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Chapter 12: Structure Determination: Mass Spectrometry and Infrared Spectroscopy

Every problem in the chapter beside the book’s answer, a chapter at a time as you scroll.

Chapter 12: Structure Determination: Mass Spectrometry and Infrared Spectroscopy

11 problems · 10 figures

12-1 §12.2 source ↗

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

Answer

C19H28O2

12-2 §12.2 source ↗

Two mass spectra are shown in Figure 12.9. One spectrum is that of 2-methyl-2-pentene; the other is of 2-hexene. Which is which? Explain.
(a)
(b)
Figure 12.9 Mass spectra for Problem 12-2.
Figure 12.9 in the book ↗
Mass spectra for Problem 12-2.

Answer (2 parts)

(a)
2-Methyl-2-pentene
(b)
2-Hexene

12-3 §12.3 source ↗

What are the masses of the charged fragments produced in the following cleavage pathways?
(a)
Alpha cleavage of 2-pentanone (CH3COCH2CH2CH3)
(b)
Dehydration of cyclohexanol (hydroxycyclohexane)
(c)
McLafferty rearrangement of 4-methyl-2-pentanone [CH3COCH2CH(CH3)2]
(d)
Alpha cleavage of triethylamine [(CH3CH2)3N]

Answer (4 parts)

(a)
43, 71
(b)
82
(c)
58
(d)
86

12-4 §12.3 source ↗

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

Answer

102 (M+), 84 (dehydration), 87 (alpha cleavage), 59 (alpha cleavage)

12-5 §12.5 source ↗

Which has higher energy, infrared radiation with λ = 1.0 × 10–6 m or an X ray with λ = 3.0 × 10–9 m? Radiation with ν = 4.0 × 109 Hz or with λ = 9.0 × 10–6 m?

Answer

X-ray energy is higher; λ = 9.0 × 10−6 m is higher in energy.

12-6 §12.5 source ↗

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 kJ/mol of each of the following kinds of radiation:
(a)
A gamma ray with λ = 5.0 × 10–11 m
(b)
An X ray with λ = 3.0 × 10–9 m
(c)
Ultraviolet light with ν = 6.0 × 1015 Hz
(d)
Visible light with ν = 7.0 × 1014 Hz
(e)
Infrared radiation with λ = 2.0 × 10–5 m
(f)
Microwave radiation with ν = 1.0 × 1011 Hz

Answer (6 parts)

(a)
2.4 × 106 kJ/mol]
(b)
4.0 × 104 kJ/mol
(c)
2.4 × 103 kJ/mol
(d)
2.8 × 102 kJ/mol
(e)
6.0 kJ/mol
(f)
4.0 × 10−2 kJ/mol

12-7 §12.7 source ↗

What functional groups might the following molecules contain?
(a)
A compound with a strong absorption at 1710 cm–1
(b)
A compound with a strong absorption at 1540 cm–1
(c)
A compound with strong absorptions at 1720 cm–1 and 2500 to 3100 cm–1

Answer (3 parts)

(a)
Ketone or aldehyde
(b)
Nitro compound
(c)
Carboxylic acid

12-8 §12.7 source ↗

How might you use IR spectroscopy to distinguish between the following pairs of isomers?
(a)
CH3CH2OH and CH3OCH3
(b)
Cyclohexane and 1-hexene
(c)
CH3CH2CO2H and HOCH2CH2CHO

Answer (3 parts)

(a)
CH3CH2OH has an −OH absorption.
(b)
1-Hexene has a double-bond absorption.
(c)
CH3CH2CO2H has a very broad −OH absorption.

12-9 §12.8 source ↗

The IR spectrum of phenylacetylene is shown in Figure 12.29. What absorption bands can you identify?

Figure 12.29 The IR spectrum of phenylacetylene, Problem 12-9.
Figure 12.29 in the book ↗
The IR spectrum of phenylacetylene, Problem 12-9.

Answer

1450–1600 cm−1: aromatic ring; 2100 cm−1: C ≡ C; 3300 cm−1: C ≡ C−H

12-10 §12.8 source ↗

Where might the following compounds have IR absorptions?
(a)
(b)
(c)

Answer (3 parts)

(a)
1715, 1640, 1250 cm−1
(b)
1730, 2100, 3300 cm−1
(c)
1720, 2500–3100, 3400–3650 cm−1

12-11 §12.8 source ↗

Where might the following compound have IR absorptions?

Answer

1690, 1650, 2230 cm−1

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Problems and answers are adapted from Organic Chemistry: A Tenth Edition by John McMurry, OpenStax, and are used under the CC BY-NC-SA 4.0 licence. Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter. Illustrations: Copyright Rice University, OpenStax, under CC BY-NC-SA 4.0 license. This adaptation is released under the same licence, is non-commercial, and is neither affiliated with nor endorsed by OpenStax or Rice University.