Organic Chemistry 6 th Edition Paula Yurkanis Bruice Chapter 20 More About Oxidation –Reduction Reactions

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• Oxidation is always coupled with reduction.

• Loss of electrons is oxidation.

• Gain of electrons is reduction.

• The oxidation state of a carbon atom equals the total number of its C —O, C—N, and C—X bonds.

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• Reduction at carbon increases the number of C—H bonds or decreases the number of C —O, C—N, or C—X bonds.

• Oxidation at carbon decreases the number of C—H bonds or increases the number of C —O, C—N, or C—X bonds.

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Hydrogen, sodium borohydride, and hydrazine are the reducing agents: © 2011 Pearson Education, Inc.

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Bromine and chromic acid are the oxidizing agents: © 2011 Pearson Education, Inc.

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H 2 as a Reducing Agent

H 2 , Pd on charcoal Na or Li metal in liq NH 3 NaBH 4 or LiAlH 4

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Reduction by Catalytic Hydrogenation Addition of two hydrogen atoms: © 2011 Pearson Education, Inc.

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Only the alkene substituted to benzene is reduced: Reduction of carbon –nitrogen double and triple bonds: 8 © 2011 Pearson Education, Inc.

Reduction of Ketones and Aldehydes © 2011 Pearson Education, Inc.

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Rosenmund Reduction © 2011 Pearson Education, Inc.

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Dissolving-Metal Reduction

Addition of an electron, a proton, an electron, and a proton:

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Reduction by Addition of a Hydride Ion and a Proton © 2011 Pearson Education, Inc.

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Aldehydes, ketones, and acyl halides can be reduced to alcohols by sodium borohydrides: © 2011 Pearson Education, Inc.

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LiAlH 4 is a stronger reducing agent than NaBH 4 LiAlH 4 is used to reduce compounds that are unreactive toward NaBH 4 © 2011 Pearson Education, Inc.

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DIBALH allows the addition of one equivalent of hydride to an ester: Replacing some of the hydrogens of LiAlH 4 with –OR groups decreases the reactivity of the metal hydride: 16 © 2011 Pearson Education, Inc.

Formation of Amines by Reduction © 2011 Pearson Education, Inc.

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NaBH 4 can be used to selectively reduce an aldehyde or a keto group in a compound: Alkenes and alkynes do not possess a partial positive charge: 18 © 2011 Pearson Education, Inc.

Sodium borohydride can be used as a chemoselective reducing agent: © 2011 Pearson Education, Inc.

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Oxidation of Alcohols © 2011 Pearson Education, Inc.

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Oxidation of a Primary Alcohol © 2011 Pearson Education, Inc.

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Mechanism of Alcohol Oxidation by the Swern Oxidation © 2011 Pearson Education, Inc.

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Oxidation of Aldehydes and Ketones © 2011 Pearson Education, Inc.

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The Tollens reagent oxidizes only aldehydes: © 2011 Pearson Education, Inc.

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Both aldehydes and ketones can be oxidized by peroxyacid: the Baeyer –Villiger oxidation © 2011 Pearson Education, Inc.

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Mechanism of the Baeyer –Villiger Oxidation © 2011 Pearson Education, Inc.

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Predicting Baeyer –Villiger reaction products: © 2011 Pearson Education, Inc.

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Controlling Stereochemistry in Synthesis An enantioselective reaction forms more of one enantiomer than of another: © 2011 Pearson Education, Inc.

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Hydroxylation of Alkenes © 2011 Pearson Education, Inc.

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Mechanism for

cis-

Glycol Formation Higher yields of the diol are obtained with osmium tetroxide than with permaganate 32 © 2011 Pearson Education, Inc.

Oxidative Cleavage of 1,2-Diols © 2011 Pearson Education, Inc.

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Summary of Alkene Hydroxylation Reactions © 2011 Pearson Education, Inc.

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Permaganate Cleavage of Alkenes

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Examples of permaganate-mediated alkene cleavage reactions: © 2011 Pearson Education, Inc.

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Oxidative Cleavage of Alkenes by Ozonolysis Examples: © 2011 Pearson Education, Inc.

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Structure of Ozone © 2011 Pearson Education, Inc.

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The alkene and ozone undergo a concerted six-electron cycloaddition Mechanism of ozonide formation: The molozonide is unstable because it has two O —O bonds, but the ozonide is stable 39 © 2011 Pearson Education, Inc.

Ozonides can be cleaved to carbonyl compounds: © 2011 Pearson Education, Inc.

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Ozonolysis Mechanism

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Examples of the Oxidative Cleavage of Alkenes by Ozonolysis © 2011 Pearson Education, Inc.

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The benzene ring is not oxidized under mild ozonolysis conditions: 43 © 2011 Pearson Education, Inc.

Oxidative Cleavage of Alkynes The same reagents that oxidize alkenes also oxidize alkynes: © 2011 Pearson Education, Inc.

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Designing a Synthesis by Functional Group Interconversion Conversion of an aldehyde to other functional groups: © 2011 Pearson Education, Inc.

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Conversion of a Ketone into an Ester or an Alcohol © 2011 Pearson Education, Inc.

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