CHEM24
1.
Dr. Spence
Doestheadditionof HBr to the following alkenegive synaddition,anti additionor a mixtureof
both? Briefly explainyour answerusingthe structureof the intermediateformedin this addition.
Hodwhy is this differentfrom additionof Br2? H2?
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Rank the following compoundsfrom most reactive (1) to leastreactive (4) towards 03. Also
rank their reactivity towards BH3. Briefly explain yoril answers.
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In the additionof electrophilesto alkenes,most electrophileswill add to the most reactivealkene
first (most electronrich alkene). The two exceptionsto this generalizationarcH2 and BH3.
Provide the productswhen the following alkenereactswith the two reagentsshown below. Be
sureto draw correct stereochemistrywhere necessaryand explain why the electrophileaddedto
the positionyou indicated.
1. BH3 (1 equiv)
Br2 (1 equiv)
2. H2O2,-OH
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CHEM24
Dr. Spence
Preparationof Alkenes. The main way to preparean alkene is to perform
an elimination reaction
of an alcohol (via carbocation)or an alkyl halide. For the two reactions
shown below draw all
possibleproducts and then circle the one that is favored under
thermodynamic conditions. We
will seemechanismsfor thesereactionsin a later chapter.
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CHEM24
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Dr. Spence
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CHEM24
8.
Dr. Spence
Reactions. Give the structureof the major organicproduct(s)expectedfrom eachof the
following reactions.If necessary,indicateproductstereochemistry.
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l. OsOa
-----------..-_2. NaHSO3
3. HIO4
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tBu
,-Ja,
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1. RCO3H (1 eq)
2. Brt, HrO
\--\-.2
(CH3)3C p
a*Y'n'
\-.\-,/
l HBr (l equiv)
2. H2,Pil/C
l . H g ( O A c ) 2 ,H 2 O
2. NaBHa
H
*Y
- $ t
or{
1 RCO3H
2. H-,H2O
Lj-/
a.
OH
1. 03 (l eq)
2. Zn,H+
3. BH3
4. H2O2,HO'
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trod
9. Supply the missing reagent, reactant, or product. If more than one product can formed, give all
possible products, and indicate which would be the major product. Be sure to indicate the
stereochemistry and regiochemistry of the products where appropriate. The last three are
particularly challenging.
a.
H
1. BH3-THF
OH
2. NaOH, H2O2
syn addition of H, OH
b.
Br
NaOCH3
we used NaOH in class
c.
H2, Pd/C
O
O
note: these conditions only reduce ordinary alkenes -- not carbonyls or benzene rings
d.
OH
1. Hg(OAc)2, H2O
2. NaBH4
e.
H
1. O3
2. Zn, HOAc
O
O
H
f.
H 2O
CH3
CH3
H2SO4
OH
carbocation intermediate at more substituted carbon, so OH ends up there
g.
OH
Br2, H2O
Br
anti addition (but product is racemic mixture)
h.
OH
1. BH3-THF
2. NaOH, H2O2
note that 1-butene would give 1-butanol, not 2-butanol as given
i.
Br
Br2
Br
anti addition (but product is racemic mixture)
j.
H2, Pd/C
H2 gas always in excess, so both  bonds will react
k.
1. OsO4 (excess)
2. NaHSO3
HO
OH
HO
OH
HO
OH
+
HO
OH
major product
syn addition for each diol; second addition more likely from opposite side
l.
1. O3
O
O
+
2. Zn, HOAc
O
O
ozone always in excess; 2 moles of each product are formed
m.
H3C
H3C
CH3
H3C
1. BH3-THF
CH3
CH3
H3C
H
H
CH3
CH3
CH3
2. NaOH, H2O2
H
OH
borane addition from less sterically-hindered side of alkene
10. Give the missing products, reactants, or reagents for the following reactions.
a.
1) OsO4
OH
2) NaHSO3
OH
b.
OH
1) BH3, THF
2) HOOH, -OH
don't worry about
THF, just a solvent
c.
H2, PtO2
we use Pd/C in class
d.
O
1) O3
2) Zn, H3O+
O
H
H
e.
Br
Br2, CCl4
Br
don't worry about CCl4, just
solvent
f.
Cl
HCl
+
Cl
and likely get a third product via carbocation rearrangement
g.
Br
OH
Br2, H2O
h.
OH
1) Hg(OAc)2, H2O
2) NaBH4
i.
H2, Pd/C
j.
Cl
Cl2, CCl4
Cl
k.
Br2, H2O
OH
Br
l.
1) BH3, THF
OH
2) HOOH, -OH
m.
H2, Pd/C
n.
mCPBA
O
O
+
o.
1) O3
O
O
2) Zn, H3O+
H
p.
Cl2, CCl4
Cl
Cl
11. Using mechanistic arrows, draw a detailed mechanism for the following reactions.
a.
I
I
I
N3
N3
N3
b.
OH
OH2
H
loss of
H2O
protonation
loss of
H+
H
H
hydride
shift
12. Draw a reasonable mechanism to explain the outcome of the following reaction. Be sure that
your mechanism explains the observed stereochemistry.
O
Br2
Br
O
OH
O
H
Br
Br
Br
O
O
Br
OH
O
H
H
13. Given the results from the oxymercuration reaction below, predict the products from treating
the same alkene with Br2 and H2O. Draw a mechanism that explains the regiochemistry and
stereochemistry of the product.
H3C
CH3
CH3
H3C
1. Hg(OAc)2, H2O
OH
CH3
2. NaBH4
CH3
H3C
CH3
H3C
CH3
H3C
Br2 , H2O
CH3
OH
CH3
CH3
CH3
Br
Br
bromine must add to bottom face, so water must add from the top
14. Give the stereochemical relationship of the products formed (i.e. enantiomers, diastereomers,
etc) in the reactions below. Recall that the method we use is to look at the products from both top
and bottom face attacks and then to determine the stereochemical relationship between them.
a.
H
H
1. OsO4, pyridine
CH3
2. NaHSO3
HO
OH
H3C
CH3
HO
OH
H3C
CH3
+
H3C
the two products are the same -- so product is single, achiral, meso compound
b.
H
H3C
CH3
1. OsO4, pyridine
H
2. NaHSO3
HO
CH3
HO
CH3
H3C
OH
+
H3C
OH
(R, R)
The two products are enantiomers; overall we get a racemic mixture
(S, S)
15. Provide a synthetic sequence to go from the given starting material to the desired product.
Show all reagents and synthetic (not reaction) intermediates. All these syntheses can be
accomplished in two or three steps.
a.
?
Br
OH
OH
KOtBu
1) OsO4
2) NaHSO3
we use NaOH for step 1 in a
and b and c
b.
?
H
Br
O
KOtBu
1) O3
2) Zn, H+
c.
?
Br
Br
KOtBu
HBr
d.
?
OH
OH
1) Hg(OAc)2,
H2O, THF
2) NaBH4
CH3SO2Cl, NEt3
KOtBu
OSO2CH3
don't worry about this
step 1
e.
?
OH
OH
H2SO4
1) BH3, THF
2) HOOH, -OH
f.
?
Cl
KOtBu
H2, Pd/C
again, we use NaOH in step 1
g.
?
SCH3
HBr
NaSCH3
Br
don't worry about step 2
h.
?
O
O
I
H
KOtBu
H
1) O3
2) Zn, H+
again, NaOH for step 1
16. Propose syntheses to accomplish the following transformations. Each transformation can be
done in two or three steps.
a.
?
Br
OH
1) BH3, THF
2) HOOH, -OH
KOtBu
or
1) Hg(OAc)2,
H2O, THF
2) NaBH4
NaOH
step 1
b.
Br
Br
?
OH
KOtBu
Br2, H2O
NaOH step 1
c.
OH
?
+
OH
OH
1) Hg(OAc)2,
H2O, THF
2) NaBH4
CH3SO2Cl,
NEt3
KOtBu
O
SO2CH3
Don't worry about step 1; use NaOH for step 2
or 1) BH3, THF
2) HOOH, -OH
17. Treatment of compound A (C6H14O) with H2SO4 gave two alkene products, with B as the
major product. Catalytic hydrogenation of B gave 2-methyl-pentane as the only product.
Hydroboration (1. BH3-THF; 2. H2O2, NaOH) of B gave a single compound, alcohol C.
Ozonolysis of B gave a ketone and an aldehyde (no formaldehyde): compounds D and E. Show
the structures of all the compounds: A, B, C, D, and E.
H2SO4
Alcohol
H2, Pd/C
B
A
Alcohol
Dehydration
C
Alkene
Alkane
1.BH3-THF
2. H2O2, NaOH
OH
1. O3
2. Zn, HOAc
Alcohol C
Alkene B
must be
R
D and E must be:
O
O
R
+
R
H
E
D
O
O
H
OH