the compounds in Problem 3 .

Natalie Johns

Numerade Educator

Problem 26

Identify the following compounds. (Relative integrals are given from left to right across the spectrum.)
a. The $^{1} \mathrm{H}$ NMR spectrum of a compound with molecular formula $\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}_{2}$ has two singlets with an area ratio of 2: 3.
b. The $^{1} \mathrm{H}$ NMR spectrum of a compound with molecular formula $\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}_{2}$ has two singlets with an area ratio of 2: 3.
c. The $^{1} \mathrm{H}$ NMR spectrum of a compound with molecular formula $\mathrm{C}_{8} \mathrm{H}_{6} \mathrm{O}_{2}$ has two singlets with an area ratio of 1: 2.

Natalie Johns

Numerade Educator

Problem 27

Describe the $^{1} \mathrm{H}$ NMR spectrum you would expect for each of the following compounds, using relative chemical shifts rather than absolute chemical shifts:

Ian Kaigh

Numerade Educator

Problem 28

Why is there no coupling between $\mathrm{H}_{a}$ and $\mathrm{H}_{c}$ or between $\mathrm{H}_{b}$ and $\mathrm{H}_{c}$ in cis- or trans- 3 chloropropenoic acid?

Tyler Walter

Numerade Educator

Problem 29

Identify the compound with molecular formula $\mathrm{C}_{8} \mathrm{H}_{10} \mathrm{O}$ that gives the IR and $^{1} \mathrm{H} \mathrm{NMR}$ spectra shown in Figure 14.23.

George Bennett

Numerade Educator

Problem 30

The two hydrogens of a methylene group adjacent to an asymmetric carbon are not equivalent hydrogens because they are in different environments due to the asymmetric carbon. (You can verify this statement by examining molecular models.) Applying the $N+1$ rule to these two diastereotopic hydrogens (Section 5.16 ) separately in determining the multiplicity of the signal for the adjacent methyl hydrogens indicates that the signal should be a doublet of doublets. The signal, however, is a triplet. Using a splitting diagram, explain why it is a triplet rather than a doublet of doublets.
SOLUTION The observation of a triplet means that the $N+1$ rule did not have to be applied to the diastereotopic hydrogens separately but could have been applied to the two protons as a set $(N=2, \text { so } N+1=3) .$ This means that the coupling constant for splitting of the methyl signal by one of the methylene hydrogens is similar to the coupling constant for splitting by the other methylene hydrogen.

George Bennett

Numerade Educator

Problem 31

Draw a splitting diagram for $\mathrm{H}_{b},$ where
a. $J_{b a}=12 \mathrm{Hz}$ and $J_{b c}=6 \mathrm{Hz}$
b. $J_{b a}=12 \mathrm{Hz}$ and $J_{b c}=12 \mathrm{Hz}$

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 32

Explain why the chemical shift of the OH proton of a carboxylic acid is at a higher frequency than the chemical shift of an OH proton of an alcohol.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 33

Which would show the signal for the OH proton at a greater chemical shift, the $^{1} \mathrm{H}$ NMR spectrum of pure ethanol or the $^{1} \mathrm{H}$ NMR spectrum of ethanol dissolved in $\mathrm{CH}_{2} \mathrm{Cl}_{2} ?$

Mena Botros

Numerade Educator

Problem 34

Propose a mechanism for base-catalyzed proton exchange.

Zubair Abdulla

Numerade Educator

Problem 35

Identify the compound with molecular formula $\mathrm{C}_{3} \mathrm{H}_{7}$ NO responsible for the $^{1} \mathrm{H}$ NMR spectrum in Figure 14.29 .

Tyler Walter

Numerade Educator

Problem 36

Answer the following questions for each of the compounds:
a. How many signals are in the $^{13} \mathrm{C}$ NMR spectrum?
b. Which signal is at the lowest frequency?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 37

Describe the proton-coupled $^{13} \mathrm{C}$ NMR spectrum for compounds $1,3,$ and 5 in Problem 36 showing relative values (not absolute values) of chemical shifts.

Tyler Walter

Numerade Educator

Problem 38

How can $1,2-, 1,3-$, and 1,4 -dinitrobenzene be distinguished by
a. $^{1} \mathrm{H}$ NMR spectroscopy?
b. $^{13} \mathrm{C}$ NMR spectroscopy?

George Bennett

Numerade Educator

Problem 39

Identify each compound in Figure 14.35 from its molecular formula and its $^{13} \mathrm{C} \mathrm{NMR}$ spectrum.

George Bennett

Numerade Educator

Problem 40

Identify pairs of coupled protons in 2 -methyl- 3 -pentanone, using the COSY spectrum in Figure 14.39.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 41

How many signals are produced by each of the following compounds in its
a. 'H NMR spectrum?
b. $^{13} \mathrm{C}$ NMR spectrum?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 42

Draw a splitting diagram for the $\mathrm{H}_{b}$ proton and indicate its multiplicity if.
a. $\quad J_{b a}=J_{b c}$
b. $\quad J_{b a}=2 J_{b c}$

Tyler Walter

Numerade Educator

Problem 43

Label each set of chemically equivalent protons, using $a$ for the set that will be at the lowest frequency (farthest upfield) in the $^{1} \mathrm{H}$ NMR spectrum, $b$ for the next, etc. Indicate the multiplicity of each signal.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 44

Match each of the $^{1} \mathrm{H}$ NMR spectra on page 575 with one of the following compounds:

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 45

Determine the ratios of the chemically nonequivalent protons in a compound if the steps of the integration curves measure 40.5,27 , 13, and 118 $\mathrm{mm}$, from left to right across the spectrum. Give the structure of a compound whose $1 \mathrm{H}$ NMR spectrum would show these integrals in the observed order.

Zubair Abdulla

Numerade Educator

Problem 46

How could $^{1} \mathrm{H}$ NMR distinguish between the compounds in each of the following pairs?

Zubair Abdulla

Numerade Educator

Problem 47

Answer the following questions:
a. What is the relationship between chemical shift in ppm and operating frequency?
b. What is the relationship between chemical shift in hertz and operating frequency?
c. What is the relationship between coupling constant and operating frequency?
d. How does the operating frequency in NMR spectroscopy compare with the operating frequency in IR and UV/Vis spectroscopy?

Zubair Abdulla

Numerade Educator

Problem 48

The $^{1} \mathrm{H}$ NMR spectra of three isomers with molecular formula $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{Br}$ are shown here. Which isomer produces which spectrum?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 49

Identify each of the following compounds from the 1 H NMR data and molecular formula. The number of hydrogens responsible for each signal is shown in parentheses.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 50

Identify the compound with molecular formula $\mathrm{C}_{7} \mathrm{H}_{14} \mathrm{O}$ that gives the following proton-coupled $^{13} \mathrm{C}$ NMR spectrum.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 51

Compound A, with molecular formula $\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{Cl}$, shows two signals in its $^{13} \mathrm{C}$ NMR spectrum. Compound $\mathrm{B}$, an isomer of compound A, shows four signals, and in the proton-coupled mode, the signal farthest downfield is a doublet. Identify compounds $A$ and $B$.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 52

The $^{1} \mathrm{H}$ NMR spectra of three isomers with molecular formula $\mathrm{C}_{7} \mathrm{H}_{14} \mathrm{O}$ are shown here. Which isomer produces which spectrum?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 53

Would it be better to use $^{1} \mathrm{H}$ NMR or $^{13} \mathrm{C}$ NMR to distinguish among 1 -butene, cis - 2 -butene, and 2 -methylpropene? Explain your answer.

Tyler Walter

Numerade Educator

Problem 54

Determine the structure of each of the following unknown compounds based on its molecular formula and its IR and 'H NMR spectra.

George Bennett

Numerade Educator

Problem 55

There are four esters with molecular formula $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}$. How could they be distinguished by $1 \mathrm{H}$ NMR?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 56

An alkyl halide reacts with an alkoxide ion to form a compound whose 1 H NMR spectrum is shown here. Identify the alkyl halide and the alkoxide ion. (Hint: see Section 11.9.)

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 57

Determine the structure of each of the following compounds based on its molecular formula and its $^{13} \mathrm{C}$ NMR spectrum.
a. $C_{4} H_{10} O$
b. $C_{6} H_{12} O$

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 58

The $^{1} \mathrm{H}$ NMR spectrum of 2 -propen-1-ol is shown here. Indicate the protons in the molecule that give rise to each of the signals i e spectrum.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 59

How could the signals in the $6.5-8.1$ -ppm region of their $^{1} \mathrm{H}$ NMR spectra distinguish among the following compounds?

Tyler Walter

Numerade Educator

Problem 60

The $^{1} \mathrm{H}$ NMR spectra of two compounds with molecular formula $\mathrm{C}_{11} \mathrm{H}_{16}$ are shown here. Identify the compounds.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 61

Draw a splitting diagram for the $\mathrm{H}_{b}$ proton if $J_{b c}=10$ and $J_{b a}=5.$

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 62

Sketch the following spectra that would be obtained for 2 -chloroethanol:
a. The $^{1} \mathrm{H}$ NMR spectrum for a dry sample of the alcohol.
b. The $^{1} \mathrm{H}$ NMR spectrum for a sample of the alcohol that contains a trace amount of acid.
c. The $^{13}$ C NMR spectrum.
d. The proton-coupled $^{13}$ C NMR spectrum.
e. The four parts of a DEPT $^{13} \mathrm{C}$ NMR spectrum.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 63

How could $^{1} \mathrm{H}$ NMR be used to prove that the addition of $\mathrm{HBr}$ to propene follows the rule that says that the electrophile adds to the $s p^{2}$ carbon bonded to the greater number of hydrogens.

Tyler Walter

Numerade Educator

Problem 64

Identify each of the following compounds from its molecular formula and its $^{1} \mathrm{H}$ NMR spectrum.
a. $C_{8} H_{8}$
b. $C_{6} H_{12} O$
c. $C_{9} H_{18} O$
d. $C_{4} H_{8} {O}$

Zubair Abdulla

Numerade Educator

Problem 65

Dr. N. M. Arr was called in to help analyze the $^{1} \mathrm{H}$ NMR spectrum of a mixture of compounds known to contain only $\mathrm{C}, \mathrm{H}$, and $\mathrm{Br}$. The mixture showed two singlets -one at 1.8 ppm and the other at 2.7 ppm $-$ with relative integrals of 1: 6 , respectively. Dr. Arr determined that the spectrum was that of a mixture of bromomethane and 2 -bromo-2-methylpropane. What was the ratio of bromomethane to 2 -bromo- 2 -methylpropane in the mixture?

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 66

Calculate the amount of energy (in calories) required to flip an $^{1} \mathrm{H}$ nucleus in an $\mathrm{NMR}$ spectrometer that operates at $60 \mathrm{MHz}$.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 67

The following $^{1} \mathrm{H}$ NMR spectra are for four compounds with molecular formula $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}$. Identify the compounds.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 68

When compound $\mathrm{A}\left(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\right)$ is treated with $\mathrm{HBr}$, it forms compound $\mathrm{B}\left(\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Br}\right)$. The $^{1} \mathrm{H}$ NMR spectrum of compound $\mathrm{A}$ has one singlet (1), two doublets (3, 6), and two multiplets (both 1). (The relative areas of the signals are indicated in parentheses.) The 'H NMR spectrum of compound B has a singlet (6), a triplet (3), and a quartet (2). Identify compounds A and B.

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 69

Determine the structure of each of the following compounds based on its molecular formula and its IR and $^{1} \mathrm{H}$ NMR spectra.
a. $C_{6} H_{12} O$
b. $C_{6} H_{14} O$
c. $C_{10} H_{13} NO_{3}$
d. $C_{11} H_{14} O_{2}$

Sharfa Farzandh

Sharfa Farzandh

Numerade Educator

Problem 70

Identify the compound with molecular formula $\mathrm{C}_{3} \mathrm{H}_{5} \mathrm{Cl}_{3}$ that gives the following $^{13} \mathrm{C}$ NMR spectrum.

George Bennett

Numerade Educator

Problem 71

Determine the structure of each of the following compounds based on its mass, IR, and 'H NMR spectra.

George Bennett

Numerade Educator

Problem 72

Identify the compound with molecular formula $\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}$ that is responsible for the following $\mathrm{DEPT}^{13} \mathrm{C}$ NMR spectrum.

George Bennett

Numerade Educator

Problem 73

Identify the compound with molecular formula $\mathrm{C}_{6} \mathrm{H}_{14}$ that is responsible for the following $1 \mathrm{H}$ NMR spectrum.

George Bennett

Numerade Educator