Organic Chemistry

862 CHAPTER 20 More About Oxidation–Reduction Reactions

molozonide. (The name “molozonide”indicates that one mole of ozone has added to

the alkene.) The molozonide is unstable because it has two bonds; it immedi-

ately rearranges to a more stable ozonide.

Ozonides are explosive, so they are seldom isolated. In solution, they are easily

cleaved to carbonyl compounds. If the ozonide is cleaved in the presence of a reducing

agent such as zinc or dimethyl sulfide, the products will be ketones and/or aldehydes.

(The product will be a ketone if the carbon of the alkene is bonded to two carbon-

containing substituents; the product will be an aldehyde if at least one of the sub-

stituents bonded to the carbon is a hydrogen.) The reducing agent prevents

aldehydes from being oxidized to carboxylic acids. Cleaving the ozonide in the pres-

ence of zinc or dimethyl sulfide is referred to as “working up the ozonide under reduc-

ing conditions.”

If the ozonide is cleaved in the presence of an oxidizing agent such as hydrogen perox-

ide the products will be ketones and/or carboxylic acids. Carboxylic acids are

formed instead of aldehydes because any aldehyde that is initially formed will be im-

mediately oxidized to a carboxylic acid by hydrogen peroxide. Cleavage in the pres-

ence of is referred to as “working up the ozonide under oxidizing conditions.”

The following reactions are examples of the oxidative cleavage of alkenes by

ozonolysis:

O

O

H

CH 3 CH 2 CH CHCH 2 CH 3

1. O3, – 78 °C


2. Zn, H 2 O

CH 3 CH 2 H

1. O3, – 78 °C


2. (CH 3 ) 2 S

CH 3

2 C

O

CH 3 CH 2 CH CCH 2 CH 3 +

CH 3

CH 3 CH 2

HO

C O

1. O3, – 78 °C


2. H 2 O 2

CH 2 CH 3

CH 3

O C

H 2 O 2

(H 2 O 2 ),

C

R

R

R

R

C

C

R

R

OH

R

H

R

C

H

R

C

ozonide

a ketone an aldehyde

OO

Zn, H 2 O

or

(CH 3 ) 2 S

H 2 O 2

+

a ketone a carboxylic acid

OOC+

O

O O

oxidizing

conditions

reducing

conditions

sp^2

sp^2

O¬O

To determine the product of ozonolysis,

replace with

If work-up is done under oxidizing

conditions, convert any aldehyde

products to carboxylic acids.

C“C C“OO“C.

R

C

R

O H

C

O

R

O

O

−O O

−

+

+

molozonide ozonide

mechanism for ozonide formation

C

R

C

H R

R R

C

R RR

C

H

C

R

RHC

O

O O

O

O O

3-D Molecules:

Ozone;

Molozonide;

Ozonide

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