Alkenes can be oxidized with ozone to form
alcohols,
aldehydes or
ketones, or
carboxylic acids. In a typical procedure, ozone is bubbled through a solution of the alkene in
methanol at until the solution takes on a characteristic blue color, which is due to unreacted ozone. Industry however recommends temperatures near . This color change indicates complete consumption of the alkene. Alternatively, various other reagents can be used as indicators of this endpoint by detecting the presence of ozone. If ozonolysis is performed by introducing a stream of ozone-enriched oxygen through the reaction mixture, the effluent gas can be directed through a
potassium iodide solution. When the solution has stopped absorbing ozone, the excess ozone oxidizes the iodide to
iodine, which can easily be observed by its violet color. For closer control of the reaction itself, an indicator such as
Sudan Red III can be added to the reaction mixture. Ozone reacts with this indicator more slowly than with the intended ozonolysis target. The ozonolysis of the indicator, which causes a noticeable color change, only occurs once the desired target has been consumed. If the substrate has two alkenes that react with ozone at different rates, one can choose an indicator whose own oxidation rate is intermediate between them, and therefore stop the reaction when only the most susceptible alkene in the substrate has reacted. Otherwise, the presence of unreacted ozone in solution (seeing its blue color) or in the bubbles (via iodide detection) only indicates when all alkenes have reacted. After completing the addition, a reagent is then added to convert the intermediate ozonide to a carbonyl derivative.
Reductive work-up conditions are far more commonly used than oxidative conditions. The use of
triphenylphosphine,
thiourea,
zinc dust, or
dimethyl sulfide produces aldehydes or ketones. While the use of
sodium borohydride produces alcohols. (R group can also be hydrogens) : The use of
hydrogen peroxide can produce carboxylic acids. : Amine
N-oxides produce aldehydes directly. Other
functional groups, such as
benzyl ethers, can also be oxidized by ozone. It has been proposed that small amounts of acid may be generated during the reaction from oxidation of the solvent, so
pyridine is sometimes used to
buffer the reaction.
Dichloromethane is often used as a 1:1 cosolvent to facilitate timely cleavage of the ozonide.
Azelaic acid and
pelargonic acids are produced from ozonolysis of
oleic acid on an industrial scale. An example is the ozonolysis of
eugenol converting the
terminal alkene to an aldehyde: : By controlling the reaction/workup conditions, unsymmetrical products can be generated from symmetrical alkenes: • Using
TsOH;
sodium bicarbonate (NaHCO3);
dimethyl sulfide (DMS) gives an aldehyde and a dimethyl
acetal • Using
acetic anhydride (Ac2O),
triethylamine (Et3N) gives a methyl
ester and an aldehyde • Using TsOH; Ac2O, Et3N, gives a methyl ester and a dimethyl acetal.
Reaction mechanism : In the generally accepted mechanism proposed by
Rudolf Criegee in 1953, the alkene and ozone form an intermediate
molozonide in a
1,3-dipolar cycloaddition. Next, the molozonide reverts to its corresponding carbonyl oxide (also called the
Criegee intermediate or Criegee
zwitterion) and aldehyde or ketone (
3) in a retro-1,3-dipolar cycloaddition. The oxide and aldehyde or ketone react again in a 1,3-dipolar cycloaddition, producing a relatively stable
ozonide intermediate (
4). : Evidence for this mechanism is found in
isotopic labeling. When 17O-labelled
benzaldehyde reacts with carbonyl oxides, the label ends up exclusively in the ether linkage of the ozonide. There is still dispute over whether the molozonide collapses via a concerted or radical process; this may also exhibit a substrate dependence.
History Christian Friedrich Schönbein, who discovered ozone in 1840, also did the first ozonolysis: in 1845, he reported that ethylene reacts with ozone – after the reaction, neither the smell of ozone nor the smell of ethylene was perceivable. The ozonolysis of alkenes is sometimes referred to as "Harries ozonolysis", because some attribute this reaction to
Carl Dietrich Harries. Before the advent of modern spectroscopic techniques, the ozonolysis was an important method for determining the structure of organic molecules. Chemists would ozonize an unknown alkene to yield smaller and more readily identifiable fragments.
Ozonolysis of alkynes Ozonolysis of
alkynes generally gives an
acid anhydride or
diketone product, not complete fragmentation as for
alkenes. A reducing agent is not needed for these reactions. The mechanism is unknown. If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two
carboxylic acids. :
Other substrates Although rarely examined,
azo compounds () are susceptible to ozonolysis.
Nitrosamines () are produced. ==Applications==