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1Chemistry 303
fall, 2000
SECOND EXAMINATION
7:30 PM, NOVEMBER 13th, 2000
Duration: 2.0 hr
Name____________________________________________________________
Lab
TA__________________________________________________________(if you
do not know his/her name, give day of lab section. NOT Hooley nor
Chlenov)
This is an "open book" examination; you may use anything which
is not alive.
Note: if you do not know the complete or specific answer, give a
partial or general answer--
WRITE SOMETHING
Write only in the space provided for each question.
Score:
I__________/28
II__________/36
III_________ /22
IV_________ /14
Total: /100
There are 9 pages in this exam, followed by five pages of
spectra. All 13C NMR spectra are "proton-decoupled",as usual.
Please check now to be sure you have a complete set. You may detach
the Data Sheets for yourconvenience, but remember to re-staple the
rest of the exam securely when you hand it in.
If you are using a resonance argument in your answer, draw the
relevant resonance structures.
Be aware that in Problem I and II, you may "purchase" the
unknown structure (X, A) for a point penalty. Go tothe front and
request the structure of the proctor and he will deduct the
appropriate points.
Please be aware that a small number of students will be taking
the exam at different times up until the afternoon onTuesday. It
would be well not to discuss the exam until after that time.
PLEDGE:_________________________________________________________________
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2I. (28 pts) Consider the spectral data for molecule X on
p 11/12.
A. (4 pts) From the 4000-1500 cm-1 region of
the infrared spectrum alone, which of the following
functional groups are ruled out? (circle those which are ruled out,
and explain in one sentence how you came to that
conclusion.):
carbonyl group (C=O)
_______________________________________________________________________
alcohol
(-OH)______________________________________________________________________________
amino
(-NH2)______________________________________________________________________________
cyano
(-CN)________________________________________________________________________________
aldehyde
(RCHO)___________________________________________________________________________
ether
(R-O-R)______________________________________________________________________________
monosubstituted alkyne
(R-C!CH)______________________________________________________________
disubstituted alkyne (e.g.,
Me-C!C-Me)_________________________________________________________
B. (2 pts).
1. From the mass spectral data, what is the
molecular weight of the compound? ______
2. Give an estimate of the number of carbon atoms.
4 5 6 7 8 9 (circle one)
C. (6 pts) For each set of equivalent protons, give the
1 H NMR chemical shift, the relative area,
and describethe splitting pattern (singlet, doublet, triplet,
quartet, quintet, sextet, etc) including approximate
couplingconstants. You should estimate the J values as best you
can.
Chemical Shift Rel. Area Splitting pattern Approx. J
.
1.
_________________________________________________________________________________
2.
_________________________________________________________________________________
3.
_________________________________________________________________________________
4.
_________________________________________________________________________________
5.
_________________________________________________________________________________
6.
_________________________________________________________________________________
7.
_________________________________________________________________________________
D. (2 pts). From the UV data alone, which of the following
functional groups are ruled out? Cross out yourchoices. Assume
there could be more than one functional group in the molecule.
R OR RR R C N
OR
E. (1 pt) How many non-equivalent carbon atoms are
indicated in the 13C NMR spectrum? _______
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3
F. 1. (8 pts) Draw the structure for X which best
fits all the data (review your IR conclusions). Explain in
detail how your structure fits the pattern of peaks (splitting) in
the 1 H NMR spectrum. Label thegroups of equivalent H in your
structure for reference. You need not show calculation of chemical
shift positions, but you might do this to confirm your
structure. You can buy structure X (6 pt penalty).
2. (3 pts) (a) Specify which carbon in your structure gives the
peak at " 175 ppm in the 13C NMR spectrum.
(b) Specify which carbon in your structure gives the peak
at " 70 ppm in the 13C NMR spectrum.
3. (2 pts) Explain how your structure fits the IR spectrum,
based on the peaks from 4000-1500 cm-1.
II. (36 pts) Consider the reaction of B in the presence of
triethylamine, which leads to A (isomer of B).(spectral data
for A in the Data Tables).
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4
Et3N
[A]
O
Hb
Ha
B
Hd
Hc
A. (1 pt) In the IR spectrum for B, approx. where would you
expect to see the absorptiondue to the C=O stretch?
______cm-1
B. (1 pt) In the IR spectrum for B, approx. where would you
expect to see the absorption
due to the C=C stretch? ______cm-1
C. (2 pts) What do you predict as # max for the UV
spectrum of B, considering only $%$* type absorptions?
215 nm cannot tell ( circle single best
answer)______________________________________________
D. (3 pts) Consider protons Ha and Hb in B.1. Are they
equivalent?
2. What do you predict for their signal(s) in the 1 H NMR
spectrum [approx. chemical shift and pattern,including approximate
coupling constants]?
_______________________________________________E. (3 pts)
Consider protons Hc and Hd in B.
1. Are they equivalent?
2. What do you predict for their signal(s) in the 1 H NMR
spectrum [calculated chemical shift and pattern,
including approximate coupling constants]? Show any
arithmetic.
continued
F. (2 pts) Consider all of the spectral data and the
mechanism of the reaction to form A, and
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5 write a [large] structure for A. Draw in the
groups of hydrogens and label groups of equivalent H as
a,b,c, etc.
[if you do not know the structure for A, you may
request it from the proctor for a cost of 10 pts.
Alternatively, you may answer the following questions in
general
terms]
G. (3 pts) Write the steps involved in the formation
of A from B in the presence of
triethylamine. Be sure toshow the role of the
triethylamine.
H. Explain how parts of the 1H NMR spectrum for A are
consistent with your structure for A.1. (2 pts) To which H do you
assign the peak at " 1.05? Specify the number of Hs giving
this peak.
2. (3 pts) To which H do you assign the peak at " 1.95?
Give number of Hs. Calculate the expectedchemical shift based on
your structure and compare with the observed number.
3. (4 pts) To which H do you assign the peaks at " 2.20?
Give number of Hs.
4. (2 pts) to which H do you assign the pattern at " 5.88?
Give number of Hs.
continued
I. (2 pts) Explain how the UV data for A is
consistent with your structure for A and not
with B.
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J. (3 pts) Explain how the IR spectrum for A, in the
region from 1500-1800 cm-1, is consistent with your
structure. Be as precise as you can in defining the structural
type suggested by the IR peak position(s).
____________________________________________________
K. Consider the
13 C NMR spectrum for
A.Re-draw the structure of A here, for reference and
number each carbon.
1. (1 pt) How many non-equivalent carbons are there in
A?5 6 7 8 9 10 11 12 (circle single best answer)
2. (1 pt) To which carbon do you assign the peak at
"199?
3. (3 pts) To which carbon do you assign the peak at
" 160? Explain in terms of the structure how you madeyour
assignment and take into account the peak at " 125 as
well.
III. (22 pts)
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7A. Consider the actual equilibrium constants for the
chair flip of the monosubstituted cyclohexanes, a.
Note that there is not a linear change in Keq with atomic
number of halogen atom, and that Cl, Br, and I have similareffect
on Keq.
X
X
X Keq
F 0.53 0.25
Cl 0.41 0.53
Br 0.42
0.48 I 0.42 0.47
a b
Keq !G (Kcal/mol)
1. (1 pt) Which isomer has the equatorial halogen? a ____
b______ neither____
2. (2 pts) If X = H, the equilibrium constant Keq would be:
zero__ 1.0__ >1.0__
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82. (3 pts) The &G for this equlibrium is -2.0 Kcal/mol and
Keq is 30. Specify which
is the less stable isomer and explain carefully why it is
less stable than the other, in terms of the important types
of repulsive interactions. You may want to re-draw the less
stable isomer here.
3. (2 pts) Consider the more stable isomer. Are there any
elements of torsional or bond angle strain inthis molecule?
Explain.
4. (3 pts) Use the structure below to draw the diaxial version
of 1,2-dichlorocyclohexane. Then show thering flip (conformational
isomer) at the other side of the equilibrium. The &G for this
process is +0.9 Kcal/mol,and Keq is ca 0.20. Specify which is
the less stable isomer and explain carefully why it is less stable
than the other,comparing repulsive interactions in both
isomers.
Keq!G = +0.9 Keq = 0.20
diaxial
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IV. (14 pts) The conformational analysis for cyclohexane
derivatives also is useful with related compounds such as the
ketone, cyclohexanone, but with some added features. It gets
particularly interesting with several
new dipolar substituents, as in
trans-2-chloro-5-methylcyclohexanone with chair conformers a and
b.
O
Cl
Me
O
ClMe
in hexane: Keq = 0.3
in methyl alcohol: Keq = 1.4
Keq
O
cyclohexanonea b
A. (5 pts) Rationalize why the equilibrium as written is
unfavorable in hexane solvent. Analyzeconformers a and b with
regard to the usual repulsive interactions as a start, but there is
obviously more to it.
B. (3 pts) Rationalize why the reverse is true in methyl
alcohol solvent.
C. (6 pts) The infrared spectrum for cyclohexanone shows a
strong peak at 1719 cm-1. However, trans-2-
chloro-5-methylcyclohexanone shows two peaks, at 1745 and 1726
cm-1. Explain carefully the origin of the two
peaks for this compound, including paying attention to the
larger change in frequency for one and smaller change
in the other, both toward higher wavenumbers.
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10end exam
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