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OXIDATION
Oxidation refers to an increase in bonds to oxygen and/or decrease in bonds to hydrogen
• The carbon must be bonded to at least one H atom for oxidation to occur
• During an oxidation, a C-H bond is broken and the electrons from this bond are used to
form a new C-O bond
• Carbon can be converted to progressively higher oxidation states as illustrated below.
H
OH
C
H
[O]
[O]
C
H
alkane
O
O
[O]
C
H
H
C
H
if H = R, ketone( from 20 alcohol)
Lowest Oxidation
State
•
carboxylic acid
aldehyde (from1 0 alcohol)
alcohol
OH
Highest Oxidation
State
Different oxidizing agents can be used to perform specific types of oxidations
OXIDATIVE CLEAVAGE
Oxidative Cleavage refers to an increase in bonds to oxygen accompanied by breaking of a C-C
bond
• Oxidative cleavage generally applies to alkenes
• The carbons of the alkene do not need to be bonded to any hydrogen atoms for
oxidative cleavage to occur.
• During an oxidative cleavage reaction, a C-C bond is broken and the electrons from this
bond are used to form new C-O bonds. Generally both carbon atoms of the original
alkene undergo the oxidative process.
• The carbon atoms are progressively oxidized to higher oxidation states as illustrated
below.
C
alkene
C
OH
OH
C
C
C
O
+
O C
carbonyls
(aldehydes/ketones)
cis-diol
HO
OH
C O
+
O C
carboxylic acids
•
Different oxidizing agents can be used to perform specific types of oxidative cleavage
reaction
All oxidations and oxidative cleavage reactions occur in stages or steps by increasing the
oxidation state one C-O bond at a time. Some milder oxidizing agents “stop” at a specific
oxidation state and other more powerful oxidizing agents continue to oxidize to higher
oxidation states.
OXIDATION OF BENZYLIC AND ALLYLIC CARBONS
Alkyl carbons of alkanes or alkane substitutents are generally inert and do not undergo oxidation.
Allylic and benzylic carbons are more susceptible to oxidation than alkyl carbons.
•
Benzylic carbons (w/at least one H) can be oxidized to carboxylic acids with KMnO4.
Some of the oxidative steps are straight oxidations, and some are oxidative cleavage
reactions, depending on the substitution of the benzylic carbon.
For primary benzylic carbons, all of the steps to convert the benzylic carbon to the carboxylic
acid are oxidations (i.e., involve breaking C-H bond)
OH
CH3
C
KMnO4
O
primary benzylic carbon
H
H
H
C
CH3
OH
C
OH
C
O
O
These are intermediate oxidation states involved in the conversion of
the benzylic carbon to a carboxylic acid using KMnO4
For secondary benzylic carbons, the first two steps to convert the benzylic carbon to the
carboxylic acid are oxidations (i.e., involve breaking C-H bond) but the last step (ketone to
carboxylic acid) is an oxidative cleavage (i.e., involves breaking a C-C bond)
H
H
OH
C
CH3
secondary benzylic
carbon
H
H
oxidation
C
KMnO4
O
oxidation
HO
CH3
H
C
C
CH3
C
CH3
O
ketone
oxidative
cleavage
OH
C
O
carboxylic acid
These are intermediate oxidation states involved in the conversion of
the benzylic carbon to a carboxylic acid using KMnO4
Tertiary benzylic carbons, that have no H, do not undergo oxidation or oxidative cleavage with
KMnO4.
OXIDATION OF ALCOHOLS (These are all oxidations, not oxidative cleavage reactions)
A. Oxidation of primary alcohols
Primary alcohols can be oxidized to aldehydes or carboxylic acids
H
H
HO
[O]
H
C
[O]
C
OH
C
Primary alcohol
O
C
C
C
Aldehyde
Carboxylic Acid
O
1. PCC (pyridinium chlorochromate) or PDC (pyridiunium dichromate) in CH2Cl2
oxidizes primary alcohols to aldehydes
2. Jones reagent (CrO3, H2SO4) or sodium dichromate (Na2Cr2O7)
oxidizes primary alcohols to carboxylic acids (through the aldehyde and aldehyde
hydrate)
3. KMnO4, NaOH and heat oxidizes primary alcohols to carboxylic acids (through the
aldehyde and aldehyde hydrate)
B. Oxidation of secondary alcohols
Secondary alcohols can be oxidized to ketones
OH
C
[O]
C
O
C
C
H
C
Secondary Alcohol
C
Ketone
1. PCC (pyridinium chlorochromate) or PDC (pyridiunium dichromate) in CH2Cl2
oxidizes secondary alcohols to ketones
2. Jones reagent (CrO3, H2SO4) oxidizes secondary alcohols to ketones
C. Tertiary alcohols do not undergo oxidation
OXIDATION AND OXIDATIVE CLEAVAGE OF ALKENES
A. Alkenes can be converted to cis-diols via an oxidative cleavage. The C-C pi bond is broken
and is replaced by two C-O bonds on each carbon atom.
C
C
OH
OH
C
C
cis-diol
alkene
1. Cold, dilute, permanganate (KMnO4) can be used to convert alkenes to cis-diols
2. Osmium tetroxide (toxic, expensive, used in catalytic amount) (OsO4) can be used to
convert alkenes to cis diols
B. Alkenes can be converted to carbonyl compounds (aldehydes and ketones) through a cis-diol
intermediate via an oxidative cleavage reaction.
1. Alkenes can be converted to aldehydes and ketones by ozonolysis via a diol-like
intermediate(1. O3 2. Zn, HCl)
C
C
OH
OH
C
C
C
+
O
C
carbonyls
(aldehydes/ketones)
cis-diol
alkene
O
2. Cis diols can be converted to carbonyls (aldehydes and ketones) using periodic acid
(HIO4, H2O, THF))
OH
OH
C
C
C O
+
O
C
carbonyls
(aldehydes/ketones)
cis-diol
C. Alkenes can be converted to carboxylic acids through cis-diol and carbonyl intermediates via
an oxidative cleavage reaction.
OH
C
alkene
C
C
OH
C
cis-diol
HO
C
O
+
O C
carbonyls
(aldehydes/ketones)
OH
C
O
+
O C
carboxylic acids
1. Alkenes can be converted to carboxylic acids with concentrated KMnO4, with heat and
acidic conditions. A cis diol and carbonyls are intermediates in this reaction.
(Also responsible for epoxidation of alkenes)