Oxidative Cleavage of Alkenes and Alkynes by
Ozonolysis or Potassium Permanganate
In Chapter 10 of Fox and Whitesell (pp 520-524), there is a relatively short discussion of
reactions that cut alkenes into two pieces. The details of the mechanisms for these
processes are outside the scope of 2.222, but you need to be able to use reactions of this
type in “predict the product” questions. This document provides a little more detail to
help you understand these reactions.
The Basic Pattern of Oxidative Cleavage
In all cases the reaction follows a simple pattern: The C=C bond is broken, and
each of the former alkene carbons becomes a carbonyl.
R
R1
2
C
2
R
C
R
4
R
3
R1
C
+
O
R3
C
R4
O
If the alkene is included in a ring in the starting material, the two carbonyl groups
remain attached to one another in the product.
R1
C
C
R
3
R1
C
O
C
O
3
R
+
Acidic Potassium Permanganate – KMnO4/H3O
When KMnO4 reacts with an alkene in cold, alkaline (basic) conditions, it is possible to
obtain cis-diol products just as one would from oxidation using OsO4. However, this
reaction is not easy to control, and yields are typically low because permanganate is such
a strong oxidizing agent that it readily oxidizes the diol product further.
When the reaction is carried out in acidic solutions, the diol is not formed. Instead,
the alkene is cleaved. The reaction proceeds by the same mechanism at the start, forming
a cyclic manganate ester (although since the reaction is under acidic conditions the
structure is protonated). In the schemes below, the alkene carbons are highlighted
throughout, so you can see where they end up in the product.
H
H
O
OH this is just the conjugate
KMnO4
C
C
Mn
acid of the structure shown
+
C
C
H3 O
on page 521 in the text
O
O
H
H
Now, under these conditions the manganate ester is not very stable and it undergoes a
cyclic fragmentation process, which results in breaking the C-C bond between the two
oxygens. Notice that in this case, since there was a hydrogen atom attached to each of
the alkene carbons in the starting material, there is a hydrogen attached to the carbonyl
carbon in the product and therefore the product is an aldehyde.
O
H
C
OH
O
OH
C
H H
+ Mn
Mn
C
C
O
O
O
O
H
Aldehydes are very easily oxidized to carboxylic acids, and thus the aldehydes formed
in the cleavage reaction do not survive. They are rapidly transformed into carboxylic
acid groups, by a complex reaction whose mechanism you need not worry about.
O
O
C
C
H KMnO4
H
C
C
O
OH
OH
O
Now, if the alkene had not had any hydrogens attached, the product in that case would
have been a ketone rather than an aldehyde. Ketones are not easily oxidized further, and
the reaction would have stopped at that stage.
CH3
O
C
CH3
KMnO4
CH3
O
OH
OH
H3O+
C
C
CH3
+ Mn
Mn
C
C
C
O
O
CH3
O
O
CH3
If one of the alkene carbons had a hydrogen substituent, while the other did not, then we
would get both acid and ketone groups in our product, as shown below.
C
C
CH3
H
KMnO4
H3O +
CH3
O
OH
C
Mn
C
O
O
H
O
C
CH3
C
CH3
OH
H
C
O
O
C
Ozonolysis
Ozone (O3) is a much better behaved reagent than permanganate, at least at low
temperatures. The fundamental pattern is identical to that of the permanganate reaction,
although the details of the reaction mechanism are quite different. Ozone also cleaves
alkenes, but it will not oxidize aldehyde groups to carboxylic acids. The reaction of
O
ozone with an alkene does not directly form carbonyl groups, however. It is necessary to
reduce an intermediate ozonide, so this is always a 2-stage process. The reduction step
simply cleaves the relatively weak peroxidic O-O bond in the ozonide.
O
CH3
C
CH3
CH3
O3
Zn
CH3
O
CH
Cl
2 2
C
C
HOAc
C O
H
O
O
C
C
-78 oC
reduction
C O
O
H
C
O
H
H
"molozonide"
"ozonide"
Don’t worry about exactly how the molozonide rearranges to form the ozonide. Simply
notice that the alkene carbons of the starting material still appear as the carbonyl carbons
in the product.
Oxidative Cleavage Reactions of Alkynes
An alkyne is more highly oxidized than an alkene, so it should be no surprise that when it
is oxidatively cleaved, the products obtained are also at a higher oxidation level.
Ozonolysis/reduction cleaves alkynes, forming carboxylic acid groups.
1) O3, CH2Cl2
R
1
C
C
2
R
O
o
-78 C
2) Zn, HOAc
R1
O
OH + HO
C
C
R2
The intermediate structures are similar to those formed in the ozonolysis of alkenes, but
simply involve addition of a second molecule of ozone. Notice that the net result is to
form 3 C-O bonds at each of the former alkyne carbons, and the result of reduction with
Zn/HOAc is to cleave the peroxidic bonds, just as before. You are not expected to be
able to write detailed stepwise mechanisms for the rearrangement or reduction steps in
this reaction.
1
R
C
C
R2
O3
O
R
1
C
O
O
C
R2
O3
O
1
R
O
C
O
C
O
O
O
R
2
O
ozonide intermediates
R
1
O
C
O
O
C
O
O
2
R