Download 20_Lecture - Ventura College

Chapter 20
Organic Chemistry
6th Edition
Paula Yurkanis Bruice
More About
Oxidation–Reduction
Reactions
1
© 2011 Pearson Education, Inc.
• 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.
2
© 2011 Pearson Education, Inc.
• 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.
3
© 2011 Pearson Education, Inc.
Hydrogen, sodium borohydride, and hydrazine are the
reducing agents:
4
© 2011 Pearson Education, Inc.
Bromine and chromic acid are the oxidizing agents:
5
© 2011 Pearson Education, Inc.
H2 as a Reducing Agent
H2, Pd on charcoal
Na or Li metal in
liq NH3
NaBH4 or LiAlH4
6
© 2011 Pearson Education, Inc.
Reduction by Catalytic Hydrogenation
Addition of two hydrogen atoms:
7
© 2011 Pearson Education, Inc.
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
9
© 2011 Pearson Education, Inc.
Rosenmund Reduction
10
© 2011 Pearson Education, Inc.
11
© 2011 Pearson Education, Inc.
Dissolving-Metal Reduction
Addition of an electron, a proton, an electron, and a proton:
12
© 2011 Pearson Education, Inc.
Reduction by Addition of
a Hydride Ion and a Proton
13
© 2011 Pearson Education, Inc.
Aldehydes, ketones, and acyl halides can be reduced to
alcohols by sodium borohydrides:
14
© 2011 Pearson Education, Inc.
LiAlH4 is a stronger reducing agent than NaBH4
LiAlH4 is used to reduce compounds that are
unreactive toward NaBH4
15
© 2011 Pearson Education, Inc.
DIBALH allows the addition of one equivalent of hydride
to an ester:
Replacing some of the hydrogens of LiAlH4 with –OR
groups decreases the reactivity of the metal hydride:
16
© 2011 Pearson Education, Inc.
Formation of Amines by Reduction
17
© 2011 Pearson Education, Inc.
NaBH4 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:
19
© 2011 Pearson Education, Inc.
Oxidation of Alcohols
20
© 2011 Pearson Education, Inc.
Oxidation of a Primary Alcohol
21
© 2011 Pearson Education, Inc.
Mechanism of Alcohol Oxidation
by the Swern Oxidation
22
© 2011 Pearson Education, Inc.
Oxidation of Aldehydes and Ketones
23
© 2011 Pearson Education, Inc.
The Tollens reagent oxidizes only aldehydes:
24
© 2011 Pearson Education, Inc.
Both aldehydes and ketones can be oxidized by
peroxyacid: the Baeyer–Villiger oxidation
25
© 2011 Pearson Education, Inc.
26
© 2011 Pearson Education, Inc.
Mechanism of the
Baeyer–Villiger Oxidation
27
© 2011 Pearson Education, Inc.
Predicting Baeyer–Villiger reaction products:
28
© 2011 Pearson Education, Inc.
Controlling Stereochemistry
in Synthesis
An enantioselective reaction forms more of one
enantiomer than of another:
29
© 2011 Pearson Education, Inc.
30
© 2011 Pearson Education, Inc.
Hydroxylation of Alkenes
31
© 2011 Pearson Education, Inc.
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
33
© 2011 Pearson Education, Inc.
Summary of Alkene Hydroxylation Reactions
34
© 2011 Pearson Education, Inc.
Permaganate Cleavage of Alkenes
35
© 2011 Pearson Education, Inc.
Examples of permaganate-mediated alkene cleavage
reactions:
36
© 2011 Pearson Education, Inc.
Oxidative Cleavage of Alkenes by
Ozonolysis
Examples:
37
© 2011 Pearson Education, Inc.
Structure of Ozone
38
© 2011 Pearson Education, Inc.
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:
40
© 2011 Pearson Education, Inc.
Ozonolysis Mechanism
41
© 2011 Pearson Education, Inc.
Examples of the Oxidative Cleavage of
Alkenes by Ozonolysis
42
© 2011 Pearson Education, Inc.
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:
44
© 2011 Pearson Education, Inc.
Designing a Synthesis by Functional
Group Interconversion
Conversion of an aldehyde to other functional groups:
45
© 2011 Pearson Education, Inc.
Conversion of a Ketone into
an Ester or an Alcohol
46
© 2011 Pearson Education, Inc.