Chem 281 Problem Set 11 Key
1. Provide the products for each reaction below and label the major product. If a
mixture of stereoisomers is formed, state the total number of stereoisomers of the
major product. Consider possible carbocation rearrangements.
Br
Br
HBr
a.
major
b.
H 2O
OH
OH
major
H3O+
Cl
HCl
c.
Cl
O
Br
O
HBr
major
d.
H2SO4
e.
major
O
Br
O
OH
2. Provide a mechanism for the major product in each reaction below. Consider
possible carbocation rearrangements. Describe the 1H NMR for the product of a.
HCl
a.
Cl
H Cl
Cl
1H NMR: singlet 6H, 1.2 ppm, septet 1H, 1.5 ppm, 6H doublet 0.9 ppm.
H OH2
b.
H 2O
HO
H3O+
H 2O
H 2O
H
H O
3. Provide the major products for each reaction below. If a mixture of stereoisomers is
formed, state the total number, and their stereoisomeric relationships
1. BH3
a.
OH
2. NaOH
H2O2
OH
1. BH3
2. NaOH
H2O2
b.
two enantiomers.
OH
two diastereomers
4. Provide the alkene and the reagents required to make the alcohol below via a
hydroboration-oxidation1.reaction.
BH3
racemic
2.1.NaOH
BH3
H2O2
2. NaOH, H2O2
OH
5. Provide the major product for each reaction below. If a mixture of stereoisomers is
formed, draw all stereoisomers using wedged and dashed bonds, and state the
relationship. (enantiomers or diastereomers). Draw the curved arrow mechanisms
for reactions a, b, and c.
a.
Br Br
Br
Br
Br
Br
enantiomers
Br
Br
OH
OH
Br
Br
Br Br
b.
enantiomers
HO H
Br
H 2O
H 2O
Br
c.
HO
HO
H OH2
H 2O
H 2O
enantiomers
H
OH
Br
Br2, CH2Cl2
d.
Br
Br
Br
enantiomers
Cl
Cl
Cl2, H2O
OH
e.
OH
enantiomers
f.
OH
1. BH3
2. H2O2, NaOH, H2O
no stereoisomers
6. Provide the product for each reaction below. If a mixture of stereoisomers is
formed, draw all stereoisomers using wedged and dashed bonds, and state the
relationship. (enantiomers or diastereomers). If the reaction is stereoselective,
clearly draw the major stereoisomer(s).
Cl
Cl
Cl2
Cl
a.
mCPBA
b.
enantiomers
Cl
O
O
enantiomers
Lindlar Catalyst
c.
+ H2 (excess)
d.
Pd/C
+ H2 (excess)
F
F
F
enantiomers
7. Provide the product for the below. If a mixture of stereoisomers is formed, draw all
stereoisomers using wedged and dashed bonds, and state the relationship.
(enantiomers or diastereomers). Name the products. What 1H NMR signals
(chemical shift, splitting, integration) would distinguish the product from the starting
material?
Br
Br
HBr
Enantiomers
(S) and (R) -2-bromo-4-methylpentane
The 1H NMR of the product would be missing the signals for the alkene protons (3 signals
around 5-6 ppm, integrating to 1H each, multiplet, doublet, doublet). There would be 3 new
signals, one around 3-4 ppm , a multiplet with integration = 1H. The second would be around 1.2
ppm, a doublet with integration = 3H, and 2H multiplet around 1.5 ppm
8. For the following reactions, provide the starting alkene and the reagents needed to
complete the reactions. None of the starting alkenes should have existing chiral
centers.
H2SO4, H2O
a.
1. BH3, THF
2. H2O2, NaOH, H2O
b.
OH
OH
H2, Pd/C
c.
H2, Pd/C
d.
Cl2, CH2Cl2
e.
Cl
Cl
Br2, H2O
f.
OH
Br
mCPBA
g.
CH2Cl2
O
HBr
h.
O
Br
OH
9. Provide a sequence of two to three reactions for the transformations below, starting
from the given starting material. You’ll need to use reactions from throughout the
course. A good approach is to use retrosynthetic analysis:
a. Start at the product, and think about a single reaction that would give you
the final product.
b. From there, if it’s a two reaction sequence, you should be able to make the
reactant you just drew from the given starting material, in one reaction. If
not, repeat step a, but the product is now the reactant you just drew.
Cl
1. TsCl, Pyridine
a.
OH
b.
2. t-BuOK
3. Cl2, CH2Cl2
Cl
(racemic)
OCH3
1. NaOCH3
Br
2. mCPBA
3. NaOCH3
HO
(racemic)
Br
c.
1. H2, Lindlar Catalyst
2. Br2, CH2Cl2
d.
1. BH3
2. NaOH, H2O2
3. TsCl, pyridine
4. (CH3)2CHONa
1. HCl
e.
2. NaOCH3
Br
(racemic)
O
(racemic)
10. Provide the products of the following sequences of reactions. For (a) and (b),
indicate the number of signals and how many singlets, doublets, triplets, quartets,
multiplets or combinations of these splittings you would expect in the 1H NMR of
the final product.
1. NaNH2
2. CH3CH2CH2I
3. H2, Lindlar cat.
a.
Br
b.
1. KOtBu, hexane
2. MCPBA
+ enantiomer
O
O
c.
1. Cl2, H2O
+ enantiomer
2. NaH, hexane
Cl
1. Cl2, CH2Cl2
+ enantiomer
2. KOCH3, hexane
Reaction conditions were incorrect. Should’ve said 2 eq of KOtBu,hexane then the
product would be the diene shown above. It would be difficult to eliminate one Cl and
not the other.