CHEM 633: Advanced Organic Chem: Physical
Problem Set 3
ANSWER KEY
1. (a) Please draw the lowest energy conformations of these two methyl-substituted decalins.
Me
Me
H
H
(b) Estimate the energy difference between the lowest energy conformations.
Me
CH3
H
favored
CH3
H
CH3
H
CH3
H
H
4 gauche butane interactions: 4(0.88 kcal/mol) = 3.52 kcal/mol
Me
CH3
CH3
H
CH3
H
CH3
H
CH3
H
H
H
5 gauche butane interactions: 5(0.88 kcal/mol) = 4.4 kcal/mol
!E = +0.88 kcal/mol
2. Predict the stereochemistry of the product and explain your reasoning.
Me
Me
(i) LiN(i-Pr)2
O
O
(ii) MeI
*
Me
Me
CH3
Me
O
H
Me
H
N(i-Pr)2
LiO
H
H
CH3
MeI
H
OLi
H 3C
H
H
O
O
Me
Back face is blocked by
cyclooctene ring.
1. 3. Edwards, Montoya-Peleaz and Crudden have studied the mechanism of epoxidation of benzaldehyde
(B) with sulfonium SM (Org. Lett. 2007, 9, 5481). The general mechanism for this transformation is
believed to proceed via sulur ylide 1 and then intermediate 2.
N
H3C
S
Ph
CH3
BF4
N
(DBU)
step 1
H3C
S
Ph
CH3
1
SM
O
PhCHO (B)
O
Ph
Ph
step 2
S
2
step 3
Ph
Ph
P
(a) Under pseudo-first-order conditions (large excess of benzaldehyde B and DBU), the authors
monitored the disappearance of sulfonium tetrafluoroborate SM. What is the kinetic order of [SM]
in this reaction? First order (linear plot of ln[SM] vs time)
(b) Under pseudo-first-order conditions, the following data was obtained. In each of these reactions,
[B] = 0.0174 M and temperature = 298 K. What is the kinetic order of [DBU]?
Entry
[DBU] (M)
Trendline from plot of ln[SM] vs. time (seconds/100)
1
0.0291
y = –0.0068x – 1.8296
2
0.0751
y = –0.0199x – 1.6405
3
0.113
y = –0.0329x – 1.6278
4
0.156
y = –0.0466x – 6.3816
m
Slope of the trendline = –kobs = k [DBU] [B]
entry
1
2
3
4
[DBU] (M)
0.0291
0.0751
0.113
0.156
kobs
0.0068
0.0199
0.0329
0.0466
n
kobs vs. [DBU]
0.05
y = 0.3159x - 0.0029
R2 = 0.9987
0.045
0.04
0.035
kobs
0.03
0.025
0.02
0.015
0.01
0.005
0
0
0.05
0.1
0.15
0.2
[DBU] (M)
Linear relationship between kobs & [DBU], so m = 1… first-order rate dependence on [DBU].
(c) Under pseudo-first-order conditions, the following data was obtained. In each of these reactions,
[DBU] = 0.156 M and temperature = 298 K. What is the kinetic order of [B]?
Entry
1
2
3
4
5
6
entry
1
2
3
4
5
6
[B] (M)
0.0174
0.0433
0.0866
0.130
0.173
0.260
[B] (M)
0.0174
0.0433
0.0866
0.13
0.173
0.26
kobs
0.0466
0.1103
0.1986
0.2474
0.3022
0.3192
Trendline from plot of ln[SM] vs. time (seconds/100)
y = –0.0466x – 6.3816
y = –0.1103x – 1.8947
y = –0.1986x – 1.7842
y = –0.2474x – 1.7569
y = –0.3022x – 1.7994
y = –0.3192x – 1.7368
kobs bs. [B]
0.35
0.3
0.25
kobs
0.2
0.15
0.1
0.05
0
0
0.05
0.1
0.15
0.2
0.25
0.3
[B] (M)
Saturation kinetics (first-order at low [B] going to zero-order at high [B])
Answer to (d)
2.
5.
3.
4.
5.
7.
(a)
(b)
(d)
(e)
(f)
(g)
(h)
(i)
12.
9.
10.
11.
(13)
(14)