Chem 201
Name: _______________________
Exam 1 - Key
Clearly indicate stereochemistry unless otherwise indicated. In each of the mechanisms draw
cyclopropylcarbinyl or norbornyl resonance structures as a series of 1,2 rearrangements.
You may use molecular models. If you get stuck, try numbering atoms and/or working backwards.
Remember, we reward partial credit. Good luck!
1. (10 pts)
a. The following reaction has reached equilibrium.1 What is the difference in free energies (± 0.1 kcal/mol)
between starting material and product?
MeO2C
CO2Me
Ph
Ph
33%
25 °C
+
S.M.
66%
Answer: (ΔH° 5.4 kcal/mol; ΔS° 16.8 eu; ΔG° +0.39 kcal/mol)
1:2 ratio of S.M. to products suggests an equilibrium constant of 1/2, or +0.4 kcal/mol.
b. The following cycloaddition is irreversible. Circle the product that is formed through the lower energy
transition state. What is the difference in free energies (±0.1 kcal/mol) between the two competing
transition states that lead to exo and endo products?2
N
+
CO2Et
cat. BF3
N CO2Et +
Ph
benzene
25 °C
Ph
N CO2Et
exo
80%
Ph
endo
20%
Answer:
The ratio of exo/endo is 4:1 so that is the same as two factors of 2 (0.4 kcal/mol + 0.4 kcal/mol) or 0.8
kcal/mol. As you might have guessed from the exo/endo ratio, this reaction does not proceed through a
concerted, pericyclic reaction.
2. (12 pts) Estimate the reaction enthalpy (going from starting materials to products) using Hess’ law and
circle the side of the equilibrium that you would expect to be favored.
a.
broken
•
sp C-H 131
C C•
C CH
131
-
formed
sp2 C-H 110
110 = +21 kcal/mol
b.
O
O
broken
! C O 79
" C C 64
143
formed
! C C 81
" C O 94
-
semiempirical calcns:
#EMINI = -18.2 kcal/mol
175 = -32 kcal/mol
Chem 201
Name: _______________________
3. (20 pts)
a. Draw a box around the least stable cation. Circle the most stable cation
O
O
O
+
+
O
+
+31.1 kcal/mol
+44.1 kcal/mol
+
+0 kcal/mol
+31.9 kcal/mol
semiemipirical
(MINI basis set)
b. Redraw the most stable cation from part a in the space below. Draw and label the canonical filled
orbitals (e.g., πC-N) that donate into the empty p orbital of the carbocation.
nO
pC
:
+
O :
nO
pC
pC
!CC
O+
c. Circle the substrate that would undergo the fastest SN1 reaction. Draw a box around the substrate that
would undergo the slowest SN1 reaction
O
O
Cl
O
Cl
Cl
4. (18 pts) Suggest a plausible arrow-pushing mechanism for the following reaction.3 Note: Ag+ ions
activate iodine by forming an intermediate (I—I+—Ag) that generates insoluble AgI.
I2
AgOAc
O
H
O
O
O
CH3CO2H
23 °C
O
I
75%
H
O
Answer:
I
I:
Ag
Ag+
I
O
+I
O
O
Ag OAc
O
H
O
I +
H
O
.. +
I
O
O
O
O
H
O
I
H
O
Chem 201
Name: _______________________
5. (18 pts) The following epoxide was found to be so unstable that it decomposed within 1 day of storage.4
Suggest a plausible acid-catalyzed arrow-pushing mechanism that explains the two products.
unstable
O
cat. HA
Ph
OH
CHO
Ph
20 °C, 24 h
Ph
24%
65%
Answer:
H A
O:
OH
cat. HA
Ph
20 °C, 24 h
Ph
24%
+ H
O
Ph
OH
+
CHO
Ph
65%
HO
Ph
+
-A :
Ph
H
A..
HO:
and...
+
Ph
+O
+
H
Ph
Chem 201
Name: _______________________
6. (12 pts) Suggest a plausible arrow-pushing mechanism for the following reaction.5
OH
4:1
HO
H
O
cat. SnCl4
CO2H
CH3NO2
0 °C, 1.5 h
H
3:2 mixture
of epimers
H
HO
H
O
64%
O
Answer:
..
O
Cl4Sn
SnCl4
R
HO
O
HO
Cl4Sn
HO
H
H
H
H
H
R
=
H
..
O
H
+
HO O
+
4
HO
O
HO
H
+
H
H
H
OH
-A :
H
+O
O
H O
O
H
R
7. (12 pts) Suggest a plausible arrow-pushing mechanism for the following reaction.6
OMe
O
N
O
H 2O
23 °C, 12 h
MsO
H N
NaHCO3
O
MeO
HO
O
90%
Answer
OMe
N
MsO
O
O
OMe phenonium ion
O
+
N
O
OMe
N
HO :
-
+
H
HO
H
H
H
H
R
+
- O-SnCl
Cl4Sn
H
H
HO
R
Cl4Sn
or
4
HO
H
H
Actually, the tin probably
chelates between both
oxygen atoms.
- O-SnCl
O
+
O
O
Chem 201
Name: _______________________
References
1
Ciganek, E. J. Am. Chem. Soc. 1971, 93, 2207–2215.
Grant. Krow, Ron. Rodenbaugh, R. Carmosin, W. Figures, H. Pannella, G. DeVicaris, M. Grippi
J. Am. Chem. Soc. 1973, 95, 5273–5280.
3
Brian A. Keay, Christine Rogers, Jean-Louis J. Bontront J. Chem. Soc., Chem. Commun. 1989, 1782-1784.
4
Coxon, J.M.; Hartshorn, M.P.; Lewis, A.J. Aust. J. Chem. 1971, 24 1009-1015.
5
White, J.D.; Jensen, M.S. J. Am. Chem. Soc. 1995, 117, 6224–6233. The starting material was a 4:1 mixture of E and Z isomers.
2
H
the poorly drawn T.S. in the paper
suggests that
both of the H's on the cyclopropane
should be syn
6
Cl
Cl
Sn O
H
O
H
H
R
Inubushi, Y.; Fales, H. M.; Warnhoff, E.W.; Wildman, W.C. J. Org. Chem. 1960, 25, 2153–2164.