Organic Chemistry With Biological Applications 3rd Edition By John E. McMurry – Test Bank

1. Nuclear magnetic resonance spectroscopy provides information about a molecule’s:
a. conjugated pi electron system.
b. size and formula.
c. carbon-hydrogen framework.
d. functional groups.
ANSWER: c
POINTS: 1

2. Which of the following would not produce nuclear magnetic resonance?
a. 2H
b. 14N
c. 16O
d. 19F
ANSWER: c
POINTS: 1

3. Explain why all protons in a molecule do not absorb rf energy at the same frequency.
ANSWER: All nuclei in molecules are surrounded by electron clouds. When a uniform external magnetic field is applied to a molecule, the circulating electron clouds set up tiny local magnetic fields of their own. These local magnetic fields act in opposition to the applied field, so that the effective field actually felt by a nucleus is a bit smaller than the applied field.
Beffective = Bapplied − Blocal
This effect is termed shielding. Each nucleus is shielded to a slightly different extent, so each unique kind of proton in a molecule resonates at a slightly different frequency and gives rise to a unique NMR signal.
POINTS: 1

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled 13C NMR spectra.

14. Refer to instructions. What is the splitting pattern for the hydrogens in 3-methylbutan-2-one labeled A, B, and C, respectively?
a. singlet, singlet, singlet
b. singlet, septet, quartet
c. singlet, septet, doublet
d. singlet, septet, doublet, doublet
ANSWER: c
POINTS: 1

15. Refer to instructions. What is the ratio of peak areas upon integration of the spectrum for A, B, and C respectively?
a. 3:1:3:3
b. 1:1:6
c. 1:1:6
d. 3:1:6
ANSWER: d
POINTS: 1

16. Treatment of tert-butyl alcohol with hydrogen chloride yields a mixture of tert-butyl chloride and 2-methylpropene.

a) After chromatographic separation, how would you use 1H NMR to help you decide which was which?

b) How would the 13C NMR for the two compounds differ?
ANSWER: a) 2-Methylpropene has two kinds of hydrogens. It has a vinylic absorption (4.5−6.5 δ) representing two hydrogens and an unsplit signal (1.0−1.5 δ) due to the six equivalent methyl hydrogens.

tert-Butyl chloride has only one kind of hydrogen, which results in one unsplit signal.

b) 2-Methylpropene would have three signals with the signals due to the alkene carbons being further downfield than any of the signals in tert-butyl chloride. tert-Butyl bromide would have only two signals.

POINTS: 1

19. Refer to instructions. Based on the mass spectral data and the IR data, what functional groups are present in this compound?
ANSWER: A molecular ion of mass 113 indicates an odd number of nitrogen atoms in the compound. Coupled with the IR absorption at 2270 cm−1, a nitrile functional group is indicated. The IR absorption at 1735 cm−1 indicates a carbonyl group, possibly an ester.
POINTS: 1

21. Refer to instructions. Describe each signal in terms of its integration, splitting and chemical shift.
ANSWER: The signals at 1.05 and 2.45 δ are characteristic, in their integration and splitting (a 2H quartet and a 3H triplet), for an ethyl group. The shift of the CH2 to 2.45 indicates that the ethyl group is adjacent to a carbonyl group. The signal at 2.1 δ, a 3H singlet, is characteristic for a CH3 adjacent to a carbonyl group.
POINTS: 1

22. Refer to instructions. Propose a structure for this compound.
ANSWER: butan-2-one, CH3COCH2CH3
POINTS: 1

24. Which feature in the 1H NMR spectrum provides information about the relative numbers of hydrogen atoms of each type found in a compound?
a. number of signals
b. integration of signals
c. splitting of signals
d. chemical shift
ANSWER: b
POINTS: 1

25. Which feature in the 1H NMR spectrum provides information about the electronic environment of the protons in a compound?
a. number of signals
b. integration of signals
c. splitting of signals
d. chemical shift
ANSWER: d
POINTS: 1

26. Which feature in the 1H NMR spectrum provides information about the number of neighboring protons of each proton in the compound?
a. number of signals
b. integration of signals
c. splitting of signals
d. chemical shift
ANSWER: c
POINTS: 1

27. How many sets of equivalent protons are there in hexane, 2-methylhexane, and 3-methylhexane, respectively?
ANSWER: 3, 6, and 7
POINTS: 1

28. Which of the following combinations of peaks appears in the 1H NMR spectrum of diethyl ether, CH3CH2OCH2CH3?
a. a triplet and a doublet
b. a quartet and a sextet
c. two singlets
d. a triplet and a quartet
ANSWER: d
POINTS: 1