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OZONE. OZONE. electric discharge. or. cosmic rays. . . . . :. . . . -. +. +. :. :. . . -. EQUIVALENT RESONANCE STRUCTURES. OZONOLYSIS. Ozonolysis. FORMATION OF AN OZONIDE. unstable. HYDROLYSIS OF THE OZONIDE (WORKUP). aldehydes or ketones.

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OZONE

electric discharge

or

cosmic rays

..

..

..

..

:

..

..

..

-

+

+

:

:

..

..

-

EQUIVALENT RESONANCE STRUCTURES



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Ozonolysis

FORMATION OF AN OZONIDE

unstable

HYDROLYSIS OF THE OZONIDE (WORKUP)

aldehydes or ketones


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FORMATION OF AN OZONIDE - MECHANISM

-

..

..

UNSTABLE

:

:

O

O

..

..

C

H

C

l

2

2

+

:

:

O

O

..

o

0

C

:

O

O

..

..

ozone

Addition is

concerted

molozonide

carbonyl

oxide

..

..

..

+

+

O

O

..

..

O

C

C

..

:

:

O

O

..

..

-

-

:

O

:

:

O

:

..

C

O

..

:

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

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 (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

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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REDUCTIVE

WORKUP

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” =

before spectroscopy.

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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oxidative

workup

oxidized

during

work-up

WHAT WAS THE ORIGINAL STRUCTURE ?



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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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O3

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-CH=CH2

incompletely burned

hydrocarbons

Temperature Inversion:

Air above land is cooler than air above.


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O3

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

more vigorous

ozonolysis :

cinnamaldehyde

H2O2

O3 / 20o/ CH2Cl2

oxidizes

aldehyde

2 hours

1)

O3 / CH2Cl2 /0o

oxalic

acid

2)

H3O+

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)


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