Name (please print):
KEY
Signature (Honor Pledge):
CH 231 Examination 5
Chapters 12 & 13
Summer 2007
July 6, 2007
Pages 2/3:
IR and MS
/16
Page 4:
True/False
/15
Pages 5/6:
Isomers and IR
/23 + 3 (Bonus)
Page 6:
NMR Structural Elucidation
/14
Pages 7/8:
NMR Spectroscopy
/32
Total:
/100
1
1)
Multiple Choice (4 pts ea)
a) Which of the following corresponds to the molecular ion in a mass spectrum?
______ The most abundant ion
______ Any ion formed by fragmentation of the molecule
______ The ion with the smallest m/z
__X__ The ion formed by removal of an electron from the molecule
b) Which of the following gives rise to a prominent peak in the mass spectrum corresponding
to 18 mass units less than the molecular ion?
______ octane
__X__ 1-heptanol
______ 1-chloropentane
______ 3-methylpentane
c) Which of the following labeled positions would have protons with the most downfield
chemical shift?
b
F
c
d
a
______ Not enough information to determine
______ a
______ b
__X__
c
______ d
2
d) An unknown molecule yields the following IR spectrum. What functional groups are
definitely present in the unknown molecule?
__X__ Alkane
__X__ Alkene
______ Carboxylic acid
__X__ Alcohol
______ None of these functional groups are present
3
2)
True / False (3 pts ea)
a)
Mass spectrometry requires the absorption of photons with the correct energies.
FALSE
b)
Alpha cleavage of the 3-pentanone (CH3CH2C=OCH2CH3) produces ethyl radicals
(which are not detected) and a resonance-stabilized cation (CH3CH2C=O+) which is
detected.
TRUE
c)
IR spectroscopy measures the absorption of photons corresponding to the energies
required to break bonds in organic molecules.
FALSE
d)
The use of the δ scale standardizes chemical shift data collected from magnets of
differing strengths.
TRUE
e)
Spin-spin splitting arises from the effects of nearby nuclear spins on the local magnetic
field around an effected nucleus.
TRUE
4
3)
Isomers and IR Spectroscopy (23 pts)
The following questions deal with the eleven possible constitutional and stereo-isomers with the formula
C3H6O:
OH
A
B
O
OH
OH
O
C
D
O
O
OH
O
E
F
O
H
OH
G
a)
H
I
J
Which compounds have an intense, broad IR absorbance at about 3500 cm-1?
A, C, D, E, K
b)
Which compounds have an intense absorbance near 1700 cm-1?
F, G
c)
Which compounds have an absorbance at both 2950 and 3010 cm-1?
A, B, C, D, E
d)
Which compounds have an absorbance at ~2950 cm-1?
A, B, C, D, E, F, G, H, I, J, K
Bonus)
K
Which compounds have identical IR spectra? (3 pts)
I&J
5
4)
NMR Structural Elucidation
Match the spectrum to one of the compounds shown below. Justify your answer by assigning the
peaks to the protons in the structure you have chosen. (14 pts)
d, 3.85 ppm
m, 1.9 ppm
H3 C
HH
H
O
O
CH3
CH3
s, 2.05 ppm
d, 0.95 ppm
O
H 3C
CH3 O
CH3
O
H3C
CH3
O
O
H3 C
O
CH3
CH3
H3C
O
CH3 O
6
CH3
5)
NMR Spectroscopy
a)
How many unique resonances are expected in the 13C NMR spectrum of the
following compound? (5 pts)
CH3
13
b)
C: 4
How many unique resonances are expected in the 1H NMR spectrum of the
following compound? (5 pts)
CH3
H 3C
1
7
H: 2
b)
For each of the sets of non-equivalent protons, predict the multiplicities of the peaks.
(16 pts)
doublet of quartets
triplet of quintets
doublet
CH3
triplet
singlet
CH3
singlet
H 3C
c)
Indicate whether the indicated pairs of protons are unrelated (non-equivalent),
homotopic, enantiotopic, or diastereotopic. (6 pts)
O
H
H
C
H
C H
H
H
H
H
enantiotopic
homotopic
8
For practice for the other major NMR questions, identify the number of
non-equivalent carbons and non-equivalent protons for each of the
possible answers. For each group of equivalent protons, assign the
expected multiplicity.
These problems are from the textbook, but are formatted like those
you will see on the exam.
C4H10O
H 3C
OH
CH3
HO
Choose the correct structure:
Problem 13.21
CH3
H 3C
O
CH3
H3 C
O
CH3
H 3C
Br
Br
Br
Br
Choose the correct structure:
Problem 13.51: C3H6Br2
CH3
H 3C
CH3
Br Br
Br
same
Br
H 3C
O
CH3
H 3C
Choose the correct structure:
Problem 13.57a: C5H10O
O
CH3
H 3C
O
CH3
CH3
H 3C
OH