1 Chapter 12 Oxidation and Reduction. 2 Oxidation Oxidation results in an increase in the number of C—Z bonds; or Oxidation results in a decrease in the

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Chapter 12

Oxidation and Reduction

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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.

Chapter 4

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• 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.

Introduction

X X

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• 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).

Reducing Agent: one that is oxidized

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• catalytic hydrogenation : metal catalyst is required• catalyst : Pd, Pt, or Ni, adsorbed onto a finely divided inert solid,

such as charcoal.

• H2 adds in a syn fashion.

Reduction of Alkenes—Catalytic Hydrogenation

heterogeneous reaction

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• When hydrogenation of two alkenes gives the same alkane, the more stable alkene has the smaller heat of hydrogenation.

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Catalytic Hydrogenation : reversible !

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• The mechanism explains two facts about hydrogenation:


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Application : Structural Determination

C8H12H2, cat.

C8H14

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Reduction of other double bonds

Cl

O

NO

O

O

H

O

OH

O

OR

O

Inert except with stronger catalyst like Raney-Ni

Slow

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Oxidation and ReductionReduction of Alkynes

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Alkane formation:

Reduction of an Alkyne to an Alkane

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• 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.

Reduction of an Alkyne to a Cis Alkene

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• Reduction of an alkyne to a cis alkene is a stereoselective reaction, because only one stereoisomer is formed.

Reduction of an Alkyne to a Cis Alkene

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• dissolving metal reduction (such as Na in NH3), forms a trans alkene.

adding electrons one at a time.

Reduction of an Alkyne to a Trans Alkene

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Reduction of an Alkyne to a Trans Alkene: Mechanism

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Summary of Alkyne Reductions

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• Alkyl halides can be reduced to alkanes with LiAlH4.

• Epoxide rings can be opened with LiAlH4 to form alcohols.

Reduction of Polar C—X Bonds

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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 LiAlH4) occurs at the less substituted carbon atom.

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Summary of Reductions

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Oxidizing Reactions

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• There are two main categories of oxidizing agents:1. Reagents that contain an oxygen-oxygen bond

2. Reagents that contain metal-oxygen bonds

• 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.

Oxidizing Agents (that deliver oxygen atom or take hydrogen atom)

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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.

Oxidizing Agents

• KMnO4 (potassium permanganate), MnO2.

• OsO4 (osmium tetroxide) and Ag2O [silver(I) oxide].

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• Alcohols are oxidized to a variety of carbonyl compounds.

Oxidation of Alcohols

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• 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.

Oxidation of Alcohols: mostly chromium reagents

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• Any of the Cr6+ oxidants effectively oxidize 2° alcohols to ketones.

Oxidation of 2° Alcohols

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• 1° Alcohols are oxidized to either aldehydes or carboxylic acids, depending on the reagent.


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• Epoxidation is the addition of a single oxygen atom to an alkene to form an epoxide.

• Epoxidation is typically carried out with a peroxyacid.

Epoxidation

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• a cis alkene gives an epoxide with cis substituents. A trans alkene gives an epoxide with trans substituents.

Epoxidation

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

• more substituted, electron rich alkenes react faster.

Epoxidation

C CHH

CH3 CH3

C CCH3H

CH3 H

O

C C

H HCH3 CH3

O

C C

H CH3

CH3 H

mCPBA

mCPBA H

O

C C

CH3 HCH3H

O

C C

CH3 CH3H

+

+

cis-2-Butene

trans-2-Butene

Attack from below

Attack from belowAttack from above

Attack from above

*

****

***

Enantiomers

Achiral meso compound

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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:

The Synthesis of Disparlure

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Oxidation and ReductionThe Synthesis of Disparlure

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• Dihydroxylation is the addition of two hydroxy groups to a double bond, forming a 1,2-diol or glycol.

Dihydroxylation

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• Anti dihydroxylation : two steps—epoxidation, followed by ring

opening with ¯OH or H3O+.

Dihydroxylation

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• Syn hydroxylation : with either KMnO4 or OsO4.

Dihydroxylation

Insoluble in organic solvent

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• Each reagent adds two oxygen atoms in a syn fashion.

Dihydroxylation

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• Dihydroxylation with a catalytic amount of OsO4 : the oxidant N-methylmorpholine N-oxide (NMO)

Dihydroxylation : catalytic version

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• Oxidative cleavage forms two carbonyl compounds. Cleavage with ozone (O3) is called ozonolysis.

Oxidative Cleavage of Alkenes

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Oxidative Cleavage of Alkenes : mechanism

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• 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.

Oxidative Cleavage of Alkenes

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• Alkynes undergo oxidative cleavage of the and both bonds.

• Internal alkynes are oxidized to carboxylic acids (RCOOH).

Oxidative Cleavage of Alkynes

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• 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.

Green Chemistry

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• With Amberlyst A-26 resin-HCrO4¯, 1° alcohols are oxidized to aldehydes and 2° alcohols are oxidized to ketones.

Green Chemistry

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• 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.

The Oxidation of Ethanol

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12.40, 12.42, 12.43, 12.50, 12.51, 12.52, 12.58, 12.60,

12.61

Homework

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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?

Documents

1 Chapter 12 Oxidation and Reduction. 2 Oxidation Oxidation results in an increase in the number of C—Z bonds; or Oxidation results in a decrease in the