| Ozonolysis | |
|---|---|
| Reaction type | Organic redox reaction |
| Identifiers | |
| Organic Chemistry Portal | ozonolysis-criegee-mechanism |
Inorganic chemistry,ozonolysis is anorganic reaction where theunsaturated bonds arecleaved withozone (O3). Multiplecarbon–carbon bond are replaced bycarbonyl (C=O) groups, such as aldehydes, ketones, and carboxylic acids. The reaction is predominantly applied to alkenes, but alkynes and azo compounds are also susceptible to cleavage. The outcome of the reaction depends on the type of multiple bond beingoxidized and thework-up conditions.[1]
Detailed procedures have been reported.[2][3][4]
| Alkene Ozonolysis | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Reaction type | Organic redox reaction | ||||||||||
| Reaction | |||||||||||
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| Identifiers | |||||||||||
| Organic Chemistry Portal | ozonolysis-criegee-mechanism | ||||||||||
| RSC ontology ID | RXNO:0000344 | ||||||||||
Alkenes can be oxidized with ozone to formalcohols,aldehydes orketones, orcarboxylic acids. In a typical procedure, ozone is bubbled through a solution of the alkene inmethanol at −78 °C (−108 °F; 195 K) until the solution takes on a characteristic blue color, which is due to unreacted ozone. Industry however recommends temperatures near −20 °C (−4 °F; 253 K).[5] 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 apotassium iodide solution. When the solution has stopped absorbing ozone, the excess ozone oxidizes the iodide toiodine, which can easily be observed by its violet color.[6] For closer control of the reaction itself, an indicator such asSudan 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.[7] 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.Reductivework-up conditions are far more commonly used than oxidative conditions.
The use oftriphenylphosphine,thiourea,zinc dust, ordimethyl sulfide produces aldehydes or ketones. While the use ofsodium borohydride produces alcohols. (R group can also be hydrogens)
The use ofhydrogen peroxide can produce carboxylic acids.
AmineN-oxides produce aldehydes directly.[8] Otherfunctional groups, such asbenzylethers, 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, sopyridine is sometimes used tobuffer the reaction.Dichloromethane is often used as a 1:1 cosolvent to facilitate timely cleavage of the ozonide.Azelaic acid andpelargonic acids are produced from ozonolysis ofoleic acid on an industrial scale.
An example is the ozonolysis ofeugenol converting theterminal alkene to an aldehyde:[9]
By controlling the reaction/workup conditions, unsymmetrical products can be generated from symmetrical alkenes:[10]
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In the generally accepted mechanism proposed byRudolf Criegee in 1953,[11][12][13] the alkene and ozone form an intermediatemolozonide in a1,3-dipolar cycloaddition. Next, the molozonide reverts to its corresponding carbonyl oxide (also called theCriegee intermediate or Criegeezwitterion) 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 stableozonide intermediate, known as atrioxolane (4).
Evidence for this mechanism is found inisotopic labeling. When17O-labelledbenzaldehyde reacts with carbonyl oxides, the label ends up exclusively in the ether linkage of the ozonide.[14] There is still dispute over whether the molozonide collapses via a concerted or radical process; this may also exhibit a substrate dependence.
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.[15] The ozonolysis of alkenes is sometimes referred to as "Harries ozonolysis", because some attribute this reaction toCarl Dietrich Harries.[16] 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 ofalkynes generally gives anacid anhydride ordiketone product,[17] not complete fragmentation as foralkenes. A reducing agent is not needed for these reactions. The mechanism is unknown.[18] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give twocarboxylic acids.
Although rarely examined,azo compounds (N=N) are susceptible to ozonolysis.Nitrosamines (N−N=O) are produced.[19]
The main use of ozonolysis is for the conversion of unsaturated fatty acids to value-added derivatives. Ozonolysis ofoleic acid is an important route toazelaic acid. The coproduct isnonanoic acid:[20]
Erucic acid is a precursor tobrassylic acid, a C13-dicarboxylic acid that is used to make specialtypolyamides andpolyesters. The conversion entails ozonolysis, which selectively cleaves the C=C bond in erucic acid:[21]
A number of drugs and their intermediates have been produced by ozonolysis.[22] The use of ozone in the pharmaceutical industry is difficult to discern owing to confidentiality considerations.[5]
Ozonolysis has been used to characterize the structure of somepolyolefins. Early experiments showed that therepeat unit innatural rubber was shown to beisoprene.
Ozonolysis can be a serious problem, known asozone cracking where traces of the gas in an atmosphere degradeelastomers, such asnatural rubber,polybutadiene,styrene-butadiene, andnitrile rubber. Ozonolysis produces surface ketone groups that can cause further gradual degradation viaNorrish reactions if the polymer is exposed to light. To minimize this problem, many polyolefin-based products are treated withantiozonants.
Ozone cracking is a form ofstress corrosion cracking where active chemical species attack products of a susceptible material. The rubber product must be undertension for crack growth to occur. Ozone cracking was once commonly seen in the sidewalls oftires, where it could expand to cause a dangerousblowout, but is now rare owing to the use of modernantiozonants. Other means of prevention include replacing susceptible rubbers with resistant elastomers such aspolychloroprene,EPDM orViton.
The use of ozone in the pharmaceutical industry is limited by safety considerations.[5]