C HAPTER 12 HW S OLUTIONS : R EDOX R EACTIONS
OXIDATION STATES AND GENERAL CONCEPTS
1. Are the following statements true or false?
a. When a carbon atom changes its oxidation state from -2 to 0 it is reduced.
False (Loss of e–)
b. The oxidation state of carbon in methane is -4.
True
c. When a carbon atom “gains hydrogen,” (is connected to more hydrogen atoms than before), it is reduced.
Mostly True (except see 3d)
–
–
d. The SN2 reaction CH3Br + OH à CH3OH + Br is not a redox reaction.
True
2. Calculate the oxidation state for each atom in the transformations below. Then identify whether the organic
compound in each process is oxidized, reduced or not undergoing a redox reaction.
OH
O
a.
H
C
H
H
O
H
C
H
H: +1
OH
H
H
H: +1
OH
b.
H
Ox: 6-8 = -2
O
C
H
Ox: -2
C
H: +1 H
C
H3C
H
C: 4-7 = -3
C: 4-5 = -1
H: +1
H
c.
OH
H
H
H: +1
Ox: -2
C
O
H3C
C
H
H
H
C: 4-6 = -2
C: 4-5 = -1
C: 4-4 = 0
Carbon is oxidized Carbon is oxidized
C: 4-6 = -2
Carbon is reduced
Br
H
H
H
C: 4-4 = 0
C
H
H
O
CH3 Br: 7-8 = -1
C
H3C
Br
H
H
H: +1
CH3 Ox: -2
C: 4-4 = 0
C: 4-7 = -3
O
H
C: 4-4 = 0
Not a redox
3. Identify if each process below is a reduction, oxidation or neither (not a redox reaction).
OH
Process
Rxn Type
O
Oxidation (loss of H on central C)
Reduction (gain of H)
O
Br
Pair
HO
Rxn Type
HO
Oxidation (loss of H and gain of Ox)
Not a redox
(one C gains Br, but other C gains H)
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HYDROGENATION
4. Draw the two enantiomeric products formed in this reaction, then briefly explain why a racemic mixture is
produced.
The alkene is flat, so will approach the
palladium (Pd) surface from either face. The
Pd cat.
two H’s will add to whichever face it “docks”
onto the Pd, and there is an equal probability for either side to dock and add the H’s.
H2
+
5. Explain why reactions A + C make only or mainly one product, while reaction B makes two products.
A
D2
Pd cat.
D2
C
Pd cat.
D
D
D
D
D
(These are same)
D
In the reaction of A, there’s less steric hindrance when the alkene docks onto the palladium surface with the
methyls facing away from the surface. This will cause the D’s to add to the side opposite the methyls (on the
side it “docks”).
In the reaction of B, both faces have steric interactions with the Pd surface to some extent. There’s not a big
difference between a methyl and ethyl, so addition will happen on both faces (a mixture of products is made).
D2
B
Pd cat.
D
D
D
D
In the reaction of C, there’s no significant
difference in sterics between the two alkene
faces, so addition occurs on both sides. It
just so happens that the two addition
products are superimposable, so there’s
only one product.
6. Give the major organic product of each reaction, paying attention to stereochemistry. Indicate if a racemic
mixture is expected.
a.
excess H2
e.
Pd (cat.)
b.
c.
excess H2
Pd (cat.)
H2
(meso)
Pt (cat.)
D
D2
f.
Pd (cat.)
D
CH3
g.
Pt (cat.) D
excess H2
D2
(racemic)
Pt (cat.)
(racemic)
D
CH3
d.
D2
h.
Pd (cat.)
D
D
D2
D
Pd (cat.)
D
CH3
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7. Structures A-C are constitutional isomers of C8H14. Propose a possible structure for A-C that fits each of the
observations below.
Structure A reacts with 2 moles
of H2 (w/ Pd catalyst).
Structure B reacts with 1 mole of
H2 (w/ Pd catalyst).
Structure C does not react with
H2 /Pd.
Two alkenes or one alkyne
Ring and alkene
Any two rings
8. (Multiple choice) Which statement is false about the platinum used in a hydrogenation reaction?
1. 2. 3. 4. The platinum is regenerated after the reaction.
The platinum changes oxidation state during the course of the reaction.
The platinum lowers the activation barrier for the reaction between alkene and H2.
One mole of platinum must be used for every mole of alkene to fully hydrogenate an alkene.
mCPBA OXIDATION
9. Give the major organic product of each reaction, paying attention to stereochemistry. Indicate if a racemic
mixture is expected.
a.
mCPBA
C
a. mCPBA
CH3
c.
b. H C CNa
c. H+, H2O
O
CH
OH
(racemic)
O
OH
a. mCPBA
b.
d.
b. NaOH
c. H+, H2O
O
OCH3
a. mCPBA
b. H+, CH3OH
CH2CH3
OH
OH
(not racemic,
happens to be meso)
(racemic)
O
H
10. (Multiple choice) Which substance is most likely to react in a similar way to mCPBA? (Hint: what is the
reactive part of the mCPBA reagent?)
Has a RCO3H (peroxide)
O
O
OH
O
A
O
H
B
Cl
C
HOO
D
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OsO 4 OXIDATION
11. Give the major organic product of each reaction, paying attention to stereochemistry. Indicate if a racemic
mixture is expected.
O
a. OsO4, NMO, H2O
HO
cat. OsO4
a.
NMO, H2O
CH3
c.
HO
b. excess NaH
c. CH3CH2Br
OH
(racemic)
cat. OsO4
b.
OH
O
ONa
OH
d.
(racemic)
ONa
a. OsO4, NMO, H2O
Br
b. excess PBr3
NMO, H2O
(meso)
Br
OH
OH
OH
12. (Multiple choice) The function of NMO in the reaction between an alkene and osmium tetroxide is to:
1. 2. 3. 4. Block one face of the alkene so the oxygen atoms add to the same face.
Oxidize the osmium intermediate to regenerate the OsO4.
Lower the activation barrier between the alkene and the OsO4.
To prepare the osmium surface for the alkene.
OZONOLYSIS
13. Give all organic products of these reactions.
H
O
a.
a. O3
H
b. CH3SCH3
O
a. O3
c.
b. Zn, H2O
H
O
O
H
O
H
H
O
O
b.
a. O3
b. Zn, H2O
H
H
O
O
d.
a. O3
b. S(CH3)2
O
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14. For each problem, propose a structure that upon ozonolysis would yield only the products shown.
O
a. O3
a.
O
+
b. Zn, H2O
H
H
a. O3
b.
b. Zn, H2O
H
O
O
a. O3
c.
b. Zn, H2O
H
H
O
a. O3
d.
O
+
O
b. Zn, H2O
OR
15. Propose structures for A-D in the scheme below.
Unknown A (with molecular formula C10H18O) reacts with concentrated H2SO4 to form two compounds (B +C,
both with a molecular formula of C10H16). B + C both react with H2 in the presence of a Pd catalyst to form
decalin. Ozonolysis of B produces E and ozonolysis of C produces diketone D with a molecular formula of
C10H16O2.
B
A
C
OH
con. H2SO 4
+
heat
C10 H18 O
If A were:
OH
C10 H16
Then C would be:
C10 H16
H2
Pd cat.
a. O3
H2
Pd cat.
a. O3
b. Zn
decalin
O
And D
would be:
(not
diketone)
b. Zn
O
H
O
H
O
E
decalin
D
O
H
O
diketone, C10 H16 O2
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CHROMIUM OXIDATIONS
16. Give the major organic product of each reaction. Assume sufficient chromium reagent (excess) for complete
reaction of the organic substance.
O
OH
a.
CrO3
OH
H2SO 4 (aq)
PCC
OH
f.
O
CH2Cl2
O
OH
O
Na2Cr2O7
b.
H2SO 4 (aq)
O
HO
H
OH
d.
O
CrO3
c.
H2SO 4 (aq)
O
HO
OH
OH
OH
K2Cr2O7
O
CH2Cl2
OH
O
j.
H
H
O
CH2Cl2
O
H
O
O
CrO3
H2SO 4 (aq)
OH
O
PCC
H
O
PCC
H2SO 4 (aq)
O
i.
H2SO 4 (aq)
H
Na2Cr2O7
h.
OH
e. HO
OH PCC
CH2Cl2 HO
g. HO
O
HO
17. For each problem, draw a structure that matches each set of criteria.
Criteria
Possible Structure (many answers)
• Has a molecular formula of C5H10O à 1 d.u.
• Reacts with CrO3/H2SO4 to produce a blue-green precipitate à
1o or 2o ROH, or aldehyde
• Reacts with H2 in the presence of a Pd catalyst à Alkene or
alkyne (but not alkyne, need 2 d.u.)
• Has a molecular formula of C5H10O à 1 d.u.
• Reacts with CrO3/H2SO4 to produce a blue-green precipitate à
1o or 2o ROH, or aldehyde
• Does not react with H2 in the presence of a Pd catalyst à No
alkene
• Has a molecular formula of C6H12O
• When CrO3/H2SO4 is added, an orange solution is observed à
No reaction. 3o ROH or other unreactive groups
• When Br2 is added, the solution turns colorless à Alkene or
alkyne (but not alkyne, need 2 d.u.)
OH
OH
OH
O
H
OH
OCH3
Page 6
18. (Multiple choice) The reason a primary alcohol is oxidized to a carboxylic acid with CrO3/H2SO4, but to an
aldehyde with PCC/CH2Cl2 is:
1. 2. 3. 4. There are not enough oxygen atoms in the PCC reagent.
PCC is not a strong enough oxidizing agent to react with alcohols.
The CrO3 reagent is in an aqueous solvent while the PCC reagent is not.
The CrO3 reagent is acidic, which helps the OH to leave.
SUMMARY OF REDOX REACTIONS
19. For each reaction, decide whether the organic compound is undergoing a reduction, oxidation or neither (is not
undergoing a redox reaction).
Reaction
RCl rxn with NaOH
ROH rxn with HBr
ROH rxn with CrO3 /
H2SO4
Alkene rxn with
OsO4 / NMO, H2O
Red, Ox
or neither
Not redox
Not redox
Oxidation
Oxidation
Reaction
Alkene rxn with
mCPBA
Alkene rxn with H2 /
Pd
Alkene rxn with HCl
Red, Ox
or neither
Oxidation
Reduction
Not redox
20. Give the major organic product of each reaction. Indicate if a racemic mixture is produced.
D
OH
a.
O
CrO3
D2
d.
OCH3
OCH3 H2SO 4 (aq)
CH3
D
D
Pt cat.
CH3 D
CH3
O
b.
a. O3
b. Zn, H2O H
O
a. mCPBA
e.
b. H+, CH3OH
O
O
H
H
OCH3
CH3
OH
(racemic)
H
OH
c.
OsO4 cat.
PCC
f.
NMO, H2O
OH
(racemic)
CH2Cl2
OH
H
O
Page 7