Additional Problems for Kinetics II
1. State whether the following reactions will show a normal or inverse, primary or
secondary, kinetic isotope effect. Explain your reasoning.
O
O
N3 -
A.
H (D)
H (D)
N3
H (D)
B.
H (D)
(D) H
C.
O
OH (D)
O
+ CO2
O
2. The following isotope effects are found for the ozonolysis of various deuteriumsubstituted propenes. What do these isotope effects tell you about the mechanism?
H (D)
(D) H
KIE = 0.88
KIE = 0.88
3. The acid-catalyzed Beckmann rearrangement of oximes to amides has two
possible rate-determining steps, the first one in each path shown below. Explain
how you would use a Hammett plot analysis to distinguish these possibilities.
Note that the R group trans to the departing water is the only one that migrates.
OH2
N
Path A
N
R
R'
NHR
H2O
R+
O
R'
R'
R
OH2
N
R
R'
Path B
N
R'
NHR
H2O
O
R'
4. Explain how one would perform a Hammett plot study for the following reaction.
Do you expect a positive or negative ρ value?
Br
Br
Br-
H
Br––Br
5. How can one distinguish if the orthoester shown below hydrolyzes in acidic water
by an SN1 or SN2 mechanism? Note that the relevant SN2 mechanism would
involve nucleophilic attack at a methyl group.
OCH3
O
H3O+
OCH3
OCH3
OCH3
6. Two possible mechanisms for the Wittig rearrangement of benzyl ethers are
shown below. Path 1 involves concerted intramolecular migration of CR3,
whereas path 2 involves a heterolysis. In either mechanism, the deprotonation step
is rate –determining. How would you apply the following experiments to
distinguish between the two mechanisms: cross-over, product stereochemistry.
Ph
O
nBuLi
CR3
Ph
rds
CR3
O
CR3
path 1
O-
Ph
CR3
H2O
Ph
path 2
Ph
CHR3
O
7. In the following reactions substituent X was varied between electron-donating and
electron-withdrawing. If you use the usual σ values, do you expect a positive or
negative ρ value for each equilibrium? Should each reaction be more or less
sensitive to the X-substituent than benzoic acid? Explain your reasoning.
O-
O
NaOEt
H
X
X
HOEt
H
O
O
H+
X
X
OH
8. The relative rates of the SN1 reactions of the following two alkyl p-nitrobenzoates
are show. The reason for the dramatic difference in the rate of departure of the
nitrobenzoate leaving groups is the stabilization from the alkene to form a nonclassical carbocation (three-center, two-electron bond)
OPNB
OPNB
OPNB
1
OPNB
1011
a. When p-anisyl is placed in the reactant as shown below, the 1011 rate
enhancement by the double bond is reduced to only a factor of 3. Explain
why the introduction of this substitutent changes the relative reactivity
from the example given above.
CH3
CH3
CH3
O
CH3
O
O
O
OPNB
OPNB
OPNB
1
OPNB
3
b. The ρ values for the SN1 reaction of the following compounds were
measured using σ+ values. Interpret these values in light of the information
given above. Do these ρ values support or contradict the conclusions
based on the difference in relative rates of the two comparisons given
above?
X
X
OPNB
ρ= -5.17
OPNB
ρ= -2.30
c. When H and CF3 are the X groups shown above, the relative rates of the
SN1 reaction of the norbornene structure compared to the norbornane analog
are 41 and 35,000, respectively. Why are these relative rates higher than when
X=OMe?