Ozonolysis

Ozonolysis . It is the name given to the reaction of ozone with organic compounds dissolved in a solvent and through which ozonides are formed, for example the reaction of an alkene or alkyne with an ozone molecule. There are a large number of reactions of organic compounds with ozone, such as the production of ozonides, the reaction of ozone to form aldehydes , dialdehydes , carboxylic acids , etc.

Summary

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  • 1 Story
  • 2 Ozonolysis Reactions
  • 3 Ozonolysis of alkenes
  • 4 Mechanism of Ozonolysis
    • 1 Stage 1. Formation of the molozónido by 1,3-dipolar.
    • 2 Stage 2. Rupture of the molozonido by retro-1,3-dipolar
    • 3 Stage 3. Ozonide formation by 1,3-dipolar and reduction
  • 5 Alkyne Ozonolysis
  • 6 Precautions
  • 7 Sources

History

The ozonolysis process was used on a large scale to locate the position of the double bonds of ethylenic compounds in organic molecules. The discovery of the ozonolysis reaction is attributed to Harries in 1903 . Currently, spectroscopic techniques allow more powerful analyzes with smaller amounts of product and without destruction. This reaction serves to transform the alkenes into ketone / aldehyde / carboxylic acid in organic synthesis. Ozonolysis is a highly effective oxidation step in organic processing technology.

Ozonolysis Reactions

Ozonolysis is the addition of the complete ozone (O 3 ) molecule to the compound, that is, the three oxygen atoms ; it is not a substitution of an element or a molecule in the compound, as is the case with other oxidation reactions.

The process is exothermic and therefore implies that, to maintain stable conditions, it is an essential requirement to have an efficient refrigeration system. The ozonides thus formed are also stable only at a low temperature and require that both the reactor and the downstream processing equipment be kept at a temperature below the decay level of the ozonides.

These reactions involve the use of ozone under special conditions, such as rapid kinetic reactions, highly flammable and potentially explosive materials and with a toxic gas . Based on the experience in the design and use of ozonolysis systems (not only from the scientific point of view) the most significant parameters to take into account when applying the process are being debated.

These appear to be solvent choice, process temperature, reactor design, aerosol management, residual ozone effects, and exhaust gas management. Provided that a correct chemical engineering approach is applied, ozonolysis is an inexpensive alternative oxidant in numerous chemical processes.

As long as correct chemical engineering is applied, ozonolysis should not be considered as an exotic process. It can be done safely and can be applied as an inexpensive oxidizing alternative in many chemical processes.

Ozone is the only oxidant that can be used in a homogeneous system that does not involve adding water . All other oxidants form, in addition to the desired product, by-products that have to be separated from the product, immediately before or after the next treatment steps. For example, hydrogen peroxide can only be used with water which may not be beneficial to the overall process if, at some later stage in the process, it has to be removed.

Alkene Ozonolysis

Alkene ozonolysis consists of a first 1,3-dipolar cycloaddition that generates the molozonide. The retro-1,3-dipolar breaks the molozonide and a new 1,3-dipolar generates an ozonide that breaks to give carbonyl and an oxygen atom. Alkenes react with ozone to form aldehydes, ketones, or mixtures of both after a reduction step.

Ozonolysis Mechanism

Stage 1. Formation of the molozónido by 1,3-dipolar.

The overall result of ozonolysis is the rupture of the carbon-carbon double bond of the molecule, oxygen joins each of the two atoms that form the original double bond forming aldehydes or ketones.

Alkene reaction with ozone.

Ozonolysis occurs when each carbon in the alkene binds to one oxygen in the ozone, the third oxygen reacts with the reducer. An ozone molecule binds to the carbons of a carbon-carbon double bond to give rise to a primary monoozonide or ozonide. This very unstable ozonide splits into two molecules that react with each other to give a secondary ozonide.

The explosive ozonide is generally treated around -80 ° C (193 K), and is separated into two ketones or aldehydes, depending on the initial substituents of the olefin , and an oxygen atom (which will be able to react with the other reaction products, if not captured for example with dimethyl sulphide to give dimethyl sulfoxide (DMSO). the reactions are reduction with catalysts type hydrogen H 2 or platinum Pt (metal), or oxidation with hydrogen peroxide or hydrogen peroxide, H 2 O 2 .

Ozonolysis is an important method of preparing aldehydes and ketones, but it can also be used as an analytical method to determine alkenes. Once the products of ozonolysis are known, the structure of the alkene can be determined.

Stage 2. Rupture of the molozonido by retro-1,3-dipolar

The 1,3-dipolar reaction between ozone (dipole) and an alkene (dipolarphile). It is established to form the molozónido that breaks by means of the retro-1,3-dipolar generating new dipole and diporelafilo, that by means of a new 1,3-dipolar form the ozónido. The ozonide breaks in the reduction stage, leaving the carbonyls free

Rupture of the molozonido.

Stage 3. Ozonide formation by 1,3-dipolar and reduction

Determines the structure of the alkene that produces cyclohexanone and methanal in equimolar relation when breaking with ozone

Ozonid formation.

Alkyne Ozonolysis

Alkynes react with ozone to form carboxylic acids. In this reaction, the triple bond is broken, transforming each carbon of the alkyne into the carboxylic group.

Cautions

Ozone is a fairly dangerous compound. On the one hand it is toxic, and on the other it is explosive. Therefore, a well-ventilated fume hood must be used and the effluents must be collected at the source for disposal. On the other hand, taking into account its explosive nature, it is necessary to avoid a high concentration of ozone and therefore avoid cooling the reaction medium too much since it increases the solubility of ozone in the solvent and if the ozone liquefies, an explosion occurs.

 

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