Organic Chemistry

Leroy G. Wade

Chapter 12

Infrared Spectroscopy and Mass Spectrometry - all with Video Answers

Educators

Chapter Questions

Problem 1

Complete the following conversion table.
$$
\begin{array}{ccccccccc}
\hline \bar{\nu}\left(\mathrm{cm}^{-1}\right) & 4000 & & & & 1700 & 1640 & 1600 & 400 \\
\lambda(\mu \mathrm{~m}) & 2.50 & 3.03 & 3.33 & 4.55 & & & & 25.0 \\
\hline
\end{array}
$$

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Problem 2

Which of the bonds shown in red are expected to have IR-active stretching frequencies?
Reaction a-j can't copy

Ian Kaigh

Numerade Educator

Problem 3

For each hydrocarbon spectrum, determine whether the compound is an alkane, an alkene, an alkyne, or an aromatic hydrocarbon, and assign the major peaks above (to the left of) $1600 \mathrm{~cm}^{-1}$. More than one unsaturated group may be present.
Figure can't copy

Lottie Adams

Numerade Educator

Problem 4

Spectra are given for three compounds. Each compound has one or more of the following functional groups: alcohol, amine, ketone, aldehyde, and carboxylic acid. Determine the functional group(s) in each compound, and assign the major peaks above $1600 \mathrm{~cm}^{-1}$.
Figures can't copy

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Problem 5

The infrared spectra for three compounds are provided. Each compound has one or more of the following functional groups: conjugated ketone, ester, amide, nitrile, and alkyne. Determine the functional group(s) in each compound, and assign the major peaks above $1600 \mathrm{~cm}^{-1}$.
Figures can't copy

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Problem 6

For each spectrum, interpret all the significant stretching frequencies above $1580 \mathrm{~cm}^{-1}$.
Figures can't copy

Ian Kaigh

Numerade Educator

Problem 7

Identify which of these four mass spectra indicate the presence of sulfur, chlorine, bromine, iodine, or nitrogen. Suggest a molecular formula for each.
Figures can't copy

Zubair Abdulla

Numerade Educator

Problem 8

Show the fragmentation that accounts for the cation at $m / z 57$ in the mass spectrum of 2-methylpentane. Explain why this ion is less abundant than those at $m / z 71$ and 43.

Ian Kaigh

Numerade Educator

Problem 9

Show the fragmentations that give rise to the peaks at $m / z 43,57$, and 85 in the mass spectrum of 2,4-dimethylpentane (Figure 12-17).

Ian Kaigh

Numerade Educator

Problem 10

Ethers are not easily differentiated by their infrared spectra, but they tend to form predictable fragments in the mass spectrum. The following compounds give similar but distinctive mass spectra.
Reaction can't copy
Both compounds give prominent peaks at $m / z 116,73,57$, and 43 . But one compound gives a distinctive strong peak at 87 , and the other compound gives a strong peak at 101 . Determine which compound gives the peak at 87 and which one gives the peak at 101 . Propose fragmentations to account for the ions at $m / z 116,101,87$, and 73 .

Lottie Adams

Numerade Educator

Problem 11

Account for the peaks at $m / z 87,111$, and 126 in the mass spectrum of 2,6-dimethyl-heptan-4-ol.
Figure can't copy

Zubair Abdulla

Numerade Educator

Problem 12

Predict the characteristic infrared absorptions of the functional groups in the following molecules.
(a) cyclohexene
(b) pentan-2-ol
(c) pentan-2-one
(d) pent-1-yne
(e) diethylamine
(f) pentanoic acid
(g) pentanenitrile
(h) ethyl acetate
(i) pentanamide

Zubair Abdulla

Numerade Educator

Problem 13

Convert the following infrared wavelengths to $\mathrm{cm}^{-1}$.
(a) $6.24 \mu \mathrm{~m}$, typical for an aromatic $\mathrm{C}=\mathrm{C}$
(b) $3.38 \mu \mathrm{~m}$, typical for a saturated $\mathrm{C}-\mathrm{H}$ bond
(c) $5.85 \mu \mathrm{~m}$, typical for a ketone carbonyl
(d) $5.75 \mu \mathrm{~m}$, typical for an ester carbonyl
(e) $4.52 \mu \mathrm{~m}$, typical for a nitrile
(f) $3.03 \mu \mathrm{~m}$, typical for an alcohol $\mathrm{O}-\mathrm{H}$

MT

Monica Tran

Numerade Educator

Problem 14

All of the following compounds absorb infrared radiation between 1600 and $1800 \mathrm{~cm}^{-1}$. In each case,
1. show which bonds absorb in this region.
2. predict the approximate absorption frequencies.
3. predict which compound of each pair absorbs more strongly in this region.
Compounds a-d can't copy

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Problem 15

Describe the characteristic infrared absorption frequencies that would allow you to distinguish between the following pairs of compounds.
(a) 2,3-dimethylbut-2-ene and 2,3-dimethylbut-1-ene
(b) cyclohexa-1,3-diene and cyclohexa-1,4-diene
Compounds c-h can't copy

Lottie Adams

Numerade Educator

Problem 16

Four infrared spectra are shown, corresponding to four of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.
Compounds can't copy

Lottie Adams

Numerade Educator

Problem 17

Predict the masses and the structures of the most abundant fragments observed in the mass spectra of the following compounds.
(a) 2-methylpentane
(b) 3-methylhex-2-ene
(c) 4-methylpentan-2-ol
(d) 2-methyl-1-phenylpropane
(e) cyclohexyl isopropyl ether [cyclohexyl-O-CH(CH3) 2 ]
(f) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{NHC}\left(\mathrm{CH}_3\right)_3$ tert-butyl propyl amine
g Compounds can't copy
(h) 3-bromo-2-methylpentane

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Problem 18

Give logical fragmentation reactions to account for the following ions observed in these mass spectra.
(a) n-octane: $114,85,71,57$
(b) methylcyclohexane: 98,83
(c) 2-methylpent-2-ene: 84, 69
(d) pentan-1-ol: 70, 55, 41, 31
(e) $N$-ethylaniline $\left(\mathrm{PhNHCH}_2 \mathrm{CH}_3\right): 121,106,77$
*(f) 1-bromo-2-methylbutane: 152, 150, 123, 121, 71 (base)

Ian Kaigh

Numerade Educator

Problem 19

A common lab experiment is the dehydration of cyclohexanol to cyclohexene.
(a) Explain how you could tell from the IR spectrum whether your product was pure cyclohexene, pure cyclohexanol, or a mixture of cyclohexene and cyclohexanol. Give approximate frequencies for distinctive peaks.
(b) Explain why mass spectrometry might not be a good way to distinguish cyclohexene from cyclohexanol.

Lottie Adams

Numerade Educator

Problem 20

(A true story.) While organizing the undergraduate stockroom, a new chemistry professor found a half-gallon jug containing a cloudy liquid (bp $100-105^{\circ} \mathrm{C}$ ), marked only "STUDENT PREP." She ran a quick mass spectrum, which is printed below. As soon as she saw the spectrum (without even checking the actual mass numbers), she said, "I know what it is."
Structure can't copy
(a) What compound is the "student prep"? Any uncertainty in the structure?
(b) Suggest structures for the fragments at 136,107 , and 93 . Why is the base peak (at $m / z 57$ ) so strong?

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Problem 21

A C-D (carbon-deuterium) bond is electronically much like a $\mathrm{C}-\mathrm{H}$ bond, and it has a similar stiffness, measured by the spring constant, $k$. The deuterium atom has twice the mass $(m)$ of a hydrogen atom, however.
(a) The infrared absorption frequency is approximately proportional to $\sqrt{k / m}$, when one of the bonded atoms is much heavier than the other, and $m$ is the lighter of the two atoms ( H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use $3000 \mathrm{~cm}^{-1}$ as a typical C-H absorption frequency.
(b) A chemist dissolves a sample in deuterochloroform $\left(\mathrm{CDCl}_3\right)$ and then decides to take the IR spectrum and simply evaporates most of the $\mathrm{CDCl}_3$. What functional group will appear to be present in this IR spectrum as a result of the $\mathrm{CDCl}_3$ impurity?

Tianyu Li

Numerade Educator

Problem 22

The mass spectrum of $n$-octane shows a prominent molecular ion peak ( $m / z 114$ ). There is also a large peak at $m / z 57$, but it is not the base peak. The mass spectrum of 3,4-dimethylhexane shows a smaller molecular ion, and the peak at mass 57 is the base peak. Explain these trends in abundance of the molecular ions and the ions at mass 57 , and predict the intensities of the peaks at masses 57 and 114 in the spectrum of 2,2,3,3-tetramethylbutane.

Lottie Adams

Numerade Educator

Problem 23

An unknown, foul-smelling hydrocarbon gives the mass spectrum and infrared spectrum shown.
(a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there?
(b) Use the IR spectrum to determine the functional group(s), if any.
(c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives $n$-octane, would the structure still be uncertain?
(d) Propose structures for the major fragments at $39,67,81$, and 95 in the mass spectrum.
Structure can't copy

Zubair Abdulla

Numerade Educator

Problem 24

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert trans-2,5-dimethylhex-3-ene to 2,5-dimethylhex-3-yne by adding bromine across the double bond and then doing a double elimination. The infrared and mass spectra of the major product are shown here.
Structure can't copy
(a) Do the spectra confirm the right product? If not, what is it?
(b) Explain the important peaks in the IR spectrum
Structure can't copy

Zubair Abdulla

Numerade Educator

Problem 25

Three IR spectra are shown, corresponding to three of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.
Structures can't copy

Lottie Adams

Numerade Educator

Problem 26

A laboratory student added 1-bromobutane to a flask containing dry ether and magnesium turnings. An exothermic reaction resulted, and the ether boiled vigorously for several minutes. Then she added acetone to the reaction mixture and the ether boiled even more vigorously. She added dilute acid to the mixture and separated the layers. She evaporated the ether layer, and distilled a liquid that boiled at $143^{\circ} \mathrm{C}$. GC-MS analysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.
(a) Draw out the reactions that took place and show the product that was formed.
(b) Explain why the molecular ion is or is not visible in the mass spectrum, and show what ions are likely to be responsible for the strong peaks at $m / z 59$ and 101 .
Structure can't copy

Ian Kaigh

Numerade Educator

Problem 27

(Another true story.) A student who was checking into her lab desk found an unlabeled sample from a previous student. She was asked to identify the sample. She did an IR spectrum and declared, "It looks like an alkane." But it seemed too reactive to be an alkane, so she did a GC-MS. The mass spectrum is shown next. Identify the compound as far as you can, and state what part of your identification is uncertain. Propose fragments corresponding to the numbered peaks.
Structure can't copy

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Problem 28

Three common lab experiments are shown. In each case, describe how the IR spectrum of the product would differ from that of the reactant. Give approximate frequencies for distinctive peaks in the IR spectrum of the reactant and also that of the product.
Structure can't copy

Lottie Adams

Numerade Educator

Problem 29

The ultimate test of fluency in MS and IR is whether you can determine a moderately complex structure from just the MS and the IR, with no additional information. The IR and MS of a compound are shown below. Use everything you know about IR and MS, plus reasoning and intuition, to determine a likely structure. Then show how your proposed structure is consistent with these spectra.
Structure can't copy

George Bennett

Numerade Educator

Problem 30

These five structures all have distinguishing absorptions in the IR. Match each structure with its characteristic absorption.
Structure can't copy
(a) sharp, $2254 \mathrm{~cm}^{-1}$
(b) very broad, centered about $3330 \mathrm{~cm}^{-1}$
(c) strong, slightly broadened, $1645 \mathrm{~cm}^{-1}$
(d) broad with spikes at 3367 and $3292 \mathrm{~cm}^{-1}$
(e) strong, sharp $1717 \mathrm{~cm}^{-1}$

Tom Rutherford

Numerade Educator

Problem 31

Consider the following four structures, followed by mass spectral data. Match each structure with its characteristic molecular ion or fragment. In each case, give a likely structure of the ion responsible for the base peak.
Structure can't copy
(a) base peak at 105
(b) base peak at 72
(c) $\mathrm{M}^{+}$doublet at 198 and 200, base peak at 91
(d) base peak at 91, large peak at 43

Lottie Adams

Numerade Educator