Download Chapter 12: Oxidations In order to discuss the oxidation

Chapter 12: Oxidations
In order to discuss the oxidation and reduction
reactions of organic molecules we must first learn
how to recognize the oxidation state of any
particular carbon.
How do we assign the oxidation state of a carbon atom?
1) Examine the groups attached to carbon
2) For each atom attached to carbon that is less
electronegative than carbon we give the carbon a -1
oxidation number
3) For each atom attached to carbon that is more
electronegative than carbon we give the carbon a +1
oxidation number
4) Multiple bonds to an atom are counted as thought the
carbon were bonded to multiple atoms of that type
Examine molecules containing a single carbon.
4 C-H bonds. H is less electronegative than C
oxidation number: -4
3 C-H bonds, 1 C-O bond. O is more electronegative than C
oxidation number: -2
2 C-H bonds, 2 C-O bonds
oxidation number: 0
1 C-H bond, 3 C-O bonds
oxidation number: +2
4 C-O bonds
oxidation number: +4
4 C-O bonds
oxidation number: +4
1
carbonic acid
carbon dioxide
In the equilibrium above, note that no change in oxidation number
has occurred. In general hydration and dehydration reactions
do not lead to overall changes in oxidation state.
Ethane derived molecules
Each carbon has three C-H bonds and one C-C bond
Each carbon has an oxidation number of -3
The oxidation number of both carbons together is -6
The alcohol carbon has 2 C-H bonds, 1 C-O bond, and
1 C-C bond. The oxidation number of the alcohol carbon is – 1
The oxidation number of both carbons together is -4
The carbon attached to chlorine has 2 C-H bonds, 1 C-Cl bond, and
1 C-C bond. The oxidation number of the carbon attached to chlorine is – 1
The oxidation number of both carbons together is -4
Each carbon has two C-H bonds and two C-C bonds
Each carbon has an oxidation number of -2
The oxidation number of both carbons together is -4
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In the equilibrium above no overall change in oxidation state is occurring in either
direction, however the oxidation numbers of the individual carbons change. An alkene
can be seen as having the same overall oxidation state as an alcohol. Many other
functional groups have the same oxidation state as an alcohol. Any time a carbon
has a single bond to an atom more electronegative than itself, we will consider
that carbon to have the oxidation state of an alcohol.
Each carbon possesses a single C-H bond and three C-C bonds
oxidation number: -1
total oxidation number of both carbons together: -2
The carbon bearing a single C-O bond has an oxidation number = 0
The other carbon has an oxidation number = -2
total oxidation number of both carbons together: -2
The carbon bearing a double C-O bond has an oxidation number = +1
The other carbon has an oxidation number = -3
total oxidation number of both carbons together: -2
The ketone carbon has an oxidation number of +2. This oxidation number
is different from the aldehyde carbon above (+1), however for our purposes
we will consider ketones and aldehydes to have essentially the same
oxidation state
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In the equilibrium above the oxidation numbers of the two carbons change
but the overall oxidation state remains the same. In general a carbon that has
two bonds to atoms more electronegative than itself will be in the aldehyde/ketone
oxidation state. An alkene that bears a heteroatom will also be in the aldehyde/
ketone oxidation state.
Increasing oxidation state
As degrees of unsaturation are introduced into organic molecules
the overall oxidation state of the molecules is increased.
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acetic acid - a carboxylic acid
Any carbon with three bonds to atoms more electronegative
than itself can be considered to be at the carboxylic acid
oxidation state. Alkenes that are bound geminally to heteroatoms
are also at the carboxylic acid oxidation state.
Any carbon with 4 bonds to atoms more electronegative
than itself is considered to have the same oxidation state
as carbon dioxide.
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Manipulation of the oxidation states of carbon is a very important
aspect of synthetic organic chemistry.
Perhaps the most commonly oxidized functional group in organic chemistry is
the alcohol functionality
In general secondary and primary alcohols are oxidized to ketones
and aldehydes respectively.
Both of these reaction involve the formal loss of H 2. We have seen that
increasing the degrees of unsaturation lead to higher oxidation states
by definition degrees of unsaturation define how many equivalents of H 2
a molecule is missing relative to an alkane. Adding H 2 to an organic molecule
constitutes a reduction in oxidation state
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Chromium (VI) oxidations of alcohols
Perhaps the oldest and most widely known chromium (VI) oxidant is
chromic acid, historically referred to as the Jones reagent
Limitations of the Jones reagent
In general primary alcohols are oxidized to carboxylic acids. This can be an
efficient transformation.
Other chromium (VI) reagents that offer convenient handling and good
selectivity have been developed. In general these reagents will oxidize
primary alcohols to aldehydes.
Collins reagent
Pyridinium
chlorochromate
(PCC)
Pyridinium
dichromate
(PDC)
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Lactols are oxidized to lactones (cyclic esters)
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In DMF, PDC oxidizes primary alcohols to
carboxylic acids
Manganese dioxide can be a useful oxidizing reagent, however its utility
Is generally limited to benzylic and allylic alcohols
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Dimethylsulfoxide-mediated oxidations
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TEMPO is an efficient catalyst for the oxidation of alcohols in the
presence of stoichiometric oxidants such as N-chlorosuccinimide,
sodium hypochlorite (bleach) as well as others. These procedures
Can be selective for primary alcohols in the presence of secondary
alcohols
Periodinane can be an exceptionally mild oxidant
Mechanism:
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Sodium chlorite is a mild oxidant for the transformation of aldehydes
to carboxylic acids.
Oxidation of Alkenes
Oxidation of alkenes usually results in the loss of the pi bond And formation of
two new sigma bonds between the carbons of the alkene and atoms more
electronegative than carbon
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Major oxidation reactions of alkenes
Epoxidations: Peroxy Acids
The related peroxyimidic acids probably operate through an analogous mechanism
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Commonly available peroxyacids
m-chloroperoxybenzoic acid (mCPBA) -stable solid
peroxyacetic acid – supplied as a 37% w/w solution
In acetc acid
peroxytrifluoroacetic acid – used for electron deficient
(unreactive alkenes)
Epoxidation is generally a syn-stereospecific processboth new carbon oxygen bonds are created from the same side
of the alkene
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The alcohol functional group can “direct” peroxy acid epoxidations
These reactions are particularly useful with cyclic alcohols
Proposed transition state
Hydrogen bond
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Other hydrogen bonding substituents
have been used.
The substituent does not necessarily
need to be in the allylic position.
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