OZONE

1
OZONE
2
OZONE
electric discharge
or
cosmic rays
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..
..
..

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..
-




..
..
-
EQUIVALENT RESONANCE STRUCTURES
3
OZONOLYSIS
4
Ozonolysis
FORMATION OF AN OZONIDE
unstable
HYDROLYSIS OF THE OZONIDE (WORKUP)
aldehydes or ketones
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FORMATION OF AN OZONIDE - MECHANISM
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-
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UNSTABLE


O
O
..
..
C
H
C
l
2
2



O
O
..
o
0
C

O
O
..
..
ozone
Addition is concerted
molozonide
..
carbonyl oxide
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..


O
O
..
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O
C
C
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O


O
..
..
-
-

O


O

..
O
C
..

O
C
..
ketone or aldehyde
ozonide
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OVERALL RESULT OF THE REACTION WITH OZONE
The double bond is replaced with the ozonide ring.
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OZONIDE AND MOLOZONIDE STRUCTURES
molozonide
ozonide
forms initially
forms after rearrangement
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SOME EVIDENCE FOR THE MECHANISM
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EVIDENCE FOR THE MECHANISM
When ozonolysis is performed in an aliphatic
hydrocarbon solvent (hexane) dimers of the
carbonyl oxide intermediate sometimes form.
insoluble
hexane
carbonyl oxide dimer
This dimerization proves the existence of the
carbonyl oxide intermediate.
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YET MORE MECHANISTIC EVIDENCE
If a foreign ketone is placed in the solution
(e.g., benzophenone) it becomes incorporated into
a portion of the ozonides formed.
EXPECTED

benzophenone
traps the carbonyl oxide
Can you explain this with a mechanism ? HINT The
benzophenone reacts with the carbonyl oxide
intermediate.
TRY THIS ON YOUR OWN
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MORE EVIDENCE
When an unsymmetrical alkene undergoes
ozonolysis, it is not uncommon for three
different ozonides to form.
EXPECTED
The alkene must break in two and recombine.
Can you draw mechanisms that explain the
formation all three ozonides ?
cis trans
TRY THIS ON YOUR OWN
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WORKUP PROCEDURES FOR OZONOLYSIS
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WORKUP PROCEDURES FOR OZONOLYSIS
After the ozonide is formed it is hydrolyzed
(work-up).
Two types of work-up (decomposition of the
ozonide) are possible
Oxidative Workup
Add aqueous acid (H3O)
H2SO4 H2O
Reductive Workup
A) Zn, acetic acid or Zn, H2O
Two methods
CH3COOH
B) Pd/H2 followed by aqueous acid
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OXIDATIVE WORKUP
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OXIDATIVE WORKUP (acid)
Just add H3O ( H20 and acid )
O
O
H3O
O
O
O
Aldehydes are oxidized to carboxylic
acids. Formaldehyde is oxidized to carbon
dioxide, which
is lost as a gas.
These oxidations occur because H2O2 is a
hydrolysis product. The general hydrolysis
mechanism follows.
You do not have to known this mechanism.
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OXIDATIVE WORKUP
(acid water)
continued ...
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HYDROGEN PEROXIDE IS A PRODUCT OF THE
OXIDATIVE WORKUP
Aldehydes are easily oxidized by the H2O2 that is
produced.
oxidizing agent
Ketones are not oxidized by hydrogen peroxide.
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OXIDATIVE METHODS DESTROY FORMALDEHYDE
If formaldehyde is produced, it is oxidized to
CO2 and H2O.
gas
two moles
Carbonic acid is unstable and decomposes to CO2
and H2O.
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REDUCTIVE WORKUP
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REDUCTIVE WORKUP
There are two methods of reductive workup.
Add Zn and H2O (or acetic acid)
METHOD A
Reduce the ozonide with Pd / H2 , and
then add acid ( H3O ).
METHOD B
With either method, aldehydes survive intact and
are not oxidized.
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REDUCTIVE WORKUP - METHOD A
With the REDUCTIVE WORKUPS, no H2O2 is produced.
The zinc scavenges the peroxide before it can
act.
Notice that the O-O bond is broken so that no
H2O2 is formed during the subsequent hydrolysis.
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REDUCTIVE WORKUP - METHOD B
With the REDUCTIVE WORKUPS, no H2O2 is produced.
The hydrogenation step cleaves the O-O bond.
Since the O-O bond is broken, no H2O2 is
formed during the hydrolysis.
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EXAMPLES
aldehyde survives
O3
Zn / H2O
OR
O3
1) Pd/H2
1) O3
2) H3O
2) H3O
OXIDATIVE WORKUP
formaldehyde CO2
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USING OZONOLYSIS FOR STRUCTURE PROOF
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AT ONE TIME OZONOLYSIS WAS WIDELY USED FOR
STRUCTURE PROOF BY DEGRADATION
Simpler Known Compounds
Broken apart ( or degraded ) to simpler pieces
that are easier to identify.
Unknown compound
The original structure can be deduced by
reassembling the pieces.
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PROBLEM TO SOLVE
6-ketoheptanoic acid
or 6-oxoheptanoic acid
1) O3 / CH2Cl2
C7H12
2) H3O
Pd / H2
C7H14
answer
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WHAT WAS THE ORIGINAL STRUCTURE ?
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OXIDATION OF ACETYLENES
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ACETYLENES
1) O3, CH2Cl2 2) H3O
KMnO4 or
Oxidation of acetylenes, whether by KMnO4 or
ozone, normally yields carboxylic acids.
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OZONE AND SMOG
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FORMATION OF SMOG - OZONE IS A COMPONENT
WARMER AIR
temperature inversion traps pollutants
COOLER AIR
SO2
CO2
H2O
reacts with unburned hydrocarbons
NO2
NO
R-CHCH2
incompletely burned hydrocarbons
Temperature Inversion
Air above land is cooler than air above.
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NATURAL SOURCES
temperature inversion traps bioemissions
reacts with terpenes
Terpenes
Spruce, Cedar, Fir or Pine Forest
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OXIDATION OF SIDE CHAINS ON AROMATIC RINGS
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BENZENE RINGS
Under normal conditions of ozonolysis, or
treatment by KMnO4, benzene rings are not
oxidized. They are quite resistant to reaction
due to the presence of aromatic ring resonance.
When using KMnO4 on a benzene ring that has a
side chain, the side chain is oxidized before
the ring is affected.
KMnO4 / 50o C /2 hours
The exception is rings with -OH, -OCH3, -NH2 and
similar O/N groups, which oxidize quite readily.
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BENZENE RINGS
Ozone, will not attack the saturated side chain.
O3 / CH2Cl2 /0o
O3 / CH2Cl2 /20o
long time
However, under more vigorous conditions
the benzene ring can be ozonized.
5

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SELECTIVITY
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SELECTIVITY
more vigorous ozonolysis
cinnamaldehyde
H2O2
O3 / 20o/ CH2Cl2
oxidizes aldehyde
2 hours
O3 / CH2Cl2 /0o
1)
oxalic acid
H3O
2)
cinnamic acid
cleaves benzene ring cleaves double bond oxidizes
aldehydes
benzoic acid
cleaves double bond
oxidizes aldehydes (oxidative work-up)
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MORE SELECTIVITY
RCO3H
epoxidation
1) OsO4
cinnamaldehyde
KMnO4
2) NaHSO3
30 min
H2SO4
H2O
1) O3 / CH2Cl2 / 0o
2) Zn / CH3COOH
aldehyde survives (OsO4 is mild)
aldehydes are oxidized by KMnO4
benzaldehyde
aldehydes survive (reductive work-up)