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Organic Chemistry

8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds

Organic Chemistry8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds

8.8 • Oxidation of Alkenes: Cleavage to Carbonyl Compounds

In all the alkene addition reactions we’ve seen thus far, the carbon–carbon double bond has been converted into a single bond but the carbon skeleton has been unchanged. There are, however, powerful oxidizing reagents that will cleave C═CC═C bonds and produce two carbonyl-containing fragments.

Ozone (O3) is perhaps the most useful double-bond cleavage reagent. Prepared by passing a stream of oxygen through a high-voltage electrical discharge, ozone adds rapidly to a C═CC═C bond at low temperature to give a cyclic intermediate called a molozonide. Once formed, the molozonide spontaneously rearranges to form an ozonide. Although we won’t study the mechanism of this rearrangement in detail, it involves the molozonide coming apart into two fragments that then recombine in a different way.

Oxygen in an electric discharge produce ozone. An alkene reacts with ozone to form a molozonide, which rearranges into an ozonide. Ozonide is reduced with zinc to two carbonyl compounds.

Low-molecular-weight ozonides are explosive and therefore not isolated. Instead, the ozonide is immediately treated with a reducing agent, such as zinc metal in acetic acid, to produce carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the C═CC═C bond is cleaved and an oxygen atom becomes doubly bonded to each of the original alkene carbons. If an alkene with a tetrasubstituted double bond is ozonized, two ketone fragments result; if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result; and so on.

First reaction: Isopropylidenecyclohexane reacts with ozone, zinc, and hydronium to form cyclohexanone and acetone. Second reaction: Methyl 9-octadecenoate reacts with ozone, zinc, and hydronium to form nonanal and methyl 9-oxononanoate.

Several oxidizing reagents other than ozone also cause double-bond cleavage, although such reactions are not often used. For example, potassium permanganate (KMnO4) in neutral or acidic solution cleaves alkenes to give carbonyl-containing products. If hydrogens are present on the double bond, carboxylic acids are produced; if two hydrogens are present on one carbon, CO2 is formed.

A reaction shows 3,7-dimethyl-1-octene reacting with potassium permanganate in the presence of hydronium ion to form 2,6-dimethylheptanoic acid with 45 percent yield and carbon dioxide.

In addition to direct cleavage with ozone or KMnO4, an alkene can also be cleaved in a two-step process by initial hydroxylation to a 1,2-diol, as discussed in the previous section, followed by treatment of the diol with periodic acid, HIO4. If the two −OH groups are in an open chain, two carbonyl compounds result. If the two −OH groups are on a ring, a single, open-chain dicarbonyl compound is formed. As indicated in the following examples, the cleavage reaction takes place through a cyclic periodate intermediate.

Two reactions show the formation of 6-oxoheptanal (86 percent yield) and cyclopentanone (81 percent yield) from the parent 1,2-diol. Cyclic periodate intermediates are formed in both reaction.

Worked Example 8.3

Predicting the Reactant in an Ozonolysis Reaction

What alkene would yield a mixture of cyclopentanone and propanal on treatment with ozone followed by reduction with zinc?

An unknown reactant, depicted by a question mark, reacts with ozone in first step and zinc in acetic acid in second step to form cyclopentanone and propanal.


Reaction of an alkene with ozone, followed by reduction with zinc, cleaves the C=CC=C bond and becomes two C=OC=O bonds. Working backward from the carbonyl-containing products, the alkene precursor can be found by removing the oxygen from each product and joining the two carbon atoms.


A backward reaction shows a cyclopentane double bonded to a 3-carbon chain forming from cyclopentanone and propanal. The oxygen atoms of cyclopentanone and propanal are circled.
Problem 8-15
What products would you expect from reaction of 1-methylcyclohexene with the following reagents?
Aqueous acidic KMnO4
O3, followed by Zn, CH3CO2H
Problem 8-16
Propose structures for alkenes that yield the following products on reaction with ozone followed by treatment with Zn:
(CH3)2C =  O + H2C =  O
2 equiv CH3CH2CH = O
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