Visualizing Chemistry

Problem 12-12

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

(a)

(b)

(c)

Problem 12-13

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

(a)

(b)

Mass Spectrometry

Problem 12-14

Propose structures for compounds that fit the following mass-spectral data:

(a) A hydrocarbon with M⁺ = 132

(b) A hydrocarbon with M⁺ = 166

(c) A hydrocarbon with M⁺ = 84

Problem 12-15

Write molecular formulas for compounds that show the following molecular ions in their high-resolution mass spectra, assuming that C, H, N, and O might be present. The exact atomic masses are: 1.007 83 (¹H), 12.000 00 ⁽¹²C), 14.003 07 (¹⁴N), 15.994 91 (¹⁶O).

(a) M⁺ = 98.0844

(b) M⁺ = 123.0320

Problem 12-16

Camphor, a saturated monoketone from the Asian camphor tree, is used among other things as a moth repellent and as a constituent of embalming fluid. If camphor has M⁺ = 152.1201 by high-resolution mass spectrometry, what is its molecular formula? How many rings does camphor have?

Problem 12-17

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 M⁺ peak. Explain.

Problem 12-18

In light of the nitrogen rule mentioned in Problem 12-17, what is the molecular formula of pyridine, M⁺ = 79?

Problem 12-19

Nicotine is a diamino compound isolated from dried tobacco leaves. Nicotine has two rings and M⁺ = 162.1157 by high-resolution mass spectrometry. Give a molecular formula for nicotine, and calculate the number of double bonds.

Problem 12-20

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

Problem 12-21

Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Recall that chlorine occurs as ³⁵Cl (75.8%) and ³⁷Cl (24.2%); and bromine occurs as ⁷⁹Br (50.7%) and ⁸¹Br (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, CH₃Br

(b) Chlorohexane, C₆H₁3Cl

Problem 12-22

By knowing the natural abundances of minor isotopes, it’s possible to calculate the relative heights of M⁺ and M + 1 peaks. If ¹³C has a natural abundance of 1.10%, what are the relative heights of the M⁺ and M + 1 peaks in the mass spectrum of benzene, C₆H₆?

Problem 12-23

Propose structures for compounds that fit the following data:

(a) A ketone with M⁺ = 86 and fragments at m/z = 71 and m/z = 43

(b) An alcohol with M⁺ = 88 and fragments at m/z = 73, m/z = 70, and m/z = 59

Problem 12-24

Methylpentane (C₆H₁₄) has the mass spectrum shown. Which peak represents 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?

Problem 12-25

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

Problem 12-26

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

(a)

(b)

(c)

Infrared Spectroscopy

Problem 12-27

How might you use IR spectroscopy to distinguish among the three isomers 1-butyne, 1,3- butadiene, and 2-butyne?

Problem 12-28

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

Problem 12-29

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

Problem 12-30

Propose structures for compounds that meet the following descriptions:

(a) C₅H₈, with IR absorptions at 3300 and 2150 cm^–1

(b) C₄H₈O, with a strong IR absorption at 3400 cm^–1

(c) C₄H₈O, with a strong IR absorption at 1715 cm^–1

(d) C₈H₁₀, with IR absorptions at 1600 and 1500 cm^–1

Problem 12-31

How could you use infrared spectroscopy to distinguish between the following pairs of isomers?

(a) HC≡CCH₂NH₂ and CH₃CH₂C≡N

(b) CH₃COCH₃ and CH₃CH₂CHO

Problem 12-32

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.

(a)

(b)

Problem 12-33

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

(a)

(b)

(c)

(d)

(e)

Problem 12-34

How would you use infrared spectroscopy to distinguish between the following pairs of constitutional isomers?

Problem 12-35

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

Problem 12-36

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?

Problem 12-37

Assume that you are carrying out the base-induced dehydrobromination of 3-bromo-3- methylpentane (Section 11.7) to yield an alkene. How could you use IR spectroscopy to tell which of three possible elimination products is formed, if one includes E/Z isomers?

General Problems

Problem 12-38

Which is stronger, the C=O bond in an ester (1735 cm^–1) or the C=O bond in a saturated ketone (1715 cm^–1)? Explain.

Problem 12-39

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

Problem 12-40

Carvone (Problem 12-39) has an intense infrared absorption at 1690 cm^–1. What kind of ketone does carvone contain?

Problem 12-41

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

(a)

(b)

Problem 12-42

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

(a)

(b)

Problem 12-43

Propose structures for compounds that meet the following descriptions:

(a) An optically active compound C₅H₁₀O with an IR absorption at 1730 cm^–1

(b) A non-optically active compound C₅H₉N with an IR absorption at 2215 cm^–1

Problem 12-44

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

Problem 12-45

Grignard reagents (alkylmagnesium halides) undergo a general and very useful reaction with ketones. Methylmagnesium bromide, for example, reacts with cyclohexanone to yield a product with the formula C₇H₁4O. What is the structure of this product if it has an IR absorption at 3400 cm^–1?

Problem 12-46

Ketones undergo a reduction when treated with sodium borohydride, NaBH₄. What is the structure of the compound produced by reaction of 2-butanone with NaBH₄ if it has an IR absorption at 3400 cm^–1 and M⁺ = 74 in the mass spectrum?

Problem 12-47

Nitriles, R–C≡N, undergo a hydrolysis reaction when heated with aqueous acid. What is the structure of the compound produced by hydrolysis of propanenitrile, CH₃CH₂C≡N, if it has IR absorptions from 2500–3100 cm^–1 and at 1710 cm^–1, and has M⁺ = 74?

Problem 12-48

The infrared spectrum of the compound with the following mass spectrum lacks any significant absorption above 3000 cm^–1. There is a prominent peak near 1740 cm^–1 and another strong peak near 1200 cm^–1. Propose a structure.

Problem 12-49

The infrared spectrum of the compound with the following mass spectrum has a medium-intensity peak at about 1650 cm^–1. There is also a C–H out-of-plane bending peak near 880 cm^–1. Propose a structure.

Problem 12-50

The infrared spectrum of the compound with the following mass spectrum has strong absorbances at 1584, 1478, and 1446 cm^–1. Propose a structure.