303 09 3rd exam.pdf
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Chemistry 303
fall, 2009
THIRD EXAMINATION
7:30 PM, DECEMBER 16TH, 2009
Duration: 2.5 hr
Name___________________________________________________________
This is an “open book” examination; you may use anything that is not alive or connected to the Web.
Note: if you do not know the complete or specific answer, give a partial or general answer—
We love to give partial credit.
If there seems to be more than one good answer, explain your thinking.
If you invoke resonance delocalization as part of your answer, draw the relevant resonance structures.
If you draw a chair cyclohexane, be sure to orient the bonds carefully.
If you do not know a structure and need to write a mechanism, write a general mechanism for partial credit.
You need not draw transition states as part of a mechanism unless expressly instructed to do so.
USE THE ARROW FORMALISM CAREFULLY FOR ALL MECHANISMS. SHOW ALL INTERMEDIATES.
BE SURE TO INCLUDE ALL FORMAL CHARGES.
Write only in the space provided for each question.
Score:
p 2___________/12 p3___________/11 p4___________/08
p5___________/14 p6___________/10 p7___________/15 Lab question _________/14
p8___________/10 p9___________/ 12 p10__________/08
Lecture Total: /100
There are 12 pages in this exam; please check now to be sure you have a complete set.
Pledge:_________________________________________________________________________________
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I. (12 pts). Consider the reaction of Z-2-butene under the conditions shown.
Br2
H2OC4H9BrO
NaH
(base)C4H8O; (one stereoisomer)
M1 and M2
N(two stereoisomers)
+ H2 + NaBr
[IR: no peaks 4000-3200 cm-1]
A. (06 pts). Write the mechanism for formation of the stereoisomers M 1 and M 2, drawing both carefully using
the sawhorse or Newman projection.
Are they: enantiomers diastereomers a racemic mixture meso isomers (circle all correct answers)?
B. (06 pts). Write the mechanisms for the formation of N from M 1 and M 2, drawing the structure carefully
using the sawhorse projection. Circle true characteristics about N. (circle all correct answers)
Racemic mixture meso isomer zero optical rotation
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II. (11 pts). Consider the simple SN1 reaction (no rearrangements) of the
secondary p-toluenesulfonate esters represented by X, and the relative rate data forthe cases where both R = Me, and where both R = tert-butyl.
A. (03 pts). Draw the mechanism for the reaction involving R = Me.
B. (03 pts). Explain in terms of the mechanism the single most important reason why the case with R = t-butyl
is much faster.
C. (05 pts). Now consider the SN1 reaction of substrate Y under the same
conditions with a very different outcome in relative rates.
Explain in terms of mechanism why Y with R = t-butyl
is much slower than than Y with R = Me.
R
ROTs
H2O
50o C
rates
R = Me = 1
R = t-Bu = 10,000
X
RBr
H2O
50o C
rates
R = Me = 600
R = t-butyl = 1
Y
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IV. (14 pts). A. (07 pts). Write the best mechanism to account for the following conversion
of G to H and J.
H+
+
G
H J
B. (07 pts). Draw one reaction coordinate diagram to represent this process, leading to both products. Be
sure to indicate the relative energy of intermediates and products carefully.
E
reaction progress
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V. (10 pts). Provide the starting material, reagent(s), or major product for each reaction below.
1. O3
2. Zn
O
MeH
O
A. (02 pts).
OEt
O
O
OO
HCl
B. (03 pts).
pay attention to exact stereochemistry
one mole
one mole
tBu
Me
a. OsO4
b. reduction
C. (03 pts).
most stable chair form
O
H
H
H
HH
O
H
H
H
HH
Br OEt
D. (02 pts).
O O
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VI. (15 pts). Consider the selective formation of L from H under the conditions shown.
a. NaHCO3
COOH
LH
b. I2
undefined configuration
Et3NM C9H12O2
[13C NMR: ! 23, 25, 29, 31, 42,
82, 122, 132, 176 ppm]
O
O
I
+ Et3NH I
A. (01 pts). What is the absolute configuration of H ? R or S circle one
B. (06 pts). Draw a mechanism for the reaction that accounts for formation of L as a single diastereomer.
Show clearly the stereochemistry of the substituents.
C. (02 pts). Draw L in its lowest-energy
conformation using the chair version.
D. (06 pts). Draw the mechanism for the conversion
of L to M, showing clearly the structure of M.
Explain why this particular alkene is formed in terms
of the mechanism. Use the appropriate chairrepresentation of L. L
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VII. (10 pts). Consider the following reactions of A to give B and the enantiomers C1 and C2.
While B appears as a single enantiomer with the cis configuration, C is actually a racemic mixture of isomerswith the trans configuation. No specific configuration is implied by structure C1 /C2 shown here.
NMe2
Cl
SEt
THF
NMe2
SEt
Ag2CO3
EtOH
NMe2
OEtA
BC1 /C2
racemic trans isomers [!] = +19.5o [!] = +8o
[!] = 0o
A. (02 pts). Draw the best mechanism to rationalize the formation of B from A, including the selective
formation of the isomer shown. What is the name of this mechanism?
B. (02 pts). Draw the two isomers C1 and C 2
that constitute the racemic mixture
represented by C1 /C 2, in their most stable
chair forms.
C. (06 pts). Draw the best mechanism to rationalize the formation of the racemic C1 /C 2; show clearly why a
racemic mixture with the trans configuation is formed .
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VIII. (12 pts). Trialkylammonium ions such as F and G can undergo E2 eliminations, as shown here.
The selectivity is not always as advertised for this mechanism, as suggested by the mixtures obtained.
Et
NMe3Me
H
HD
tBuOK
DMSO
Me
Et H
D
+
95%
Me
Et D
H
5%
Ph
NMe3iPrH
HD
t
BuOK
DMSO
i
Pr
Ph H
D+
32%
Ph
iPr H
D
68%
F
G
iPr = isopropyl,
Me
Me
A. (02 pts). Why is E2 preferred over E1/S N 1 and S N 2 under these conditions? Explain briefly
B. (05 pts). Based on the data above, draw the two Newman projections for F that account best for the two
products shown. Circle the conformation that leads to the major product. Explain why this conformation is
favored, using the arrow formalism to show electron flow.
C. (05 pts). Based on the data above, draw the two Newman projections for G that account best for the two
products shown. Circle the conformation that leads to the major product. Explain why this conformation is
favored.
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IX. (08 pts). A. (04 pts). Draw here all four stereoisomers of 1,2,4-trimethylcyclopentane and
label them 1, 2, 3 and 4. Then answer the following questions using your labels. If the answer is "none", write
"none".
B. (01 pts). Give one pair of enantiomers.
C. (01 pts). Give one pair of diastereomers.
D. (01 pts). Give one meso structure.
E. (01 pts). List all chiral structures.
_______________________________________________________________________________________
End Lecture Exam Portion
1,2,4-trimethylcyclopentane (no stereochemistry intended)
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X. (14 pts). Laboratory: Hugh B. DeMann, former orgo lab student, attempted to prepare 3-
methyl-2-butenyl acetate (2) from 1-chloro-3-methyl-2-butene (1) and acetic acid, as shown below.
H3C
H3C
Cl H3C
H3C
OCCH3
O
!
1 2
+ 3
CH3CO2H
To his chagrin, the reaction also produced isomer 3. The 1H NMR spectrum of isomer 3 is summarized below:
Isomer 3 1H NMR (CDCl3, 400 MHz): " 1.52 (s, 6H), 1.99 (s, 3H), 5.07 (dd, J = 10.9 Hz, J = 0.9 Hz, 1H)
5.17 (dd, J = 17.5 Hz, J = 0.9 Hz, 1H), 6.08 (dd, J = 17.5 Hz, J = 10.9 Hz, 1H)
Hint: read both parts of the question before analyzing the data. The mechanism and the spectral data should
work together to help you.
A. (11 pts). Deduce the structure of isomer 3. Draw the structure of isomer 3 and label each unique
hydrogen on your proposed structure. Under the structure, indicate your chemical shift assignments for the
resonances at ! 5.07, 5.17, and 6.08 ppm. Also, carefully explain the splitting patterns and make coupling
constant assignments for these resonances.
Continued….
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B. (3 pts). Tell Hugh (and us) why he should not have been surprised to obtain two isomeric products from
this reaction. (Hint: Formal mechanisms are not required here, but a brief mechanistic analysis of the reaction
seems warranted.)