Organic chemistry with biological applications

John E. McMurry

Chapter 10

Structure Determination: Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet Spectroscopy - all with Video Answers

Educators

Chapter Questions

Problem 1

The male sex hormone testosterone contains only $\mathrm{C}, \mathrm{H},$ and $\mathrm{O}$ and has a mass of 288.2089 amu, as determined by high-resolution mass spectrometry. What is the likely molecular formula of testosterone?

Dominique Joiner

Dominique Joiner

Numerade Educator

Problem 2

Two mass spectra are shown in FIGURE 10.7. One spectrum corresponds to 2-methylpent-2-ene; the other, to hex-2-ene. Which is which? Explain.

Dominique Joiner

Dominique Joiner

Numerade Educator

Problem 3

What are the masses of the charged fragments produced in the following cleavage pathways?
(a) Alpha cleavage of pentan-2-one $\left(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\right)$
(b) Dehydration of cyclohexanol
(c) McLaffertyrearrangementof4-methylpentan-2-one[CH $\left._{3} \mathrm{COCH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\right]$
(d) Alpha cleavage of triethylamine $\left[\left(\mathrm{CH}_{3} \mathrm{CH}_{2}\right)_{3} \mathrm{~N}\right]$

Ian Kaigh

Numerade Educator

Problem 4

List the masses of the parent ion and of several fragments that you might expect to find in the mass spectrum of the following molecule:

Ian Kaigh

Numerade Educator

Problem 5

Which has higher energy, infrared radiation with $\lambda=1.0 \times 10^{-6} \mathrm{~m}$ or an X ray with $\lambda=3.0 \times 10^{-9} \mathrm{~m}$ ? Radiation with $\nu=4.0 \times 10^{9} \mathrm{~Hz}$ or with $\lambda=9.0 \times 10^{-6} \mathrm{~m} ?$

Nicole Krahulik

Nicole Krahulik

Numerade Educator

Problem 6

It's useful to develop a feeling for the amounts of energy that correspond to different parts of the electromagnetic spectrum. Calculate the energies in $\mathrm{kJ} / \mathrm{mol}$ of each of the following kinds of radiation:
(a) A gamma ray with $\lambda=5.0 \times 10^{-11} \mathrm{~m}$
(b) $\operatorname{An} X$ ray with $\lambda=3.0 \times 10^{-9} \mathrm{~m}$
(c) Ultraviolet light with $\nu=6.0 \times 10^{15} \mathrm{~Hz}$
(d) Visible light with $\nu=7.0 \times 10^{14} \mathrm{~Hz}$
(e) Infrared radiation with $\lambda=2.0 \times 10^{-5} \mathrm{~m}$
(f) Microwave radiation with $\nu=1.0 \times 10^{11} \mathrm{~Hz}$

Ian Kaigh

Numerade Educator

Problem 7

What functional groups might the following molecules contain?
(a) A compound with a strong absorption at $1710 \mathrm{~cm}^{-1}$
(b) A compound with a strong absorption at $1540 \mathrm{~cm}^{-1}$
(c) A compound with strong absorptions at $1720 \mathrm{~cm}^{-1}$ and at 2500 to $3100 \mathrm{~cm}^{-1}$

Ian Kaigh

Numerade Educator

Problem 8

How might you use IR spectroscopy to distinguish between the following pairs of isomers?
(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$ and $\mathrm{CH}_{3} \mathrm{OCH}_{3}$
(b) Cyclohexane and hex-1-ene
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}$ and $\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{CHO}$

Ian Kaigh

Numerade Educator

Problem 9

The IR spectrum of phenylacetylene is shown in. What absorption bands can you identify?

Ian Kaigh

Numerade Educator

Problem 10

Where might the following compounds have IR absorptions?

Ian Kaigh

Numerade Educator

Problem 11

Where might the following compound have IR absorptions?

Lottie Adams

Numerade Educator

Problem 12

Calculate the energy range of radiation in the UV region of the spectrum from 200 to $400 \mathrm{nm} .$ How does this value compare with the value calculated previously for IR radiation in Section $10-6 ?$

Lottie Adams

Numerade Educator

Problem 13

If pure vitamin A has $\lambda_{\max }=325(\epsilon=50,100)$, what is the vitamin A concentration in a sample whose absorbance at $325 \mathrm{nm}$ is $A=0.735$ in a cell with a pathlength of $1.00 \mathrm{~cm} ?$

Lottie Adams

Numerade Educator

Problem 14

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range?

Lottie Adams

Numerade Educator

Problem 15

Show the structures of the likely fragments you would expect in the mass spectra of the following molecules:

Lottie Adams

Numerade Educator

Problem 16

Where in the IR spectrum would you expect each of the following molecules to absorb?

Lottie Adams

Numerade Educator

Problem 17

Which, if any, of the compounds shown in Problems 10.15 and 10.16 have UV absorptions?

Nima Gharibi

Numerade Educator

Problem 18

Propose structures for compounds consistent with the following massspectral data:
(a) A hydrocarbon with $\mathrm{M}^{+}=132$
(b) A hydrocarbon with $\mathrm{M}^{+}=166$
(c) A hydrocarbon with $\mathrm{M}^{+}=84$

Lottie Adams

Numerade Educator

Problem 19

Write molecular formulas for compounds that show the following molecular ions in their high-resolution mass spectra, assuming that $\mathrm{C}, \mathrm{H},$ $\mathrm{N},$ and $\mathrm{O}$ might be present. The exact atomic masses are: $1.00783\left({ }^{1} \mathrm{H}\right)$ $12.00000(12 \mathrm{C}), 14.00307(14 \mathrm{~N}), 15.99491\left({ }^{16} \mathrm{O}\right)$
(a) $\mathrm{M}^{+}=98.0844$
(b) $\mathrm{M}^{+}=123.0320$

Lottie Adams

Numerade Educator

Problem 20

Camphor, a saturated monoketone from the Asian camphor tree, is used as a moth repellent and as a constituent of embalming fluid, among other things. If camphor has $\mathrm{M}^{+}=152.1201$ by high-resolution mass spectrometry, what is its molecular formula?

Lottie Adams

Numerade Educator

Problem 21

The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogens has an odd-numbered molecular ion. Conversely, a compound containing an even number of nitrogens has an even-numbered $\mathrm{M}^{+}$ peak. Explain.

Lottie Adams

Numerade Educator

Problem 22

In light of the nitrogen rule mentioned in Problem 10.21 , what is the molecular formula of pyridine, $\mathrm{M}^{+}=79 ?$

Lottie Adams

Numerade Educator

Problem 23

In light of the nitrogen rule mentioned in Problem 10.21 , what is the molecular formula of pyridine, $\mathrm{M}^{+}=79 ?$

Lottie Adams

Numerade Educator

Problem 24

The hormone cortisone contains $\mathrm{C}, \mathrm{H},$ and $\mathrm{O},$ and shows a molecular ion at $\mathrm{M}^{+}=360.1937$ by high-resolution mass spectrometry. What is the molecular formula of cortisone? (The degree of unsaturation of cortisone is 8.)

Lottie Adams

Numerade Educator

Problem 25

Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as ${ }^{35} \mathrm{Cl}(75.8 \%)$ and ${ }^{37} \mathrm{Cl}(24.2 \%) ;$ bromine occurs as ${ }^{79} \mathrm{Br}(50.7 \%)$ and ${ }^{81} \mathrm{Br}(49.3 \%)$.
At what masses do the molecular ions occur for the following formulas? What are the relative percentages of each molecular ion?
(a) Bromomethane, $\mathrm{CH}_{3} \mathrm{Br}$
(b) 1 -Chlorohexane, $\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Cl}$

Lottie Adams

Numerade Educator

Problem 26

Propose structures for compounds that fit the following data:
(a) A ketone with $\mathrm{M}^{+}=86$ and fragments at $m / z=71$ and $m / z=43$
(b) An alcohol with $\mathrm{M}^{+}=88$ and fragments at $m / z=73, m / z=70$, and $m / z=59$

Lottie Adams

Numerade Educator

Problem 27

2-Methylpentane $\left(\mathrm{C}_{6} \mathrm{H}_{14}\right.$ ) has the mass spectrum shown. Which peak represents $\mathrm{M}^{+} ?$ Which is the base peak? Propose structures for fragment ions of $m / z=71,57,43,$ and $29 .$ Why does the base peak have the mass it does?

Lottie Adams

Numerade Educator

Problem 28

Assume that you are in a laboratory carrying out the catalytic hydrogenation of cyclohexene to cyclohexane. How could you use mass spectrometry to determine when the reaction is finished?

Lottie Adams

Numerade Educator

Problem 29

What fragments might you expect in the mass spectra of the following compounds?

Lottie Adams

Numerade Educator

Problem 30

How might you use IR spectroscopy to distinguish among the three isomers but-1-yne, buta-1,3-diene, and but-2-yne?

Lottie Adams

Numerade Educator

Problem 31

Would you expect two enantiomers such as $(R)-2$ -bromobutane and (S)-2-bromobutane to have identical or different IR spectra? Explain.

Lottie Adams

Numerade Educator

Problem 32

Would you expect two diastereomers such as meso-2,3-dibromobutane and $(2 R, 3 R)$ -dibromobutane to have identical or different IR spectra? Explain

Lottie Adams

Numerade Educator

Problem 33

Propose structures for compounds that meet the following descriptions:
(a) $\mathrm{C}_{5} \mathrm{H}_{8}$, with IR absorptions at 3300 and $2150 \mathrm{~cm}^{-1}$
(b) $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}$, with a strong IR absorption at $3400 \mathrm{~cm}^{-1}$
(c) $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}$, with a strong IR absorption at $1715 \mathrm{~cm}^{-1}$
(d) $\mathrm{C}_{8} \mathrm{H}_{10}$, with IR absorptions at 1600 and $1500 \mathrm{~cm}^{-1}$

Nicole Krahulik

Nicole Krahulik

Numerade Educator

Problem 34

How could you use infrared spectroscopy to distinguish between the following pairs of isomers?
(a) $\mathrm{HC} \equiv \mathrm{CCH}_{2} \mathrm{NH}_{2}$ and $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{N}$
(b) $\mathrm{CH}_{3} \mathrm{COCH}_{3}$ and $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}$

Lottie Adams

Numerade Educator

Problem 35

Two infrared spectra are shown. One is the spectrum of cyclohexane, and the other is the spectrum of cyclohexene. Identify them, and explain your answer.

Lottie Adams

Numerade Educator

Problem 36

At what approximate positions might the following compounds show IR absorptions?

Ian Kaigh

Numerade Educator

Problem 37

How would you use infrared spectroscopy to distinguish between the following pairs of constitutional isomers?
(a) $\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}$
and
$$
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{CH}
$$
(b)
and
(c) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHOCH}_{3}$
and $\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}$

Lottie Adams

Numerade Educator

Problem 38

At what approximate positions might the following compounds show IR absorptions?

Ian Kaigh

Numerade Educator

Problem 39

Assume that you are carrying out the dehydration of 1 -methylcyclohexanol to yield 1 -methylcyclohexene. How could you use infrared spectroscopy to determine when the reaction is complete?

Lottie Adams

Numerade Educator

Problem 40

Assume that you are carrying out an elimination reaction on 3 -bromo3 -methylpentane to yield an alkene. How could you use IR spectroscopy to tell which of two possible elimination products is formed, 3-methylpent-2-ene or 2 -ethylbut-1-ene?

Lottie Adams

Numerade Educator

Problem 41

Which is stronger, the $\mathrm{C}=\mathrm{O}$ bond in an ester $\left(1735 \mathrm{~cm}^{-1}\right)$ or the $\mathrm{C}=\mathrm{O}$ bond in a saturated ketone $\left(1715 \mathrm{~cm}^{-1}\right) ?$ Explain.

Lottie Adams

Numerade Educator

Problem 42

Would you expect allene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}$, to show a UV absorption in the 200 to $400 \mathrm{nm}$ range? Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 43

Which of the following compounds would you expect to have a $\pi \rightarrow \pi^{*}$ UV absorption in the 200 to 400 nm range?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 44

The following ultraviolet absorption maxima have been measured:
Buta- 1,3 -diene
$$
217 \mathrm{nm}
$$
2-Methylbuta-1,3-diene $\quad 220 \mathrm{nm}$ Penta-1,3-diene $223 \mathrm{nm}$
$226 \mathrm{nm}$
Hexa- 2,4 -diene $227 \mathrm{nm}$
2,4 -Dimethylpenta- 1,3 -diene $232 \mathrm{nm}$
2,5-Dimethylhexa-2,4-diene $\quad 240 \mathrm{nm}$
What conclusion can you draw about the effect of alkyl substitution on UV absorption maxima? Approximately what effect does each added alkyl group have?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 45

Hexa-1,3,5-triene has $\lambda_{\max }=258 \mathrm{nm}$. In light of your answer to Problem $10.44,$ approximately where would you expect 2,3 -dimethylhexa1,3,5 -triene to absorb? Explain.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 46

Ergosterol, a precursor of vitamin $\mathrm{D}$, has $\lambda_{\max }=282 \mathrm{nm}$ and molar absorptivity $\epsilon=11,900 .$ What is the concentration of ergosterol in a solution whose absorbance $A=0.065$ with a sample pathlength $I=1.00 \mathrm{~cm} ?$

Lottie Adams

Numerade Educator

Problem 47

Carvone is an unsaturated ketone responsible for the odor of spearmint. If carvone has $\mathrm{M}^{+}=150$ in its mass spectrum and contains three double bonds and one ring, what is its molecular formula?

Lottie Adams

Numerade Educator

Problem 48

Carvone (Problem 10.47) has an intense infrared absorption at $1690 \mathrm{~cm}^{-1}$. What kind of ketone does carvone contain?

Lottie Adams

Numerade Educator

Problem 49

The mass spectrum (a) and the infrared spectrum (b) of an unknown hydrocarbon are shown. Propose as many structures as you can.

Lottie Adams

Numerade Educator

Problem 50

The mass spectrum (a) and the infrared spectrum (b) of another unknown hydrocarbon are shown. Propose as many structures as you can.

Lottie Adams

Numerade Educator

Problem 51

Propose structures for compounds that meet the following descriptions:
(a) An optically active compound $\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}$ with an IR absorption at $1730 \mathrm{~cm}^{-1}$
(b) An optically inactive compound $\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{~N}$ with an IR absorption at $2215 \mathrm{~cm}^{-1}$

Lottie Adams

Numerade Educator

Problem 52

4-Methylpentan-2-one and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy.

Lottie Adams

Numerade Educator

Problem 53

Organomagnesium halides (R-Mg-X), called Grignard reagents, undergo a general and very useful reaction with ketones. Methylmagnesium bromide, for example, reacts with cyclohexanone to yield a product with the formula $\mathrm{C}_{7} \mathrm{H}_{14} \mathrm{O}$. What is the structure of this product if it has an IR absorption at $3400 \mathrm{~cm}^{-1} ?$

Lottie Adams

Numerade Educator

Problem 54

What is the concentration of cytosine, a constituent of nucleic acids, if its molar absorptivity is $6.1 \times 10^{3} \mathrm{~L} /(\mathrm{mol} \cdot \mathrm{cm})$ and its absorbance is 0.20 in a cell with a $1.0 \mathrm{~cm}$ pathlength?

Narayan Hari

Numerade Educator

Problem 55

$\beta$ -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:
(a) How many double bonds does $\beta$ -ocimene have?
(b) Is $\beta$ -ocimene conjugated or nonconjugated?
(c) Propose a structure for $\beta$ -ocimene.
(d) Write the reactions, showing starting material and products.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 56

Myrcene, $\mathrm{C}_{10} \mathrm{H}_{16},$ is found in oil of bay leaves and is isomeric with $\beta$ -ocimene (Problem 10.55 ). 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:
Propose a structure for myrcene, and write the reactions, showing starting material and products.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 57

Benzene has an ultraviolet absorption at $\lambda_{\max }=204 \mathrm{nm},$ and $p$ -toluidine has $\lambda_{\max }=235 \mathrm{nm} .$ How do you account for this difference?

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator

Problem 58

Ketones undergo a reduction when treated with sodium borohydride, $\mathrm{NaBH}_{4}$. What is the structure of the compound produced by reaction of butan-2-one with $\mathrm{NaBH}_{4}$ if it has an IR absorption at $3400 \mathrm{~cm}^{-1}$ and $\mathrm{M}^{+}=74$ in the mass spectrum?

Lottie Adams

Numerade Educator

Problem 59

Nitriles, $\mathrm{R}-\mathrm{C} \equiv \mathrm{N},$ undergo a hydrolysis reaction when heated with aqueous acid. What is the structure of the compound produced by hydrolysis of propanenitrile, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{N},$ if it has IR absorptions at $2500-3100 \mathrm{~cm}^{-1}$ and $1710 \mathrm{~cm}^{-1}$ and has $\mathrm{M}^{+}=74 ?$

Lottie Adams

Numerade Educator

Problem 60

Enamines (C=C-N; alkene + amine) typically have a UV absorption near $\lambda_{\max }=230 \mathrm{nm}$ and are much more nucleophilic than alkenes. Assuming that the nitrogen atom is $s p^{2}$ -hybridized, explain both the UV absorption and the nucleophilicity of enamines.

Grigoriy Sereda

Grigoriy Sereda

Numerade Educator