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Chapter 12
Oxidation and Reduction
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Chapter 4
Oxidation
• Oxidation results in an increase in the number of C—Z bonds; or
• Oxidation results in a decrease in the number of C—H bonds.
• Reduction results in a decrease in the number of C—Z bonds; or
• Reduction results in an increase in the number of C—H bonds.
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Oxidation and Reduction
Introduction
X
X
• Sometimes two carbon atoms are involved in a single
oxidation or reduction reaction, and the net change in the
number of C—H or C—Z bonds at both atoms must be
taken into account.
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Oxidation and Reduction
Reducing Agent: one that is oxidized
• Addition of molecular hydrogen: with a metal catalyst.
• Addition of two protons and two electrons: H2 = 2H+ + 2e-.
dissolving metal reductions.
• Addition of a hydride (H-) and a proton (H+):
sodium borohydride (NaBH4)
lithium aluminum hydride (LiAlH4).
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Oxidation and Reduction
Reduction of Alkenes—Catalytic Hydrogenation
• catalytic hydrogenation : metal catalyst is required
• catalyst : Pd, Pt, or Ni, adsorbed onto a finely divided inert solid,
such as charcoal.
heterogeneous reaction
• H2 adds in a syn fashion.
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Oxidation and Reduction
Reduction of Alkenes—Catalytic Hydrogenation
• When hydrogenation of two alkenes gives the same alkane, the
more stable alkene has the smaller heat of hydrogenation.
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Oxidation and Reduction
Catalytic Hydrogenation : reversible !
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Oxidation and Reduction
Reduction of Alkenes—Catalytic Hydrogenation
• The mechanism explains two facts about hydrogenation:
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Application : Structural Determination
C8H12
H2, cat.
C8H14
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Oxidation and Reduction
Reduction of other double bonds
O
O
N
Cl
O
O
O
O
O
Slow
H
OH
OR
Inert except with stronger catalyst like Raney-Ni
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Oxidation and Reduction
Reduction of Alkynes
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Oxidation and Reduction
Reduction of an Alkyne to an Alkane
Alkane formation:
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Oxidation and Reduction
Reduction of an Alkyne to a Cis Alkene
• To stop at a cis alkene, a less active Pd catalyst is used—
Pd adsorbed onto CaCO3 with added lead(II) acetate and
quinoline. This is called Lindlar’s catalyst.
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Oxidation and Reduction
Reduction of an Alkyne to a Cis Alkene
• Reduction of an alkyne to a cis alkene is a stereoselective
reaction, because only one stereoisomer is formed.
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Oxidation and Reduction
Reduction of an Alkyne to a Trans Alkene
• dissolving metal reduction (such as Na in NH3), forms a
trans alkene.
adding electrons one at a time.
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Reduction of an Alkyne to a Trans Alkene: Mechanism
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Oxidation and Reduction
Summary of Alkyne Reductions
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Oxidation and Reduction
Reduction of Polar C—X  Bonds
• Alkyl halides can be reduced to alkanes with LiAlH4.
• Epoxide rings can be opened with LiAlH4 to form alcohols.
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Oxidation and Reduction
Reduction of Polar C—X  Bonds
• This reaction follows an SN2 mechanism.
• Unhindered CH3X and 1° alkyl halides are more easily reduced
than more substituted 2° and 3° halides.
• In unsymmetrical epoxides, nucleophilic attack of H¯ (from
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LiAlH4) occurs at the less substituted carbon atom.
Summary of Reductions
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Oxidation and Reduction
Oxidizing Reactions
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Oxidation and Reduction
Oxidizing Agents (that deliver oxygen atom or take hydrogen atom)
•
There are two main categories of oxidizing agents:
1. Reagents that contain an oxygen-oxygen bond
2. Reagents that contain metal-oxygen bonds
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•
Oxidizing agents containing an O—O bond include O2, O3
(ozone), H2O2 (hydrogen peroxide), (CH3)COOH (tert-butyl
hydroperoxide), and peroxyacids.
Peroxyacids (or peracids) have the general formula
RCO3H.
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Oxidation and Reduction
Oxidizing Agents
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•
Mostly chromium +6 (six Cr—O bonds) or manganese +7
(seven Mn—O bonds).
Common Cr6+ reagents include CrO3 and sodium or potassium
dichromate (Na2Cr2O7 and K2Cr2O7).
Pyridinium chlorochromate (PCC) is a more selective Cr6+ oxidant.
•
•
KMnO4 (potassium permanganate), MnO2.
OsO4 (osmium tetroxide) and Ag2O [silver(I) oxide].
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Oxidation and Reduction
Oxidation of Alcohols
• Alcohols are oxidized to a variety of carbonyl compounds.
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Oxidation and Reduction
Oxidation of Alcohols: mostly chromium reagents
• CrO3, Na2Cr2O7, and K2Cr2O7 are strong, nonselective oxidants
used in aqueous acid (H2SO4 + H2O).
• PCC is soluble in CH2Cl2 (dichloromethane) and can be used
without strong acid present, making it a more selective, milder
oxidant.
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Oxidation and Reduction
Oxidation of 2° Alcohols
• Any of the Cr6+ oxidants effectively oxidize 2° alcohols to
ketones.
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Oxidation and Reduction
Oxidation of 1° Alcohols
• 1° Alcohols are oxidized to either aldehydes
carboxylic acids, depending on the reagent.
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or
Oxidation and Reduction
Oxidation of 1° Alcohols
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Oxidation and Reduction
Epoxidation
• Epoxidation is the addition of a single oxygen atom to an alkene
to form an epoxide.
• Epoxidation is typically carried out with a peroxyacid.
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Oxidation and Reduction
Epoxidation
• more substituted, electron rich alkenes react faster.
• a cis alkene gives an epoxide with cis substituents. A
trans alkene gives an epoxide with trans substituents.
• This reaction is stereospecific because cis and trans
alkenes yield different stereoisomers as products.
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Epoxidation
Attack from below
Attack from above
CH3
C
C
CH3
O
mCPBA
CH3
H
H
H
cis-2-Butene
C * C*
+
CH3
H
CH3
H
CH3
* H
C
C*
O
Achiral meso compound
CH3
H
C
C
H
CH3
mCPBA
Attack from below
Attack from above
O
CH3
H
C*
C*
H
CH3
+
CH3
H
trans-2-Butene
Enantiomers
*
C
*
C
O
H
CH3
Oxidation and Reduction
The Synthesis of Disparlure
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Disparlure, the sex pheromone of the gypsy moth.
Carterpillars of gypsy moth eats leaves of broadleaf trees
Use: attract and trap male moths.
Retrosynthetic analysis of disparlure illustrates three key
operations:
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Oxidation and Reduction
The Synthesis of Disparlure
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Oxidation and Reduction
Dihydroxylation
• Dihydroxylation is the addition of two hydroxy groups to
a double bond, forming a 1,2-diol or glycol.
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Oxidation and Reduction
Dihydroxylation
• Anti dihydroxylation : two steps—epoxidation, followed by ring
opening with ¯OH or H3O+.
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Oxidation and Reduction
Dihydroxylation
• Syn hydroxylation : with either KMnO4 or OsO4.
Insoluble in organic solvent
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Oxidation and Reduction
Dihydroxylation
• Each reagent adds two oxygen atoms in a syn fashion.
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Oxidation and Reduction
Dihydroxylation : catalytic version
• Dihydroxylation with a catalytic amount of OsO4 : the oxidant Nmethylmorpholine N-oxide (NMO)
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Oxidation and Reduction
Oxidative Cleavage of Alkenes
• Oxidative cleavage forms two carbonyl compounds. Cleavage
with ozone (O3) is called ozonolysis.
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Oxidative Cleavage of Alkenes : mechanism
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Oxidation and Reduction
Oxidative Cleavage of Alkenes
• Ozonolysis of dienes or other polyenes results in oxidative
cleavage of all C=C bonds.
• It is important to note that when oxidative cleavage involves a
double bond that is part of a ring, the ring opens up affording a
single chain with two carbonyls at the carbons where the double
bonds were originally.
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Oxidation and Reduction
Oxidative Cleavage of Alkynes
• Alkynes undergo oxidative cleavage of the  and both 
bonds.
• Internal alkynes are oxidized to carboxylic acids (RCOOH).
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Oxidation and Reduction
Green Chemistry
• Green chemistry : use of environmentally benign methods to
synthesize compounds i.e. to use safer reagents and less solvent, and
develop reactions that form fewer by-products and generate less waste.
• many oxidation methods use toxic reagents (such as OsO4
and O3) and corrosive acids such as H2SO4, or generate
carcinogenic by-products (such as Cr3+).
One methods uses a polymer supported Cr3+ reagent—
Amberlyte A-26 resin-HCrO4—that avoids the use of strong
acid, and forms a Cr3+ by-product that can easily be removed
from the product by filtration.
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Oxidation and Reduction
Green Chemistry
• With Amberlyst A-26 resin-HCrO4¯, 1° alcohols are
oxidized to aldehydes and 2° alcohols are oxidized to
ketones.
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The Oxidation of Ethanol
• In the body, ingested ethanol is oxidized in the liver first to
CH3CHO (acetaldehyde), and then to CH3COO¯ (the acetate
anion).
• This oxidation is catalyzed by alcohol dehydrogenase.
• If more ethanol is ingested than can be metabolized, the
concentration of acetaldehyde increases. Acetaldehyde, which is
toxic, is responsible for the feelings associated with a hangover.
• If methanol is ingested, it is metabolized by the same enzyme to
formaldehyde and formic acid. These compounds are extremely
toxic since they cannot be used by the body. Blood pH decreases,
and blindness and death can follow.
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Homework
12.40, 12.42, 12.43, 12.50,
12.51, 12.52, 12.58, 12.60,
12.61
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Preview of Chapter 13 and 14
Organic Structural Identification : NMR, IR, UV, MS
What information can you obtain from MS?
What does IR spectroscopy measure?
What does 1H-NMR provide to determine the molecular structure?
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