Oxidation and Reduction

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Chapter 12 Oxidation and Reduction
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Oxidation and Reduction ( Change in O atoms)
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Oxidation and Reduction ( Change in H atoms)
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Practice Exercise
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Reduction Reactions
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• Reduction rxns completed using either
• Electron transfer ( Na dissolved in NH3)
• Hydride/Proton transfer ( LiAlH4/NaBH4)
• Catalytic Hydrogenation

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Reduction of an alkene using metal catalyst
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The mechanism explains (1) the stereochemistry (
syn addition) and (2) how alkyl substitution
affects reactivity.
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Measuring Alkene Stability Using Hydrogenation
As you can see here, the ?Ho of hydrogenation
confirms the trans isomer is more stable than the
cis isomer i.e. 27.6 kcal would be a lower
number than 28.6 kcal on an energy diagram.
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Reduction of an Alkyne to an Alkane
Alkane formation
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Reduction of Alkyne to Cis and Trans Alkenes
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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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Mechanism for the Trans Alkene Product
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Reduction of an Alkyne to a Trans Alkene
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Match the product with the correct reagent
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Reduction of RX and Epoxides
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Reduction of Polar CX ? Bonds

• Alkyl halides can be reduced to alkanes with
  LiAlH4.
• Epoxide rings can be opened with LiAlH4 to form
  alcohols.

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Reduction of Polar CX ? Bonds

• This reaction follows an SN2 mechanism.
• Unhindered CH3X and 10 alkyl halides are more
  easily reduced than more substituted 20 and 30
  halides.
• In unsymmetrical epoxides, nucleophilic attack of
  H (from LiAlH4) occurs at the less substituted
  carbon atom.

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Oxidizing Agents
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Which of the following oxidizing agents are used
in the space shuttle program.

• NaOCl
• H2O2
• NH4ClO4
• none of these
• all of these

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Oxidizing Reactions
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Oxidation and Reduction
Oxidizing Agents

• There are two main categories of oxidizing
  agents
• Reagents that contain an oxygen-oxygen bond
• Reagents that contain metal-oxygen bonds
• Oxidizing agents containing an OO 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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Oxidizing Agents

• KMnO4 (potassium permanganate).
• OsO4 (osmium tetroxide) and Ag2O silver(I)
  oxide.

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Epoxidation of Alkenes
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Epoxidation
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Epoxidation

• Epoxidation is stereospecific because cis and
  trans alkenes yield different stereoisomers as
  products.

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Anti and Syn Dihydroxylation
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Dihydroxylation
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Anti- Dihydroxylation

• Anti dihydroxylation is achieved in two
  stepsepoxidation followed by ring opening with
  OH or H2O.

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Syn-Dihydroxylation

• Syn hydroxylation results when an alkene is
  treated with either KMnO4 or OsO4.

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Mechanis of Syn-Dihydroxylation
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Ozonolysis
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Ozonolysis (Oxidative Cleavage of Alkenes)
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Ozonolysis-Mechanism

• Addition of O3 to the ? bond of an alkene forms
  an unstable intermediate called a molozonide,
  which rearranges to an ozonide in a stepwise
  process.
• The unstable ozonide is reduced to afford
  carbonyl compounds. Zn (in H2) or dimethylsulfide
  (CH3SCH3) are two common reagents used to convert
  the ozonide to carbonyl compounds.

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Oxidative Cleavage of Alkynes
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Oxidation of Alcohols
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Oxidation and Reduction
Oxidation of Alcohols
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Oxidation of Alcohols

• Recall that the oxidation of alcohols to carbonyl
  compounds is typically carried out with Cr6
  oxidants, which are reduced to Cr3 products.
• 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 10 Alcohols

• 10 Alcohols are oxidized to either aldehydes or
  carboxylic acids, depending on the reagent.

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Oxidation and Reduction
Oxidation of 10 Alcohols
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Oxidation and Reduction
Oxidation of 20 Alcohols

• Any of the Cr6 oxidants effectively oxidize 20
  alcohols to ketones.

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Oxidation and Reduction
Oxidation of 10 Alcohols

• Cr6 oxidations are characterized by a color
  change, as the red-orange Cr6 reagent is reduced
  to green Cr3.
• Some devices used to measure blood alcohol
  content make use of this color changeOxidation
  of CH3CH2OH, the 10 alcohol in alcoholic
  beverages, with orange K2Cr2O7 forms CH3COOH and
  green Cr3.
• Blood alcohol level can be determined by having
  an individual blow into a tube containing
  K2Cr2O7, H2SO4, and an inert solid.
• The alcohol in the exhaled breath is oxidized by
  the Cr6 reagent, which turns green in the tube.
• The higher the concentration of CH3CH2OH in the
  breath, the more Cr6 is reduced, and the farther
  the green Cr3 color extends down the length of
  the sample tube.
• This extent of the green color is then correlated
  with blood alcohol levels.

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Oxidation and Reduction
Oxidation of 10 Alcohols
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Oxidation and Reduction
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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The Sharpless Epoxidation (under
construction)
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Oxidation and Reduction
The Sharpless Epoxidation

• Recall that in the reactions we have discussed
  thus far, an achiral starting material has
  reacted with an achiral reagent to give either an
  achiral product, or a racemic mixture of two
  enantiomers.

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Oxidation and Reduction
The Sharpless Epoxidation

• In the Sharpless epoxidation, the double bonds of
  allylic alcohols are oxidized to epoxides.
• Since the formation of only one enantiomer is
  favored, the reaction is said to be
  enantioselective.
• An enantioselective reaction affords
  predominantly or exclusively one enantiomer.
• A reaction that converts an achiral starting
  material into predominantly one enantiomer is
  also called an asymmetric reaction.

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Oxidation and Reduction
The Sharpless Epoxidation

• The Sharpless reagent consists of three different
  components tert-butylhydroperoxide (CH3)3COOH a
  titanium catalystusually titanium(IV)
  isopropoxide, TiOCH(CH3)24 and diethyl
  tartrate (DET).
• There are two different chiral diethyl tartrate
  isomers, labeled as ()-DET or (-)-DET to
  indicate the direction in which they rotate plane
  polarized light.
• The identity of the DET isomer determines which
  enantiomer is the major product obtained in the
  epoxidation.

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Oxidation and Reduction
The Sharpless Epoxidation

• Reactions 1 and 2 are highly enantioselective
  because each has an enantiomeric excess of 95
  (97.5 of the major enantiomer 2.5 of the
  minor enantiomer).

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Oxidation and Reduction
The Sharpless Epoxidation

• To determine which enantiomer is formed from a
  given isomer of DET, draw the allylic alcohol in
  a plane, with the OH group in the bottom right
  hand corner then