Name ________________________________ _________________________ _______
February 10, 2010
Please write your name on all pages
Ch 41b Midterm Instructions:
You have 1 hour to complete the exam. The exam is closed book and closed notes. Calculators are permitted. Good luck!
Problem 1
(4 points)
____________________
Problem 2
(6 points)
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Problem 3
(8 points)
____________________
Problem 4
(8 points)
____________________
Problem 5
(8 points)
____________________
Problem 6
(8 points)
____________________
Problem 7
(3 points)
____________________
Problem 8
(3 points)
____________________
Problem 9
(4 points)
____________________
Problem 10 (8 points)
____________________
TOTAL
(60 points) ____________________
Name ________________________________ 1. Rank the following molecules in order of decreasing pK a (increasing acidity). (4 points)
R
R
H
H A
pK a
>
R
R B
B
O
H H C
A
>
2
C
D
>
D
Name ________________________________
2a. Provide the reagents necessary for the following transformation (2 points):
BH 2 H 2. H2O2, NaOH
O
2b. Draw the structure of the major organic p roduct of the following reaction (2 points):
H2 Pd/C Pyridine
2c. Draw the structure of the major organic p roduct of the following reaction (2 points):
Na/NH3
3
Name ________________________________
3. Propose a mechanism for the following transformation. (8 points)
H
O
Br 2
Br
H2O
H O
Br
H Br H Br H
Br
Br Br
O
H H
Br bromonium
H
H
H
Br
O
H
H
Br
O
H Br
Br H
H vinyl cation
4
Br
Name ________________________________
4. Design a synthetic scheme for epoxide B starting from the alkyl halide A. Show synthetic intermediates and reagents (Organic molecules with 4 carbons or less ma y be used as reagents). (8 points)
H
O H
Br
A
B
Na A
1)
1) NaNH2, NH3 2) Br
Na , THF 2) H3O+
Na0, NH3 (liq)
mCPBA
B
5
Name ________________________________
5. Design a synthetic scheme for diol B starting from acetylene A. Show synthetic intermediates and reagents. (Organic molecules with 4 carbons or less ma y be used as reagents). (8 points)
H
HO H
H
A
OH H
B
NaNH2
NaNH2
A
Br
Br
Pd/C (poisoned) H2
-orLindlar's Catalyst
HO H
B
OH H
1) OsO4
2) H20 NaHSO3 -orreductive workup
6
Name ________________________________
6. Suppose you were able to cool a sample of 1-bromo-1,1,2-trichloroethane enough that rotation about the carbon-carbon bond becomes slow on the NMR time scale. What changes in the NMR spectrum would you anticipate? Be explicit. (8 points) ( Hint: Draw Newman projections)
H H Cl
Cl Cl Br
1-bromo-1,1,2-trichloroethane
At room temperature: rapid equilibrium, NMR is a singlet
At low temperature:
one line for conformation A and two doublets for conformations B and C, for a total of five lines (assuming no overlaps)
Loudon, prob 13.28
7
Name ________________________________
1
7. In the H NMR spectrum of propylbenzene (CDCl3, 300 MHz, in ppm), the two proton triplet consists of the following peaks: 2.60, 2.57, 2.54. Express the coupling constant ( J ) for this splitting pattern in Hz. (3 points)
propylbenzene
(300 MHz)(0.03 ppm) = 9 Hz
8. A solution of 4-methyl-3-penten-2-one in ethanol shows an absorbance of 0.52 at 236 nm in a cell with a 1-cm light path. Its molar absorptivity in ethanol at that -1 -1 wavelength is 12,600 M cm . What is the concentration of the compound? (3 points)
O
4-methyl-3-penten-2-one
A = cl c
0.52
A
l
(12,600 M
1
cm
8
1
)(1cm)
4 x10
5
M
Name ________________________________ 1
9. The H NMR spectrum of bromocyclohexane shows a downfield peak (1 H ) at 4.16. This signal is a single peak at room temperature, b ut at -75°C separates into two peaks at 3.97 and 4.64. Explain this observation. (4 points)
Br H
Br H equatorial Br
axial Br
10. Determine the structures of the stereoisomeric 1,3-dibromo-1,3-dimethylcyclobutanes A and B from the spectroscopic data below. (8 points) Isomer A: Isomer B:
1 1
H NMR peaks: 2.13 (6 H , s), 3.21 (4 H , s) H NMR peaks: 1.88 (6 H , s), 2.84 (2 H , d), 3.54 (2 H , d) (Hint: doublets have equal spacing)
Br
Br
Br
Br Isomer A
Isomer B
rotational axis
reflection plane (H are diastereotopic)
9
Stoltz Ch 41c Midterm 1
Chem 41c Midterm Exam Stoltz, Spring 2011, April 29, 2011 The exam begins when you turn to page 2. You have 55 minutes to complete the exam. This is a closed note and closed book exam with no collaboration. You may use the periodic table at the front of the room or the one on the last page of this packet. You may also use a model kit if you like. You may not use any other materials. The exam has a total of 60 points. Good luck. There are 11 pages in this exam packet. Be sure to write your name on every page!
Name:_________________________________________
Stoltz Ch 41c Midterm 2
Chem 41c Midterm Exam Stoltz, Spring 2011, April 29, 2011 The exam begins now. You have 55 minutes to complete the exam. This is a closed note and closed book exam with no collaboration. You may use the periodic table at the front of the room or the one on the last page of this packet. You may also use a model kit if you like. You may not use any other materials. The exam has a total of 60 points. Good luck. There are 11 pages in this exam packet. Be sure to write your name on every page!
THERE ARE 5 PROBLEMS WORTH 10-15 POINTS EACH ON THIS EXAM. 1. (10 points) 2. (10 points) 3. (15 points) 4. (10 points) 5. (15 points) TOTAL
PLEASE WRITE ALL ANSWERS IN SPACES PROVIDED; USE BACKS IF NECESSARY. ONLY THESE WILL BE GRADED
GOOD LUCK!
Stoltz Ch 41c Midterm 3
1. Predict the major organic products of the following reactions or sequences. (2 points each) a. O
OH
1. CH3MgBr (2 equiv), THF OEt
2. H3O+, H2O
b. O OH
1. H+, HS
H
OH
SH H
2. Raney-Ni
c. O O
O Cl
O
O
OH i -Pr2NEt,
O
O
H
O
CH2Cl2
d. O
H
O
O H OCH3
O
1. LiAlH4, Et2O 2. PDC, CH2Cl2 3. THF, 0 °C P(Ph)3
O
O
PhCH3 150 °C
O
H
H H3CO
OCH3
H
CH3
HINT: heating the product in PhCH3 at 150 °C leads to this product
e. O EtO
O
O
O
NaOEt, HOEt OEt
OEt
Stoltz Ch 41c Midterm 5
3. Provide the missing products in this multi-step synthesis scheme (15 points).
1. Mg, Ether
HBr
2. CO2 3. H2O, H3O+
Br
CO2H
A C6H11Br
B C7H12O2 1. SOCl2 2. HOCH3, DMAP, Et3N
O Cl
O O
E C9H16O2
OH
Et3N
1. LiAlH4 CO2Me
2. H2O, H3O+
D C7H14O
C C8H14O2
Hint: Below is the 1 H NMR spectrum of compound E acquired in CDCl 3 . 3H
10H
2H
1H
Stoltz Ch 41c Midterm 6
4. Glycoside derivatives of the terpene steviol ( 1) are naturally occurring molecules that have an extremely sweet taste and are used in a variety of soft drinks and other products as low calorie sweeteners. The conversion of steviol ( 1) to its methyl ester 2 is a slow reaction under acid catalyzed esterification reaction conditions. Why would you expect this to be so? Under what conditions would you expect a rapid reaction to occur? Provide reagents and draw a mechanism for the successful reaction. (10 points) OH
OH CH3OH, HCl slow reaction
H
H
H
HO
H
O
O
H3C
O
Steviol (1)
2
–reaction is slow due to sterics –CH2 N2 or MeI, Base (K2 CO3 ) would work better. Mechanism as in class for either. MECHANISM OH
OH
diazomethane H
H
H
HO
H
MeO
O
O
–N2
H R1
N
R2
N
R1
H
HO
R2
O O
H3C
N
N
O
H N
N
H
OH
OH
K2CO3, MeI
H
H
acetone H
HO
H
MeO
O
O
R1
R2
R1
HO
R2
O O
CO3 –2
O
H3C
I
Stoltz Ch 41c Midterm 7
5. We have discussed the so-called “Weinreb amide” a number of times this term. Show how you could use such an approach to prepare aldehyde 4 from carboxylic acid 3. Explain mechanistically how the Weinreb amide functions and how the chemistry observed differs from what would occur using a standard amide. Finally, propose a synthesis of hydroxyester 5 (15 points)
O
O OH
H
3
4
OH
O OEt
Ph
5
a variety of possibilities
O
DIBAL or LiAlH4
O
OH
N
O
O H 4
3
O H
AlR2 N
O
stabilizes tetrahedral intermediate
O
O OH
3
OH
1. LDA 2. add 4 OEt
3. mild H+ quench
O OEt
Ph 5
