Case Western Reserve University Chapter 20 More About Oxidation–Reduction Reactions Organic Chemistry 4th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

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

Hydrogen, sodium borohydride, and hydrazine are the reducing agents

Bromine and chromic acid are the oxidizing agents

H2 as a Reducing Agent

Reduction by Catalytic Hydrogenation

Dissolving-Metal Reduction

Reduction by Addition of a Hydride Ion and a Proton

LiAlH4 is a stronger reducing agent than NaBH4 LiAlH4 is used to reduce compounds that are nonreactive toward NaBH4

DIBAL allows the addition of one equivalent of hydride to an ester Replacing some of hydrogens of LiAlH4 with OR groups decreases the reactivity of the metal hydride

Formation of Amines by Reduction

NaBH4 can be used to selectively reduce an aldehyde or a keto group in a compound

Oxidation of Alcohols

Oxidation of a Primary Alcohol

Mechanism of Alcohol Oxidation by Chromic Acid

The oxidation of a primary alcohol can be stopped at the aldehyde if pyridinium chlorochromate (PCC) is used as the oxidizing agent

The Swern Oxidation

Mechanism of the Swern Oxidation

Oxidation of Aldehydes and Ketones

The Tollens Reagent Oxidizes Only Aldehydes

Both aldehydes and ketones can be oxidized by peroxyacid: The Baeyer–Villiger oxidation

Baeyer–Villiger Oxidation Mechanism of the Baeyer–Villiger Oxidation

Therefore, the product of the Baeyer–Villiger oxidation of cyclohexyl methyl ketone will be cyclohexyl acetate, because a secondary alkyl group is more likely to migrate than a methyl group

Oxidation of Alkenes with Peroxyacids

Mechanism of Epoxidation of an Alkene

The addition of oxygen to an alkene is a stereospecific reaction

Hydroxylation of Alkenes

Mechanism for cis-Glycol Formation

Higher yields of the diol are obtained with osmium tetroxide

Oxidative Cleavage of 1,2-Diols

Oxidative Cleavage of Alkenes by Ozonolysis

The alkene and ozone undergo a concerted cycloaddition Mechanism of ozonide formation The molozonide is unstable because it has two O–O bonds Ozonide is stable

Ozonides can be cleaved to carbonyl compounds

Examples of the Oxidative Cleavage of Alkenes by Ozonolysis

A peroxyacid, OsO4, and (cold basic) KMnO4 break only the p bond of the alkene Ozone and acidic KMnO4 break both the p bond and the s bond

Table 20.1 Summary of the Methods Used to Oxidize an Alkene

Oxidative Cleavage of Alkynes

Conversion of an Aldehyde into Other Functional Groups

Conversion of a Ketone into an Ester or an Alcohol

Biological Oxidation–Reduction Reactions

NAD+ oxidizes ethanol by accepting a hydride ion

NADH reduces a carbonyl compound by donating a hydride ion