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Chemistry 303 1fall, 2001
SECOND EXAMINATION
7:30 PM, NOVEMBER 12th, 2001
Duration: 2.5 hr
Name____________________________________________________________
Lab TA__________________________________________________________(if you do not know his/her name, give day of lab section. NOT Hooley)
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__________/24
III_________ /19
IV_________ /10
V__________/19
Total: /100
There are 10 pages in this exam, followed by four pages of spectra and one page containing a Table of NaturalIsotopes, and a Table of Spin Quantum Numbers for common nuclei. 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 foryour convenience, 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.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.
Be aware that in Problem I, you may "purchase" the unknown structure (X) for a 6 point penalty. Go to the frontand request the structure from the proctor and he will deduct the appropriate points.
PLEDGE:_________________________________________________________________
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I. (28 pts) Consider the spectral data for molecule X on p 11. 2 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 structural types are ruled out? Cross out your choices.
R OR R R R
continued
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E. (1 pt) How many non-equivalent carbon atoms are indicated in the 13C NMR spectrum? _______ 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 123 ppm in the 13C NMR spectrum. Explain any ambiguity.
(b) Specify which carbon in your structure gives the peak at 11 ppm in the 13C NMR spectrum. Explain any ambiguity.
3. (2 pts) Explain how your structure fits the IR spectrum, based on the peaks from 4000-1500 cm-1.
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II. (24 pts) Consider the spectral data for compound Y on the Data Sheets, p 12 and 13. 4
With a little work, you should be able to come to the conclusion that the structure consists of a benzene ringattached to a C=C unit, a CH3 group, and -OCH3 group connected somewhere. Exactly where the CH3 and CH3Ogroups are attached will take a little more work.
with -CH3 and CH3O- attached somewhere.
A. (6 pts) There are six peaks or multiplets in the 1H NMR spectrum, centered at the chemical shiftsshown below. Fill in the rest of the table. The patterns could include: singlet, doublet, triplet, quartet, doublet of doublets, doublet of triplets, doublet of quartets, or other. entry δ area pattern coupling constant(s) (if multiplet)
1. 1.82 ppm
2. 3.80
3. 6.1
4. 6.35
5. 6.826. 7.25
B. (2 pts) How many non-equivalent carbons due to Y are evident from the 13C NMR spectrum? ____
C. (2 pts) Give two peaks in the IR spectrum which are consistent with the presence of an alkene. Explain which type of vibration gives rise to each of those two peaks.
D. (4 pts) On the next page, write large the structure which best fits the spectral data, showing all H,the location of the CH 3 and the -OCH 3 groups, and the E/Z relationships on the alkene unit.
(2 pts) On your structure, label the proton or group of equivalent protons to correlate with the tablein part A (e.g., H 1 for entry 1, H 2 for entry 2, etc.)
(6 pts) On your structure, show clearly which H is coupled to which other H, and give the magnitudeof that coupling constant.
(2 pts) On your structure, indicate clearly pairs of carbons which are equivalent and give a single peak in the 13C NMR spectrum. You may draw a second structure if it makes the answer clearer.
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5
G. YOUR STRUCTURE, labeled: (draw twice or more if necessarity for clarity)
Bonus (3 pts): What are those teeny-weeny little peaks in the 1H NMR spectrum at δ 3.60 and 3.95 ppm due to?
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6III. (19 pts).
There are stable molecules formed by replacing one of the carbon atoms in a ring with heteroatoms, and they arecalled heterocycles. For six-membered heterocycles, one can do the same conformational analysis (chair-chairinterconversion, etc) as with cyclohexane. There are differences, some due to the fact that the C-O bond is shorter(1.43Å) compared to the C-C bond (1.54Å). Consider the molecules 2-methyl-1,3-dioxane (2), 4-methyl-1,3-dioxane (4), 5-methyl-1,3-dioxane (5), methylcyclohexane and 5-tert-butyl-1,3-dioxane (5-tBu). The energydifferences for the axial/equatorial equilibria are given, in kcal/mol.
O
O OO
O
O
2 4 5
∆G -3.5 -2.9 -0.8 -1.7for the equilibrium: axial equatorial
O
O
5-tBu
-1.4
432
1 5
∆G given
A. (3 pts) Draw the equilibrium for methylcyclohexane, indicate which is the more stable conformer, andexplain why it is more stable. Label the isomers as (a) for axial or (e) for equatorial.
B. (4 pts) Draw the equilibrium for 2 , indicate which is the more stable conformer, and explain why it ismore stable. In particular, explain why the difference in energy is now larger compared to methylcyclohexane. Label the isomers as (a) for axial or (e) for equatorial.
O
OO
O
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7C. (3 pts) Draw the equilibrium for 5 , indicate which is the more stable conformer, and explain
why it is more stable. In particular, explain why the difference in energy is now smaller compared tomethylcyclohexane. Label the isomers as (a) for axial or (e) for equatorial.
O
O
O
O
D. (4 pts) Note the G value for the 5-t-butyl isomer, 5-tBu. Draw the equilibrium for 5-tBu , indicatewhich is the more stable conformer, and explain why it is more stable. In particular, explain why the difference inenergy is now smaller compared to 2-methyl-1,3-dioxane, 2. Label the isomers as (a) for axial or (e) for equatorial.
O
OO
O
E. (5 pts) Note the G value for the 5-t-butyl isomer, 5-tBu. Now consider the molecule, cis-2-methyl-5-t-butyl-1,3-dioxane. Draw the two chair forms here, circle the one which is more stable, and explain your choice
O
OO
O
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8IV. (10 pts) Consider the structures A, B, and C.
Note also the IR and 1H NMR spectral data here, entries 1 (for C), 2, and 3.
O
O
O
O
O
O
A B C
1. 1735 cm-1
δ 5.78 and 5.80 for C
vinyl H
Ha
Hb
Hb
Ha
Ha
Hb
C=O
2. 1720 cm-1
δ 5.89 for Ha and 6.94 for Hb
3. 1760 cm-1 δ 5.24 for Hb and 7.02 for Ha
A. (5 pts) Which set of spectral data is correct for compound B?1. Why is the C=O stretching frequency for B different from C ? Explain carefully.
2. Note the specific chemical shifts for H a and H b in your choice, and explain differences from thecorresponding Hs in C .
B. (5 pts) Which set of spectral data are correct for compound A?1. Why is the C=O stretching frequency for A different from C ? Explain carefully.
2. Note the specific chemical shifts for H a and H b in your choice, and explain differences from thecorresponding Hs in C .
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9V. (19 pts) Consider the spectral data for Compound D in the data sheets, p 14.
Compound D has two carbons, four hydrogens, and two halogen atoms.
1. (2 pts) Consider the mass spectral data for D and determine the molecular formula. Circle the correct formula:
C2H4BrCl C2H4Cl2 C2H4BrF C2H4F2 C2H4ClF NONE OF THESE
2. (6 pts) Consider the 1 H NMR data (p. 14) and draw the structure for D. Label the groups of equivalent Hs in your structure as H a, H b , etc.
Give the integral ratio for each group.
3. (8 pts) Correlate your structure with the 1 H NMR data. a. Which Hs give rise to the pattern around 3.5 ppm?
Explain the pattern carefully, pointing out coupling relationships, and stipulating coupling constants..
b. Which Hs give rise to the pattern around 4.7 ppm?
Explain the pattern carefully, pointing out coupling relationships, and stipulating coupling constants.
.
\
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continued... 104. (3 pts) Consider the 13C NMR data given for D on p 14.
Please explain how the 4 peaks arise in the spectrum by correlating with your structure.
_______________________________________________________________________________________ end exam
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15Table 1. Nuclear spin for common nuclei which are "NMR active"
spin natural abundance 1H 1/2 99.98 2H 1 0.01513C 1/2 1.1131P 1/2 10019
F 1/2 10011B 1/2 80
Table 2. Some common isotopes and their abundance
isotope abundance1H 99.98
2H 0/01512C 98.913C 1.1014N 99.615N 0.3716O 99.817O 0.03818O 0.20035Cl 75.837Cl 24.2379Br 50.7
81Br 49.3127I 100.0
